PATENT DOCUMENT

Publication Number: US-10866458-B2
Application Number: US-201916439453-A
Country: US
Kind Code: B2

Title: Electronic device display with a backlight and control circuitry that corrects pixel data to reduce transition artifacts

Abstract:
An electronic device may include display layers such as liquid crystal display layers and a backlight unit that provides illumination for the display layers. The backlight unit may include light-emitting diodes that emit light into the edge of a light guide film. To minimize the inactive area of the display, the light-emitting diodes may be tightly spaced to approximate a line light source instead of point light sources. Color and/or luminance compensation layers may be incorporated at various locations within the backlight structures to ensure that the backlight provided to the display layers is homogenous. A thin-film transistor layer of the display may be coupled to a printed circuit board by a flexible printed circuit. The flexible printed circuit may have additional solder mask layers to improve robustness, may include encapsulation, and may have traces with a varying pitch.

Claims:
What is claimed is: 
     
       1. An electronic device comprising:
 an array of display pixels; 
 display driver circuitry configured to provide image data to the array of display pixels; and 
 control circuitry including a content processor that is configured to:
 receive uncorrected pixel data for a current frame; and 
 generate modified pixel data for the current frame based on the uncorrected pixel data for the current frame and pixel data for a previous frame using at least one look-up table, wherein the uncorrected pixel data includes black pixel data for a first display pixel, wherein the pixel data for the previous frame includes white pixel data for the first display pixel, and wherein the white pixel data for the first display pixel includes at least one sub-pixel value that is truncated. 
 
 
     
     
       2. The electronic device defined in  claim 1 , wherein the control circuitry is configured to provide the modified pixel data to the display driver circuitry to be displayed on the array of display pixels. 
     
     
       3. The electronic device defined in  claim 1 , wherein the at least one sub-pixel value is truncated for a white point adjustment. 
     
     
       4. The electronic device defined in  claim 1 , wherein the white pixel data for the first display pixel includes a first value for a green sub-pixel, a second value for a red sub-pixel value that is lower than the first value, and a third value for a blue sub-pixel that is lower than the first value. 
     
     
       5. The electronic device defined in  claim 4 , wherein the uncorrected pixel data for the current frame has a value of 0 for the green, red, and blue sub-pixels, and wherein the modified pixel data for the current frame includes a fourth value for the green sub-pixel that is equal to 0, a fifth value for the red sub-pixel that is greater than 0, and a sixth value for the blue sub-pixel that is greater than 0. 
     
     
       6. An electronic device comprising:
 an array of display pixels; 
 display driver circuitry configured to provide image data to the array of display pixels; and 
 control circuitry including a content processor that is configured to:
 receive uncorrected pixel data for a current frame; and 
 generate modified pixel data for the current frame based on the uncorrected pixel data for the current frame and pixel data for a previous frame using at least one look-up table, wherein the uncorrected pixel data includes black pixel data for a first display pixel, wherein the pixel data for the previous frame includes white pixel data for the first display pixel, and wherein the white pixel data for the first display pixel includes a first value for a first sub-pixel, a second value for a second sub-pixel value that is lower than the first value, and a third value for a third sub-pixel that is lower than the first value. 
 
 
     
     
       7. The electronic device defined in  claim 6 , wherein the uncorrected pixel data for the current frame has a value of 0 for the first, second, and third sub-pixels, and wherein the modified pixel data for the current frame includes a fourth value for the first sub-pixel that is equal to 0, a fifth value for the second sub-pixel that is greater than 0, and a sixth value for the third sub-pixel that is greater than 0. 
     
     
       8. An electronic device comprising:
 an array of display pixels; 
 display driver circuitry configured to provide image data to the array of display pixels; and 
 control circuitry including a content processor that is configured to:
 receive uncorrected pixel data for a current frame; and 
 generate modified pixel data for the current frame based on the uncorrected pixel data for the current frame and pixel data for a previous frame using at least one look-up table, wherein generating the modified pixel data for the current frame based on the uncorrected pixel data for the current frame and the pixel data for the previous frame using the at least one look-up table comprises increasing target pixel values for red and blue sub-pixels in pixels undergoing white-to-black transitions.

Description:
This application claims the benefit of provisional patent application No. 62/751,438, filed Oct. 26, 2018, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices with displays, and, more particularly, to displays with backlights. 
     Electronic devices such as computers, cellular telephones, and tablets have displays. Some displays such as organic light-emitting diode displays have arrays of pixels that generate light. In displays of this type, backlighting is not necessary because the pixels themselves produce light. Other displays contain passive pixels that can alter the amount of light that is transmitted through the display to display information for a user. Passive pixels do not produce light themselves, so it is often desirable to provide backlight for a display with passive pixels. Passive pixels may be formed from a layer of liquid crystal material formed between two electrode layers and two polarizer layers. 
     In a typical backlight assembly for a display, a light guide plate is used to distribute backlight generated by a light source such as a light-emitting diode light source. A reflector may be formed under the light guide plate to improve backlight efficiency. 
     Conventional backlight assemblies may cause visible artifacts, may not be robust, and may occupy an undesirably large amount of space within an electronic device. 
     It would therefore be desirable to be able to provide displays with improved backlights. 
     SUMMARY 
     A display may have an array of pixels for displaying images for a viewer. The array of pixels may be formed from display layers such as a color filter layer, a liquid crystal layer, a thin-film transistor layer, an upper polarizer layer, and a lower polarizer layer. 
     A backlight unit may be used to produce backlight illumination for the display. The backlight illumination may pass through the polarizers, the thin-film transistor layer, the liquid crystal layer, and the color filter layer. The backlight unit may have a row of light-emitting diodes that are mounted on a flexible printed circuit board and that emit light into a light guide layer. 
     To minimize the size of the inactive area of the display, the light-emitting diodes that provide the backlight to the light guide layer may be positioned close together along the flexible printed circuit board. The light-emitting diodes may be flip chip bonded to the flexible printed circuit board and may approximate a line light source instead of separate point light sources. 
     Color and/or luminance compensation layers may be incorporated at various locations within the backlight structures to ensure that the backlight provided to the display layers is homogenous. For example, colored tape may be included on light guide layer tabs or a colored adhesive layer may be used to attach the light guide layer to a chassis. 
     The thin-film transistor layer of the display may be coupled to a printed circuit board by a flexible printed circuit. The display driver integrated circuit for the thin-film transistor layer may be mounted on the flexible printed circuit. The flexible printed circuit may have additional solder mask layers to improve the robustness of the flexible printed circuit. The flexible printed circuit may also include encapsulation. The flexible printed circuit may have traces with a pitch that varies across the length of the flexible printed circuit. A stainless steel stiffener may be included on the flexible printed circuit opposite the display driver integrated circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of an illustrative display in an electronic device in accordance with an embodiment. 
         FIG. 3  is a top view of an illustrative display in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of illustrative backlight structures showing the edge of a light guide film adjacent to a plastic chassis in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of illustrative backlight structures showing a light-emitting diode that emits light into the edge of a light guide film in accordance with an embodiment. 
         FIG. 6  is a top view of illustrative backlight structures showing light-emitting diode packages that each include first and second light-emitting diodes in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view of an illustrative light-emitting diode package that includes first and second light-emitting diodes and that is flip chip bonded to a flexible printed circuit in accordance with an embodiment. 
         FIG. 8  is a circuit diagram of an illustrative light-emitting diode package that includes first and second light-emitting diodes in accordance with an embodiment. 
         FIG. 9  is a top view of an illustrative light guide layer that includes light-scattering features that extend across the light guide layer in accordance with an embodiment. 
         FIG. 10  is a cross-sectional side view of illustrative light-scattering features having a cross-sectional shape of a right triangle with a ninety degree angle in accordance with an embodiment. 
         FIG. 11  is a cross-sectional side view of illustrative light-scattering features having a cross-sectional shape of a triangle with an angle greater than ninety degrees in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of illustrative light-scattering features having a cross-sectional shape with a curved upper surface in accordance with an embodiment. 
         FIG. 13  is a cross-sectional side view of illustrative light-scattering features having alternating cross-sectional shapes with different heights in accordance with an embodiment. 
         FIG. 14  is a cross-sectional side view of illustrative light-scattering features having a cross-sectional shape of a triangle with a rounded tip in accordance with an embodiment. 
         FIG. 15  is a top view of an illustrative turning film that includes prisms that extend across the turning film in accordance with an embodiment. 
         FIG. 16  is a cross-sectional side view of illustrative prims having a cross-sectional shape with one planar surface and one curved surface in accordance with an embodiment. 
         FIG. 17  is a cross-sectional side view of illustrative prims having a cross-sectional shape with two curved surfaces having the same curvature in accordance with an embodiment. 
         FIG. 18  is a cross-sectional side view of illustrative prims having a cross-sectional shape with two curved surfaces having different curvature in accordance with an embodiment. 
         FIG. 19  is a cross-sectional side view of illustrative prims having a cross-sectional shape with a curved surface in accordance with an embodiment. 
         FIG. 20  is a cross-sectional side view of illustrative backlight structures including additional ink or colored layers to correct color and/or luminance variations in the emitted backlight in accordance with an embodiment. 
         FIG. 21  is a top view of an illustrative light guide layer having tabs in accordance with an embodiment. 
         FIG. 22  is a cross-sectional side view of an illustrative light guide layer having tabs and a color and/or luminance correcting layer mounted over the tab in accordance with an embodiment. 
         FIG. 23  is a top view of illustrative display layers showing how the display layers may have a non-uniformity such as a bright band along the upper edge when in accordance with an embodiment. 
         FIGS. 24 and 25  are top views of illustrative backlight structures showing how ink may be patterned above the light guide layer to compensate for a non-uniformity of the display in accordance with an embodiment. 
         FIG. 26  is a cross-sectional side view of illustrative backlight structures showing how ink may be patterned above the light guide layer to compensate for a non-uniformity of the display in accordance with an embodiment. 
         FIG. 27  is a cross-sectional side view of illustrative backlight structures that include an additional optical film between a turning film and display layers in accordance with an embodiment. 
         FIG. 28  is a cross-sectional side view of the illustrative additional optical film between the turning film and the display layers in accordance with an embodiment. 
         FIG. 29  is a cross-sectional side view of the illustrative additional optical film between the turning film and the display layers with a diffusive pressure sensitive adhesive layer in accordance with an embodiment. 
         FIG. 30  is a diagram of an illustrative system used to capture images of the display of an electronic device for compensation in accordance with an embodiment. 
         FIG. 31  is a diagram of illustrative method steps used to determine compensation values for each pixel in a display in accordance with an embodiment. 
         FIG. 32  is a diagram of an illustrative display showing how the compensation values for each pixel may be used to modify image data before the image data is provided to display driver circuitry in accordance with an embodiment. 
         FIG. 33  is a cross-sectional side view of an illustrative display with a flexible printed circuit that attaches a thin-film transistor layer to a rigid printed circuit board in accordance with an embodiment. 
         FIG. 34  is a top view of an illustrative flexible printed circuit that attaches a thin-film transistor layer to a rigid printed circuit board and that has a stiffener in accordance with an embodiment. 
         FIG. 35  is a cross-sectional side view of an illustrative attachment region between a flexible printed circuit and a rigid printed circuit board in accordance with an embodiment. 
         FIG. 36  is a top view of an illustrative attachment region between a flexible printed circuit and a rigid printed circuit board showing island-shaped gap-filling layers in accordance with an embodiment. 
         FIG. 37  is a top view of an illustrative flexible printed circuit having traces with a pitch that varies across the flexible printed circuit in accordance with an embodiment. 
         FIG. 38  is a graph showing illustrative profiles of the pitch and density of the flexible printed circuit traces as a function of position in accordance with an embodiment. 
         FIG. 39  is a graph showing illustrative tolerance and pitch profiles of the flexible printed circuit traces as a function of position in accordance with an embodiment. 
         FIG. 40  is a cross-sectional side view of an illustrative attachment region between a flexible printed circuit and a thin-film transistor layer in accordance with an embodiment. 
         FIG. 41  is a cross-sectional side view of an illustrative flexible printed circuit in which a tin layer has been omitted in a bending region in accordance with an embodiment. 
         FIG. 42  is a cross-sectional side view of an illustrative flexible printed circuit in which a tin layer has been omitted in a bending region and only one solder resist is included in the bending region in accordance with an embodiment. 
         FIGS. 43A and 43B  shows illustrative tables of RGB values associated with different white points in accordance with an embodiment. 
         FIG. 44  is a graph showing how red, blue, and green sub-pixel brightness levels may vary during white-to-black transitions in accordance with an embodiment. 
         FIG. 45  shows how pixels may be provided with intermediate target values during transitions to mitigate visible artifacts during the transitions in accordance with an embodiment. 
         FIG. 46  is a diagram of illustrative resources that may be used in an electronic device to reduce color-artifacts such as a green appearance during white-to-black transitions in accordance with an embodiment. 
         FIG. 47  is a flowchart of illustrative operations involved in using resources of the type shown in  FIG. 46  in displaying content with reduced artifacts in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a display is shown in  FIG. 1 . As shown in  FIG. 1 , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  12  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  12  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  12  and may receive status information and other output from device  10  using the output resources of input-output devices  12 . 
     Input-output devices  12  may include one or more displays such as display  14 . Display  14  may be a touch screen display that includes a touch sensor for gathering touch input from a user or display  14  may be insensitive to touch. A touch sensor for display  14  may be based on an array of capacitive touch sensor electrodes, acoustic touch sensor structures, resistive touch components, force-based touch sensor structures, a light-based touch sensor, or other suitable touch sensor arrangements. 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may display images on display  14 . 
     Device  10  may be a tablet computer, laptop computer, a desktop computer, a television, a cellular telephone, a media player, a wristwatch device or other wearable electronic equipment, or other suitable electronic device. 
     Display  14  for device  10  includes an array of pixels. The array of pixels may be formed from liquid crystal display (LCD) components or other suitable display structures. Configurations based on liquid crystal display structures are sometimes described herein as an example. 
     A display cover layer may cover the surface of display  14  or a display layer such as a color filter layer, thin-film transistor layer, or other portion of a display may be used as the outermost (or nearly outermost) layer in display  14 . The outermost display layer may be formed from a transparent glass sheet, a clear plastic layer, or other transparent member. 
     A cross-sectional side view of an illustrative configuration for display  14  of device  10  is shown in  FIG. 2 . As shown in  FIG. 2 , display  14  may include a backlight unit such as backlight unit  42  (sometimes referred to as a backlight or backlight structures) for producing backlight  44 . During operation, backlight  44  travels outwards (vertically upwards in dimension Z in the orientation of  FIG. 2 ) and passes through pixel structures in display layers  46 . This illuminates any images that are being produced by the pixels for viewing by a user. For example, backlight  44  may illuminate images on display layers  46  that are being viewed by viewer  48  in direction  50 . 
     Display layers  46  may be mounted in chassis structures such as a plastic chassis structure and/or a metal chassis structure to form a display module for mounting in a housing in device  10  or display layers  46  may be mounted directly in an electronic device housing for device  10  (e.g., by stacking display layers  46  into a recessed portion in a metal or plastic housing). Display layers  46  may form a liquid crystal display or may be used in forming displays of other types. 
     In a configuration in which display layers  46  are used in forming a liquid crystal display, display layers  46  may include a liquid crystal layer such a liquid crystal layer  52 . Liquid crystal layer  52  may be sandwiched between display layers such as display layers  58  and  56 . Layers  56  and  58  may be interposed between lower polarizer layer  60  and upper polarizer layer  54 . 
     Layers  58  and  56  may be formed from transparent substrate layers such as clear layers of glass or plastic. Layers  56  and  58  may be layers such as a thin-film transistor layer and/or a color filter layer. Conductive traces, color filter elements, transistors, and other circuits and structures may be formed on the substrates of layers  58  and  56  (e.g., to form a thin-film transistor layer and/or a color filter layer). Touch sensor electrodes may also be incorporated into layers such as layers  58  and  56  and/or touch sensor electrodes may be formed on other substrates. 
     With one illustrative configuration, layer  58  may be a thin-film transistor layer that includes an array of pixel circuits based on thin-film transistors and associated electrodes (pixel electrodes) for applying electric fields to liquid crystal layer  52  and thereby displaying images on display  14 . Layer  56  may be a color filter layer that includes an array of color filter elements for providing display  14  with the ability to display color images. If desired, layer  58  may be a color filter layer and layer  56  may be a thin-film transistor layer. Configurations in which color filter elements are combined with thin-film transistor structures on a common substrate layer may also be used. 
     During operation of display  14  in device  10 , control circuitry (e.g., one or more integrated circuits on a printed circuit) may be used to generate information to be displayed on display  14  (e.g., display data). The information to be displayed may be conveyed to a display driver integrated circuit such as circuit  62 A or  62 B using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example). Integrated circuits such as integrated circuit  62 A and/or flexible printed circuits such as flexible printed circuit  64  may be attached to substrate  58  in ledge region  66  (as an example). 
     Backlight structures  42  may include a light guide layer such as light guide layer  78 . Light guide layer  78  may be formed from a transparent material such as clear glass or plastic. During operation of backlight structures  42 , a light source such as light source  72  may generate light  74 . Light source  72  may be, for example, an array of light-emitting diodes (e.g., a series of light-emitting diodes that are arranged in a row that extends into the page in the orientation of  FIG. 2 ). The array of light-emitting didoes may be mounted to a rigid or flexible printed circuit. The printed circuit may be adhered to adjacent layers in the electronic device. In certain embodiments, the printed circuit may be adhered to portions of light guide layer  78 . 
     Light  74  from light source  72  may be coupled into edge surface  76  of light guide layer  78  and may be distributed in dimensions X and Y throughout light guide layer  78  due to the principal of total internal reflection. Light guide layer  78  may include light-scattering features such as pits, bumps, grooves, or ridges that help light exit light guide layer  78  for use as backlight  44 . These features may be located on an upper surface and/or on an opposing lower surface of light guide layer  78 . With one illustrative configuration, a first surface such as the lower surface of light guide layer  78  has a pattern of bumps and an opposing second surface such as the upper surface of light guide layer  78  has a pattern of ridges (sometimes referred to as lenticules, lenticular structures, or lenticular ridges). Light source  72  may be located at any desired edge of light guide layer  78 . 
     Light  74  that scatters upwards in direction Z from light guide layer  78  may serve as backlight  44  for display  14 . Light  74  that scatters downwards may be reflected back in the upward direction by reflector  80 . Reflector  80  may be formed from a reflective structure such as a substrate layer of plastic coated with a dielectric mirror formed from alternating high-index-of-refraction and low-index-of-refraction inorganic or organic layers. Reflector  80  may be formed from a reflective material such as a layer of white plastic or other shiny materials. 
     To enhance backlight performance for backlight structures  42 , backlight structures  42  may include optical films  70 . Optical films  70  may include diffuser layers for helping to homogenize backlight  44  and thereby reduce hotspots. Optical films  70  may also include brightness enhancement films for collimating backlight  44 . Optical films  70  may overlap the other structures in backlight unit  42  such as light guide layer  78  and reflector  80 . For example, if light guide layer  78  has a rectangular footprint in the X-Y plane of  FIG. 2 , optical films  70  and reflector  80  may each have a matching rectangular footprint. Optical films  70  may include compensation films for enhancing off-axis viewing or compensation films may be formed within the polarizer layers of display  14  or elsewhere in display  14 . 
       FIG. 3  is a top view of a portion of display  14  showing how display  14  may have an array of pixels  90  formed within display layers  46 . Pixels  90  may have color filter elements of different colors such as red color filter elements, green color filter elements, and blue color filter elements. Pixels  90  may be arranged in rows and columns and may form active area AA of display  14 . The borders of active area AA may be slightly inboard of the borders of light-guide layer  78  to ensure that there are no visible hotspots in display  14  (i.e., no areas in which the backlight illumination for display  14  is noticeably brighter than surrounding areas). For example, border  92  of active area AA may be offset by a distance  82  from left edge  76  of light guide layer  78 . 
     It is generally desirable to minimize the size of distance  82  so that display  14  is as compact as possible for a given active area size. This minimizes the size of the inactive area (IA) of the display. Nevertheless, distance  82  should not be too small to ensure that there is adequate light mixing. In particular, distance  82  should be sufficiently large to allow light  74  that is emitted from light-emitting diodes  72  to homogenize enough to serve as backlight illumination. Distance  82  is often as long as necessary to ensure light from light-emitting diodes  72  is sufficiently mixed. Accordingly, distance  82  may sometimes be referred to as mixing distance  82 . When light  74  is initially emitted from individual light-emitting diodes  72 , light  74  is concentrated at the exits of light-emitting diodes  72  and is absent in the spaces between light-emitting diodes  72 . After light  74  has propagated sufficiently far within light-guide plate  78  (i.e., after light  74  has traversed a sufficiently large mixing distance  82 ), light  74  will be smoothly distributed along dimension X and will no longer be concentrated near the exits of respective individual light-emitting diodes  72 . To minimize mixing distance  82 , light-emitting diodes  72  may be positioned close together such that the light-emitting diodes approximate a line source instead of separate point sources. 
     The rectangular shape of light guide layer  78  and active area AA in  FIG. 3  is merely illustrative. If desired, light guide layer  78  and/or the active area AA may have a non-rectangular shape (e.g., a shape with one or more curved portions). For example, the active area and/or light guide layer may have a rectangular outline with rounded corners. The active area and/or light guide layer may have a notch along an upper edge that accommodates additional components such as sensors. The active area and light guide layer may have different shapes if desired. For example, the light guide layer may have a rectangular shape with right-angled corners (as shown in  FIG. 3 ) whereas the active area may have a rectangular shape with rounded corners. 
     Detailed cross-sections of backlight structures  42  are shown in  FIGS. 4 and 5 . In particular,  FIG. 4  is a cross-sectional side view of backlight structures  42  taken along line  98  in  FIG. 3 .  FIG. 5  is a cross-sectional side view of backlight structures  42  taken along line  97  in  FIG. 3 . 
     As shown in  FIG. 4 , backlight structures  42  include light guide layer  78  and a reflector  80  attached to a lower surface of the light guide layer. Optical film  70 - 1  may be placed on an upper surface of the light guide layer. Optical film  70 - 1  may be a turning film that directs light from the light guide layer vertically towards the overlying display layers. The optical film may be matte and therefore may sometimes be referred to as matte turning film  70 - 1 . Any desired additional optical films may be incorporated above turning film  70 - 1 . For example, a diffuser layer may be incorporated on an upper surface of turning film  70 - 1  (or positioned above 70-1 and separated from the upper surface of the turning film). In general, any desired additional optical films may be incorporated above optical film  70 - 1  in any desired locations (e.g., positioned directly on the underlying optical film or separated from the underlying optical film by a gap). Turning film  70 - 1  may help homogenize the backlight and ensure that the light is directed vertically towards the viewer. The turning film may also be used to control the viewing angle of the display. 
     Backlight structures  42  also include chassis  102 . Chassis  102  may be a plastic chassis (sometimes referred to as a p-chassis) that supports other layers (e.g., layers in backlight structures  42  and/or display layers  46 ) in the display. Chassis  102  may extend around the periphery of light guide layer  78  with a central opening in which the light guide layer  78  is positioned (e.g., chassis  102  may be ring-shaped). If desired, chassis  102  may be formed from two or more types of material. For example, chassis  102  may be formed from two or more shots of molded plastic having different colors. This example is merely illustrative and chassis  102  may be formed from a single dielectric material if desired. 
     Backlight structures  42  may include an additional chassis  103 . Chassis  103  may be a metal chassis (sometimes referred to as an m-chassis) that supports other layers (e.g., layers in backlight structures  42  and/or display layers  46 ) in the display. Chassis  103  may extend under the entire light guide layer  78  if desired (instead of having a ring-shape like chassis  102 ). In an alternate embodiment, however, chassis  103  may also be ring-shaped. An additional adhesive layer  107  may be interposed between an edge of light guide layer  78  and metal chassis  103 . 
     Adhesive layer  104  may be attached to a top surface of chassis  102 . Adhesive layer  104  may optionally be attached to a top surface of optical film  70 - 1 . Adhesive layer  104  may extend around the periphery of light guide layer  78  and may have a central opening (e.g., adhesive layer  104  may be ring-shaped). Adhesive layer  104  may therefore sometimes be referred to as ring tape. Ring tape  104  may attach backlight structures  42  to display layers  46  if desired. Ring tape may alternatively attach chassis  102  to other layers (e.g., optical films) within backlight structures  42 . Another adhesive layer  106  may attach a lower surface of chassis  102  to an upper surface of chassis  103 . Adhesive layers  104  and  106  may be pressure sensitive adhesive layers or any other desired type of adhesive layers. 
     To minimize the width of the inactive area of the display, the distance  206  between chassis  102  and light guide layer  78  may be minimized. For example, distance  206  may be less than 1 millimeter, less than 0.5 millimeters, less than 0.4 millimeters, less than 0.3 millimeters, between 0.2 and 0.5 millimeters, between 0.3 and 0.4 millimeters, between 0.25 and 0.35 millimeters, greater than 0.2 millimeters, etc. 
     The three edges of backlight structures  42  that do not include light-emitting diodes  72  may have cross-sections of the type shown in  FIG. 4 . For example, looking at  FIG. 3 , light-emitting diodes  72  are positioned along a left edge of the light guide layer. The left edge of the backlight structures has a different arrangement than the remaining three edges of the backlight structures. The remaining three edges of the backlight structures may have an arrangement of the type shown in  FIG. 4 . The example of the light-emitting diodes being positioned along the left edge of the light guide layer is merely illustrative. The light-emitting diodes may be positioned along any edge (or more than one edge) of the light guide layer. 
       FIG. 5  is a cross-sectional side view of the left edge of the backlight structures where the light sources emit light into the light guide layer. As shown in  FIG. 5 , this portion of the backlight structures still includes light guide layer  78 , a reflector layer  80  attached to a lower surface of the light guide layer, and an optical film such as turning film  70 - 1  attached to an upper surface of the light guide layer. However, light sources  72  may be included to emit light  74  in the Y-direction through edge surface  76  of the light guide layer. 
     Light sources  72  may be light-emitting diodes that are arranged in a row along the edge surface  76  of the light guide layer. Each light-emitting diode  72  may be mounted on a printed circuit board such as flexible printed circuit board  108 . Flexible printed circuit board  108  may be a printed circuit formed from sheets of polyimide or other flexible polymer layers. Flexible printed circuit board  108  (sometimes referred to as flexible printed circuit  108  or printed circuit  108 ) may include patterned metal traces for carrying signals between components on the flexible printed circuit board. Flexible printed circuit board  108  may optionally include contact pads (e.g., solder pads) on an upper surface of the flexible printed circuit board. Solder may be used to couple the light-emitting diodes to solder pads on the flexible printed circuit board. Each light-emitting diode (sometimes referred to as a light-emitting diode package) may have one or more associated solder pads. The solder may electrically and mechanically connect the light-emitting diodes to flexible printed circuit board. Optionally, an additional adhesive layer may be attached to both the upper surface of the light-emitting diodes  72  and the upper surface of light guide layer  78 . 
     Flexible printed circuit board  108  may also be coupled to light guide layer  78  by an additional adhesive layer. As shown in  FIG. 5 , flexible printed circuit board  108  may include an adhesive layer  116  that is attached between light guide layer  78  and flexible printed circuit  108 . Adhesive layer  116  may be a pressure sensitive adhesive layer or any other desired type of adhesive layer. 
     Ring tape  104  may be attached to an upper surface of chassis  103 . Chassis  103  may be attached to a lower surface of flexible printed circuit  108  by adhesive layer  130 . Adhesive layer  130  may be a pressure sensitive adhesive layer or any other desired type of adhesive layer. Backlight structures  42  may also include one or more additional adhesive layers that attach chassis  103  to reflector layer  80 . 
     The light guide layer  78  of the backlight structures in  FIGS. 4 and 5  may be a light guide film (LGF) formed from a polymer material such as polycarbonate. Using a polycarbonate light guide film may result in the light guide layer having a small thickness  202  (see  FIG. 5 ). For example, thickness  202  may be less than 1 millimeter, less than 0.5 millimeters, less than 0.4 millimeters, less than 0.3 millimeters, between 0.2 and 0.5 millimeters, between 0.3 and 0.4 millimeters, between 0.35 and 0.40 millimeters, greater than 0.2 millimeters, etc. 
     Additionally, the arrangement of  FIG. 5  minimizes the mixing distance  82  between light-emitting diodes  72  and the active area of the display. For example, distance  82  may be less than 5.0 millimeters, less than 4.0 millimeters, less than 3.5 millimeters, less than 3.0 millimeters, less than 2.0 millimeters, greater than 1.0 millimeter, between 2.0 and 4.0 millimeters, between 2.5 and 3.5 millimeters, etc. 
     The small mixing distance in  FIG. 5  may be a result of having light-emitting diodes  72  be positioned close together on flexible printed circuit  108 . By positioning the light-emitting diodes close together, the light emitted from the light-emitting diodes will better approximate a uniform line light source rather than separate point light sources. 
       FIG. 6  is a top view of backlight structures illustrating how the light-emitting diode packages may be positioned close together to minimize mixing distance. As shown in  FIG. 6 , each light-emitting diode package  208  may include first and second light-emitting diodes  72 - 1  and  72 - 2 . The edge-to-edge distance between adjacent light-emitting diode packages (e.g., distance  211 ) may be less than 1 millimeter, less than 0.5 millimeters, less than 0.4 millimeters, less than 0.3 millimeters, between 0.2 and 0.5 millimeters, between 0.3 and 0.4 millimeters, between 0.25 and 0.35 millimeters, greater than 0.2 millimeters, etc. The pitch of the light-emitting diode packages  208  (e.g., the center-to-center distance  213  between adjacent light-emitting diode packages) may be less than 5.0 millimeters, less than 4.0 millimeters, less than 3.5 millimeters, less than 3.0 millimeters, less than 2.0 millimeters, greater than 1.0 millimeter, between 2.0 and 4.0 millimeters, between 2.5 and 3.5 millimeters, between 3.0 and 4.0 millimeters, between 3.0 and 3.5 millimeters, etc. The pitch of the light-emitting diodes (e.g., the center-to-center distance  215  between adjacent light-emitting diodes) may be less than 4.0 millimeters, less than 3.0 millimeters, less than 2.0 millimeters, less than 1.5 millimeters, less than 1.0 millimeters, greater than 1.0 millimeter, between 1.0 and 2.0 millimeters, between 1.0 and 2.5 millimeters, between 1.5 and 2.0 millimeters, etc. The edge-to-edge distance between adjacent light-emitting diodes (e.g., distance  217 ) may be less than 1 millimeter, less than 0.8 millimeters, less than 0.6 millimeters, less than 0.4 millimeters, between 0.4 and 0.6 millimeters, between 0.45 and 0.55 millimeters, greater than 0.3 millimeters, etc. 
     To form the light-emitting diode packages close together as in  FIG. 6 , the light-emitting diode packages may be flip chip bonded to the underlying flexible printed circuit board ( 108 ). An example of this type is shown in  FIG. 7 . As shown in  FIG. 7 , the light-emitting diode package  208  may be attached to flexible printed circuit board  108  using first, second, and third flip chip bumps  210 - 1 ,  210 - 2 , and  210 - 3 . Each flip chip bump may be formed from solder and therefore may sometimes be referred to as a solder ball. 
     Light-emitting diode package  208  may have three recesses  212  formed in a first portion  208 - 1  of the light-emitting diode package. Each recess may receive a corresponding solder ball  210 . The second portion  208 - 2  of the light-emitting diode package may include first and second light-emitting diodes  72 - 1  and  72 - 2 . The light-emitting diodes  72 - 1  and  72 - 2  may be coupled in series between a corresponding anode (e.g., solder ball  210 - 3 ) and cathode (e.g., solder all 210-1). The intervening solder ball  210 - 2  may serve as a heat sink and may allow individual testing of light-emitting diodes  72 - 1  and  72 - 2 . 
     A solder pad may be included in each recess  212  to electrically connect the light-emitting diode package to the solder balls. Flexible printed circuit board  108  may have corresponding recesses  214 , each of which receives a solder ball. A solder pad may be included in each recess  214  to electrically connect the flexible printed circuit board to the solder balls. The example in  FIG. 7  of both light-emitting diode package  208  and flexible printed circuit board  108  having recesses to receive the solder balls is merely illustrative. In general, recesses may be formed on none, one, or both of the light-emitting diode package  208  and flexible printed circuit board  108  for each solder ball. 
     The recesses  214  may have larger widths than their corresponding recesses  212 . Additionally, the recess  212  associated with solder ball  210 - 2  may be wider than the recesses  212  associated with solder balls  210 - 1  and  210 - 3 . Similarly the recess  214  associated with solder ball  210 - 2  may be wider than the recesses  214  associated with solder balls  210 - 1  and  210 - 3 . 
       FIG. 8  is a circuit diagram showing the circuit associated with light-emitting diode package  208  of  FIG. 7 . As shown, light-emitting diodes  72 - 1  and  72 - 2  are coupled in series between the anode and the cathode. Solder ball  210 - 1  may be coupled to the cathode voltage whereas solder ball  210 - 3  may be coupled to the anode voltage. Solder ball  210 - 2  may be coupled to a node  218  between the light-emitting diodes  72 - 1  and  72 - 2 . A voltage for testing one of the light-emitting diodes may optionally be coupled to solder ball  210 - 2 . 
     As previously mentioned, light guide layer  78  may have light-scattering features such as pits, bumps, grooves, or ridges that help light exit light guide layer  78  for use as backlight  44 . Light guide layer  78  may work in combination with turning film  70 - 1  to ensure that backlight  44  is provided to the display layers  46  in a desired direction at a desired viewing angle.  FIG. 9  is a top view of light guide layer  78  showing how the light guide layer  78  may have light-scattering features  252  (sometimes referred to as light-scattering structures) that extend along a longitudinal axis orthogonal to the edge  76  of the light guide layer that receives light from light-emitting diode packages  208 . Light-scattering structures  252  extend parallel to the direction in which light is emitted from the light-emitting diodes. Light-scattering features  252  may extend entirely across the light guide layer or may only extend partially across the light guide layer. The density of light-scattering structures  252  may be uniform across the light guide layer or may vary across the light guide layer (e.g., may vary along the X-axis and/or Y-axis). Light-scattering structures  252  may sometimes be referred to as lenticular light-scattering structures. 
     Light-scattering structures  252  may have any desired shape.  FIG. 10  is a cross-sectional side view of a light guiding layer  78  having light-scattering structures  252  with a triangular cross-sectional shape. As shown in  FIG. 10 , the light-scattering structures are formed on an upper surface  75  of the light guide layer. The light-scattering structures may be formed from the same material as the light guide layer (and therefore may be formed integrally with the light guide layer) or may be formed from a different material than the light guide layer. 
     The triangular light-scattering structures have first and second surfaces that meet at a vertex defined by angle θ 1 . In  FIG. 10 , Oi is equal to 90 degrees (e.g., the triangular cross-section is a right triangle). Light may reflect off of the surfaces of the light-scattering structure due to total internal reflection. As shown in  FIG. 10 , light that starts in the positive Z-direction will be reflected by light-scattering structure  252  and follow path  254 . When the light-scattering structure has a triangular cross-section with a right-angle the light may not be mixed effectively. In  FIG. 11 , the light-scattering structures are characterized by an angle θ 2  that is greater than 90°. For example, θ 2  may be greater than 100°, greater than 110°, greater than 120°, greater than 135°, between 91° and 110°, between 95° and 105°, greater than 95°, less than 145°, etc. Light that starts in the positive Z-direction will be reflected by light-scattering structure  252  and follow path  256 . As shown in  FIG. 11 , when the angle θ 2  is greater than 90°, light may be mixed more than in  FIG. 10  when the angle is equal to 90°. 
       FIG. 12  is a cross-sectional side view of a light guiding layer  78  having light-scattering structures  252  with a semi-circular cross-sectional shape. As shown in  FIG. 12 , the light-scattering structures are formed on an upper surface  75  of the light guide layer. The light-scattering structures may be formed from the same material as the light guide layer (and therefore may be formed integrally with the light guide layer) or may be formed from a different material than the light guide layer. The light-scattering structures have an upper surface that is curved. The upper surface may have a uniform radius of curvature across the cross-section of the light-scattering structure (e.g., as in a semi-circular cross-section). The upper surface may also have a non-uniform radius of curvature across the cross-section of the light-scattering structure. The radius of curvature may vary along the length of the light-scattering structure if desired. 
     The shape of the light-scattering structure in  FIG. 12  may also mix the light more effectively than the light-scattering structures in  FIG. 10 . Light that starts in the positive Z-direction will be reflected by light-scattering structure  252  and follow path  258 . Even when the light-scattering structure has a curved upper surface, the light-scattering structure may be characterized by an angle θ 3 . The angle θ 3  associated with the curved cross-sectional shape may be the angle of the vertex of a triangle drawn between the top-most point of the curved upper surface and the edges of the curved upper surface (at the point they meet the planar upper surface  75  of the light guide layer). The angle θ 3  may be greater than 90° to promote light mixing. For example, 03 may be greater than 100°, greater than 110°, greater than 120°, greater than 135°, between 91° and 110°, between 95° and 105°, greater than 95°, less than 145°, etc. 
     If desired, the light-scattering structures  252  on the upper surface of light guide layer  78  may vary in shape. As shown in  FIG. 13 , light-scattering structures  252 - 1  having a first shape may alternate with light-scattering structures  252 - 2  having a second shape. The first light-scattering structures  252 - 1  may have a first height  260  that is greater than the height  262  of the second light-scattering structures  252 - 2 . Alternating the light-scattering structures in this way may improve the light mixing within the light guide layer. 
     Both light-scattering structures  252 - 1  and light-scattering structures  252 - 2  may have an associated angle θ. Light-scattering structures  252 - 1  have an associated angle θ 1  whereas light scattering structures  252 - 2  have an associated angle θ 2 . In this example, light-scattering structures  252 - 1  and light-scattering structures  252 - 2  have the same width. Therefore, because height  260  is greater than height  262 , angle θ 1  is less than angle θ 2 . However, both angles θ 1  and θ 2  may be greater than 90°. The difference between angles θ 1  and θ 2  may be greater than 5°, greater than 10°, greater than 20°, less than 25°, etc. 
       FIG. 14  shows yet another example for a cross-sectional shape of light-scattering structures  252 . As shown in  FIG. 14 , the light-scattering structures may have a triangular shape with a rounded tip. Each light-scattering structure includes a planar surface  264 , a planar surface  266 , and a curved surface  268  (sometimes referred to as rounded surface  268 , rounded tip  268 , rounded vertex  268 , etc.) interposed between the planar surfaces  264  and  266 . Having the rounded tip  268  instead of an angled vertex may improve light mixing. The angle θ associated with each light-scattering structure may be greater than 90° to promote light mixing. For example, θ may be greater than 100°, greater than 110°, greater than 120°, greater than 135°, between 91° and 110°, between 95° and 105°, greater than 95°, less than 145°, etc. 
     In general, each light-scattering structure may have any desired structure. The light-scattering structures may have different structures, the same structures, structures that vary in any desired pattern, etc. The light-scattering structures may themselves have a uniform cross-sectional shape or a cross-sectional shape that varies along the length of the light-scattering structure. The light-scattering structures may be formed on the upper surface of the light guide layer and/or the lower surface of the light guide layer. Any of the potential light-scattering structure arrangements described herein may be used in the light guide layer of  FIGS. 4 and 5 . 
     Turning film  70 - 1  (see  FIGS. 4 and 5 ) may also have light-redirecting features such as pits, bumps, grooves, ridges, or prisms that help redirect light in a desired direction for use as backlight  44 .  FIG. 15  is a top view of turning film  70 - 1  showing how the turning film  70 - 1  may have prisms  272  that extend along a longitudinal axis parallel to the edge  76  of the light guide layer that receives light from light-emitting diode packages  208 . Prisms  272  are orthogonal to the direction in which light is emitted from the light-emitting diodes. Prisms  272  may extend entirely across the light guide layer or may only extend partially across the light guide layer. The density of prisms  272  may be uniform across the light guide layer or may vary across the light guide layer (e.g., may vary along the X-axis and/or Y-axis). 
     Prisms  272  may have any desired shape.  FIG. 16  shows an illustrative example of a cross-sectional shape for prisms  272 . As shown in  FIG. 16 , optical film  70 - 1  may include a base film portion  270  with prisms  272  extending from a lower surface  271  of the base film portion  270 . The prisms  272  in  FIG. 16  may be formed from the same material as base film portion  270  or a different material than base film portion  270 . Each prism  272  in  FIG. 16  has a planar surface  274  that meets a curved surface  276  at a vertex  278 . Planar surface  274  may be positioned either on the side of the prism  272  closer to the light-emitting diodes or the side of the prism further from the light-emitting diodes. 
       FIG. 17  shows an illustrative example of a cross-sectional shape for prisms  272 . As shown in  FIG. 17 , optical film  70 - 1  may include a base film portion  270  with prisms  272  extending from a lower surface  271  of the base film portion  270 . The prisms  272  in  FIG. 17  may be formed from the same material as base film portion  270  or a different material than base film portion  270 . Each prism  272  in  FIG. 17  has a curved surface  280  that meets a curved surface  282  at a vertex  284 . In this example, the curvature of curved surfaces  280  and  282  is the same. However, this example is merely illustrative. In a similar example, shown in  FIG. 18 , the curvature of surface  280  is different than the curvature of surface  282 . 
       FIG. 19  shows an illustrative example of a cross-sectional shape for prisms  272 . As shown in  FIG. 19 , optical film  70 - 1  may include a base film portion  270  with prisms  272  extending from a lower surface  271  of the base film portion  270 . The prisms  272  in  FIG. 19  may be formed from the same material as base film portion  270  or a different material than base film portion  270 . Each prism  272  in  FIG. 19  has a curved surface  286  with a rounded tip  288  (as opposed to the vertices of  FIGS. 16-18 ). 
     In general, each prism in turning film  70 - 1  may have any desired structure. The prisms may have different structures, the same structures, structures that vary in any desired pattern, etc. The prisms may themselves have a uniform cross-sectional shape or a cross-sectional shape that varies along the length of the prism. The prisms may be formed on the upper surface of the light guide layer and/or the lower surface of the light guide layer. Any of the potential prism arrangements described herein may be used in the light guide layer of  FIGS. 4 and 5 . 
     Having the prisms of turning film  70 - 1  on the lower surface of the turning film and the light-scattering structures  252  of the light guide layer on the upper surface of the light guide layer may help prevent scratching of the layers and/or wetting between the layers. Additional structures (e.g., laser dots) may be included on the turning film (or other optical films in the electronic device) to prevent scratching and/or wetting. 
     Light guide layer  78  (and corresponding light-scattering structures  252 ) may be used to control the viewing angle of the display within the XZ-plane. For example, as shown in  FIG. 9  the design of the light-scattering structures  252  may control how much the light is spread within the XZ-plane when exiting the light guide layer. Turning film  70 - 1  (and corresponding prisms  272 ) may be used to control the viewing angle of the display within the YZ-plane. For example, as shown in  FIG. 15  the design of the prisms  272  may control how much the light is spread within the YZ-plane when exiting the turning film. Light guide layer  78  and turning film  70 - 1  therefore may be used to provide a high-viewing-angle display with a high viewing-angle both along the X-axis and along the Y-axis. 
     It is generally desirable for homogenous backlight to be emitted from backlight structures  42 . It is also generally desirable for homogenous light to be emitted from display layers  46  when desired. For example, if all of the display pixels within the display are set to emit white light, it is desirable for the white light to be uniform across the entire display. Numerous steps may be taken to ensure uniform emission of light from display  14  in electronic device  10 . 
       FIG. 20  is a cross-sectional side view of backlight structures including additional ink or colored layers to correct color and/or luminance variations in the emitted backlight. Similar to  FIG. 4 ,  FIG. 20  is a cross-sectional side view of backlight structures  42  taken along line  98  in  FIG. 3 . In  FIG. 20  additional ink layers that may be used to adjust color and/or luminance are shown. 
     First, adhesive layer  107  (sometimes referred to as light guide film fixing tape) may include additional ink to control the color of the adhesive layer. Gray ink may be included in or on the adhesive layer  107  to help ensure uniform luminance at the edge of the light guide layer. Alternatively, colored ink (e.g., ink that is blue, yellow, red, green, or any other desired color) may be included in or on the adhesive layer  107  to ensure uniform color of light at the edge of the light guide layer. 
     Other components near light guide layer  78  may have their color selected to correct color and/or luminance variations in the emitted backlight or may have an attached additional ink layer to correct color and/or luminance variations. For example, an additional ink layer  302  may be attached to the edge surface of light guide layer  78  or an additional ink layer  304  may be attached to the edge of chassis  102 . The additional ink layers  302  and  304  may include gray ink or colored ink (e.g., ink that is blue, yellow, red, green, or any other desired color). The additional ink layers may be, for example, layers of tape having a specific desired color. Alternatively, ink or pigment may be added to chassis  102  itself such that the edge surface of the chassis has a specific desired color. For example, gray ink or blue ink may be added to the plastic chassis  102 . 
     In another example, an additional ink layer  306  may be attached to the bottom surface of light guide layer  78  between reflector  80  and adhesive layer  107 . Alternatively, an additional ink layer  308  may be attached to the upper surface of metal chassis  103  between reflector  80  and adhesive layer  107 . The additional ink layers  306  and  308  may include gray ink or colored ink (e.g., ink that is blue, yellow, red, green, or any other desired color). The additional ink layers may be, for example, layers of tape having a specific desired color. Alternatively, ink or pigment may be added to chassis  103  such that the upper surface of the chassis has a specific desired color. For example, gray ink or blue ink may be added to the metal chassis  103 . Ink layers  302 ,  304 ,  306 , and  308  may be referred to as layers, coatings, ink layers, tape layers, color correcting layers, compensating layers, luminance correcting layers, or paint layers. Each layer ( 302 ,  304 ,  306 ,  308 ) does not necessarily need to be formed from ink. Each layer may be formed from ink, pigment, or any other material that creates a difference in color or luminance compared with the surrounding material. 
     These examples are merely illustrative. Additional layers having any desired color may be incorporated at any desired location within the display to correct luminance or color issues if desired. 
     In some cases, pixel values may be reduced (truncated) in order to correct the white point of displayed light. Additionally, sometimes during operation of the display, displayed content may need to rapidly move across the display. For example, when a user is scrolling through the display the displayed content moves rapidly across the display. To avoid visible artifacts when content is moving rapidly across the display, voltages used in operating the display may be overdriven. For example, the data lines that provide data to the display pixels or switching transistors involved in operating the display pixels may be operated with an overdriven voltage (e.g., a voltage that is higher than during normal operation). Overdriving the display pixels in this way may speed up the time it takes to refresh the pixel, reducing visible artifacts caused by transitions between brightness levels when displaying rapidly changing content. 
     When in the overdrive mode, pixels of a certain color may be overdriven less than other pixels to avoid visible artifacts. For example, a green shadow may be visible in situations in which content is moving rapidly across the display and truncation in red and blue pixels are high. To prevent this type of visible artifact, the green pixels may be overdriven less than the red and blue pixels (e.g., the overdrive voltage of the green pixels will be lower than the overdrive voltage for the red and blue pixels). In other words, pixels of different colors may be overdriven by different amounts to ensure that no visible artifacts are present. This concept will be described in more detail in connection with  FIGS. 43-47 . 
       FIG. 21  is a top view of an illustrative light guide layer that may be included in backlight structures  42 . As shown in  FIG. 21 , light guide layer  78  may have tabs  78 T. Each tab  78 T may protrude from an edge of the light guide layer. Tabs  78 T may therefore sometimes be referred to as protrusions  78 T. Protrusions  78 T may be used to secure the light guide layer within the electronic device. For example, adhesive may be attached to an upper or lower surface of the protrusions. 
     In some cases, tabs  78 T may cause visible artifacts when viewing the display. For example, at a high viewing angle the areas of light guide layer  78  adjacent to tabs  78 T in the active area may appear darker than other areas. To avoid this type of visible artifact, an additional layer may be included on the tabs to reduce the differences between the areas adjacent to the tabs and the areas not adjacent to the tabs. 
       FIG. 22  is a cross-sectional side view of backlight structures  42  showing a tab  78 T of light guide layer  78 . As shown, adhesive layer  107  may attach the tab to chassis  103 .  FIG. 22  shows an additional optical film  70 - 2  positioned above optical film  70 - 1 . Optical film  70 - 2  may be, for example, a diffuser layer, a brightness enhancement film, or any other desired type of optical film. An opaque masking layer  122  may be formed on the lower surface of optical film  70 - 2 . Opaque masking layer  122  may be formed from black ink or another desired opaque material. The presence of the opaque masking layer may contribute to the contrast difference in regions of the light guide layer adjacent to the tabs  78 T. 
     Additional ink layer  124  may be included over tab  78 T to correct contrast differences in the tab region. The additional ink layer  124  (sometimes referred to as coating  124 , layer  124 , tape layer  124 , color correcting layer  124 , compensating layer  124 , luminance correcting layer  124 , or paint layer  124 ) may have a color selected to correct contrast differences in the tab regions. The ink layer  124  may be black, gray, or any other desired color. Layer  124  may be formed only over the tab  78 T of light guide layer  78  or may be partially formed over non-tab portions of light guide layer  78 . The layer may overlap some of the active area of the display or may not overlap the active area of the display. In  FIG. 22 , layer  124  is depicted as being formed on turning film  70 - 1  (e.g., layer  124  directly contacts the upper surface of turning film  70 - 1 ). This example is merely illustrative. If desired, the layer  124  may be formed in direct contact with the light guide layer  78  (e.g., an upper surface of a tab  78 T of the light guide layer). The layer  124  may be interposed between light guide layer  78  and turning film  70 - 1 . Similarly, opaque masking layer  122  may be formed at any desired location within the display (e.g., on an upper or lower surface of an additional optical film, between and in direct contact with two adjacent optical films, etc.). Layers  122  and  124  may be formed from ink, pigment, or any other material that creates a difference in color or luminance compared with the surrounding material. 
     In some cases, a compensating layer may be used to modify the backlight provided to display layers  46  to compensate for variations in display layers  46 . For example, consider an example where display layers  46  receive homogenous backlight with a uniform color and luminance and the display layers are configured to display a uniform white color on the active area of the display. Because the backlight has a uniform color and luminance, the display layers would ideally emit a uniform white light. However, even if receiving backlight with a uniform color and luminance, variations in the display layers may cause the emitted light to have a non-uniform appearance. 
       FIG. 23  is a top view of display layers  46  attempting to display a uniform white color (when receiving uniform backlight). Despite receiving uniform backlight, the display layers may have a band  310  along an upper edge of the display that is brighter than the remaining portion  312  of the display. The bright band  310  may be caused by variations within the display layers (e.g., thickness variations at the edge of the display layers). To correct for these types of a variations within the display layers, non-uniform backlight may deliberately be provided to the display layers. For example, the backlight provided to display layers  46  in region  310  may be dimmer than the backlight provided to display layers in region  312 . 
     To dim the backlight provided to the display layers in region  310 , a compensating layer may be included between the light guide layer  78  and the display layers.  FIG. 24  is a top view of a light guide layer showing how compensating layer  314  may cover a portion of the light guide layer. The compensating layer  314  may have a length  318  and width  316 . Because the non-uniformity of the display layers (in this example) is along the upper edge of the display, the compensating layer is also formed along the upper edge of the display layer. 
     Compensating layer  314  (sometimes referred to as coating  314 , layer  314 , tape layer  314 , color correcting layer  314 , compensating layer  314 , luminance correcting layer  314 , or paint layer  314 ) may have a color selected to correct for non-uniformities in the display layers. Compensating layer  314  may be formed from ink, pigment, or any other material that creates a difference in color or luminance compared with the surrounding material. The compensating material may be formed at any desired position between the light guide layer and the display layers (e.g., on the light guide layer, on an optical film, etc.). The compensating layer may be formed from gray ink or blank ink, in one example. The density of the ink may vary within the compensating layer if desired (e.g., the ink may have a higher density closer to the edge of the light guide layer). 
     The shape of the region covered with compensating layer  314  in  FIG. 24  is merely illustrative. Compensating layer  314  may have any desired shape.  FIG. 25  shows an example where compensating layer  314  covers a smaller area of light guide layer  78  than in  FIG. 24 . In  FIG. 25 , compensating layer  314  has a length  322  that is less than the length  318  of  FIG. 24 . Similarly, compensating layer  314  has a width  320  that is less than the width  316  of  FIG. 24 . Compensating layer  314  may have curved edges and may extend only partially across the upper edge of the light guide layer, as shown in  FIG. 25 . 
     The example in  FIGS. 24 and 25  of compensating layer  314  covering the upper edge of the light guide layer is merely illustrative. In this example, the light-emitting diodes that provide light to the light guide layer are positioned along the left edge of the display and the compensating layer  314  is positioned along the upper edge of the display. However, compensating layer  314  may be positioned along the left edge of the display or any other desired location of the display. More than one compensation layer (optionally having different colors or other properties) may also be used to correct for variations within the display layers. 
       FIG. 26  is a cross-sectional side view backlight structures that include a compensating layer to correct for variations in display layers  46 . As shown in  FIG. 26 , compensation layer  314  may also be formed on the lower surface of an additional optical film  70 - 2  (e.g., a diffuser layer, brightness enhancement film, etc.). Compensation layer  314  may instead be formed on turning film  70 - 1  if desired. Compensation layer  314  may be formed at any other desired location within the display (e.g., directly on the upper surface of light guide layer  78 , on the upper surface of an additional optical film, etc.). 
     As shown in the cross-sectional side view of  FIG. 27 , an additional optical film  70 - 2  may be incorporated between turning film  70 - 1  and display layers  46 . Optical film  70 - 2  may be a diffuser layer, a brightness enhancement film, a compensation film for enhancing off-axis viewing, a polarizer layer, a combination of one or more of these etc.  FIG. 28  is a cross-sectional side view of optical film  70 - 2 . As shown in  FIG. 28 , optical film  70 - 2  may be formed from a plurality of layers (e.g., layer  324 - 1 , layer  324 - 2 , layer  324 - 3 , layer  324 - 4 , and layer  324 - 5 ). These layers may include pressure sensitive adhesive (PSA) layers, polarizing layers, and/or any other desired type of layer. To improve the defect hiding power of the optical film, the optical film may include a diffusive pressure sensitive adhesive layer.  FIG. 29  is a cross-sectional side view of optical film  70 - 2  that includes diffusive pressure sensitive adhesive layer  324 - 6 . In this example, optical film  70 - 2  may be a brightness enhancement film. Including the diffusive pressure sensitive adhesive layer in the brightness enhancement film may cause additional diffusion of light received from the light guide layer. This may hide particle defects in the underlying layers (e.g., defects caused by contaminant particles or other kinds of defects). 
     The example of diffusive pressure sensitive adhesive layer  324 - 6  being interposed between other layers in brightness enhancement film  70 - 2  is merely illustrative. Diffusive pressure sensitive adhesive layer  324 - 6  may instead be the upper-most layer in brightness enhancement film  70 - 2  or the lowest layer in brightness enhancement film  70 - 2 . In some embodiments, diffusive pressure sensitive adhesive layer  324 - 6  may be attached to and in direct contact with an additional pressure sensitive adhesive layer (e.g., a pressure sensitive adhesive layer that does not have diffusive properties). For example, layer  324 - 2  in  FIG. 29  may be a pressure sensitive adhesive layer (that does not have diffusive properties) or layer  324 - 3  in  FIG. 29  may be a pressure sensitive adhesive layer (that does not have diffusive properties). 
     During manufacturing, some electronic devices may be produced with defects that affect the quality of the display. The defects may result in artifacts that are visible if corrective action is not taken. Without corrective action, some of the manufactured electronic devices may not be suitable for use. To increase the number of manufactured devices that may be used, display compensation techniques may be applied to the electronic devices. 
       FIG. 30  shows an illustrative system  326  for capturing images of a display and determining if compensation is required. As shown in  FIG. 30 , an electronic device being tested (sometimes referred to as a device under test) may emit light in direction  328  using display  14 . The device under test (DUT) may attempt to emit a uniform white image or some other desired test image. An image sensor  330  facing direction  332  may face the display of the DUT. The image sensor may capture an image of the display to determine if any irregularities are present in the display. The detected irregularities may be used to generate a pixel compensation table that is used to correct for the irregularities during operation of the device. For example, image sensor  330  may detect that, when attempting to display an entirely white image, a portion of the display of the DUT is slightly blue. The pixels in the slightly blue region of the display may therefore be configured to show a slightly yellow image. The yellow tint of the display pixels compensates for the blue tendency of the display, resulting in a uniform white image across the display. 
       FIG. 31  is a diagram of illustrative method steps that may be used to generate compensation information for a display in an electronic device. At step  342 , an image of the display in an electronic device may be captured (e.g., using image sensor  330  in  FIG. 30 ) while the display displays a known image (e.g., all white). Next, at step  344 , the image data may be grouped into representative blocks. For example, the image sensor that captures the image in step  342  may have a given number of rows and columns pixels. Each pixel may have a corresponding image signal. At step  344 , the image signals from more than one imaging pixel may be grouped (e.g., summed, averaged, or grouped using some other technique) into a single value that represents a block of pixels within the display. 
     Next, at step  346 , a compensation gain table may be determined for each type of pixel in the display. For example, if the representative value determined in step  344  indicates that the display has a bluish tint when attempting to display white, compensation values for each pixel type (e.g., red pixels, blue pixels, and green pixels) may be determined that result in the display emitting a desired white color when attempting to display white. At step  346 , the compensation gain table may have a corresponding compensation gain table for each color of pixel in the display (e.g., gain tables for the red, blue, and green pixels for a total of three gain tables). Each compensation gain table may have a compensation value for each block of pixels grouped in step  344 . 
     Finally, at step  348 , the compensation gain tables of step  346  may be extrapolated to determine a per-pixel compensation gain table for each color of pixel in the display. During operation of the display, each pixel may have its pixel value modified by the gain value from the per-pixel compensation gain table before the pixel displays the pixel value. 
       FIG. 32  shows a schematic diagram of illustrative circuitry that may be used in implementing display  14  of device  10 . During operation of electronic device  10 , control circuitry in the device may supply image data  356  for images to be displayed on display  14 . Ultimately, the image data may be delivered to display driver circuitry  354 , which may supply the image data to data lines of the display. Display driver circuitry  354  may also include gate driver circuitry which is used to assert gate line signals on gate lines of display  14 . The display driver circuitry may be used to provide the image data to pixels  90  in the display. 
     Before being provided to display driver circuitry  354 , the image data may be multiplied by compensation factors from gain table  358  in multiplication circuit  352  (sometimes referred to herein as a gain circuit or modification circuit). Each frame of image data  356  may include a representative brightness value for each pixel  90 . Gain table  358  may include a compensation factor for each pixel  90 . The compensation factor may correct for display uniformity issues (as determined using the method of  FIG. 31 , for example). For example, if while performing the method of  FIG. 31  it is determined that the display has a bluish tint when attempting to display white, the brightness of a blue pixel in this region may be reduced to ensure a white color is displayed when desired. 
     After image data  356  is multiplied by the dimming factors from gain table  358 , the modified image data may be provided to display driver circuitry  354 . Display driver circuitry  354  will then provide the modified image data to the pixels in the display. The pixels may then display the desired image. 
     Multiplication circuit  352 , gain table  358 , display driver circuitry  354 , and pixels  90  as shown in  FIG. 32  may sometimes be collectively referred to as display circuitry. Alternatively, pixels  90  may sometimes be referred to as a display while multiplication circuit  352 , gain table  358 , and display driver circuitry  354  may sometimes collectively be referred to as control circuitry. The example of a multiplication circuit  352  that multiplies image data  356  by compensation factors from per-pixel gain table  358  is merely illustrative. Other desired types of modifications (in addition or instead of multiplication) may be used to modify image data  356  based on compensation factors  358 . For example, the per-pixel gain table may include compensation factors that are added to the image data to produce the modified image data. Circuit  352  may therefore sometimes be referred to as an image data modification circuit. 
     In  FIG. 3 , it was discussed how information to be displayed on display  14  may be conveyed to a display driver integrated circuit using a signal path such as a signal path formed from conductive metal traces in a rigid or flexible printed circuit such as printed circuit  64  (as an example).  FIG. 33  is a cross-sectional side view of a flexible printed circuit ( 64 ) that is used to convey signals from a rigid printed circuit board to a display driver integrated circuit  402  and from the display driver integrated circuit  402  to thin-film transistor layer  58 . The display driver integrated circuit may be mounted directly to flexible printed circuit  64 . This may be referred to as a chip-on-flex (COF) arrangement. The flexible printed circuit may be coupled between thin-film transistor layer  58  and printed circuit board  404 . Printed circuit board  404  may be, for example, a rigid printed circuit board (sometimes referred to as a motherboard). 
     In region  406 , flexible printed circuit  64  is coupled to thin-film transistor (TFT) layer  58 . In region  408 , flexible printed circuit  64  is coupled to rigid printed circuit board  404 . In region  406 , a conductive layer  412  may couple a contact  410  of the flexible printed circuit to a contact  414  of the thin-film transistor layer. Contacts  410  and  414  may be contact pads, traces, or any other desired conductive layer. Conductive layer  412  may be anisotropic conductive film (ACF), may be solder, or may be formed from any other desired conductive material. In region  408 , a conductive layer  418  may couple a contact  416  of the flexible printed circuit to a contact  420  of the rigid printed circuit board. Contacts  416  and  420  may be contact pads, traces, or any other desired conductive layer. Conductive layer  418  may be anisotropic conductive film (ACF), may be solder, or may be formed from any other desired conductive material. Flexible printed circuit  64  may have a bent region  407  interposed between regions  406  and  408 . 
     To add structural support for display driver integrated circuit  402 , a stiffener  422  may be included on flexible printed circuit  64 . Stiffener  422  may be formed on an opposing side of flexible printed circuit  64  as display driver integrated circuit  402 . The presence of stiffener  422  protects integrated circuit  402  from vibrations or drop events that may otherwise damage the integrated circuit. Stiffener  422  may be formed from stainless steel or another desired material. Stainless steel may be stiff enough to provide sufficient structural support for the integrated circuit  402 . 
       FIG. 34  is a top view of flexible printed circuit  64  in an unbent state. As shown, stainless steel stiffener  422  may extend along the length of the flexible printed circuit. The stiffener has a first portion (on the side of attachment region  408 ) with a first length and a second portion (on the side of bending region  407 ) with a second length that is different than the first length. The total length of the stiffener may be more than three times greater than its width, more than five times greater than its width, more than seven times greater than its width, less than twelve times greater than its width, etc. This example is merely illustrative. In general, stiffener  422  may have any desired shape. The flexible printed circuit may extend along an edge of the thin-film transistor layer. In one example, the flexible printed circuit may extend along the same edge of the thin-film transistor layer that overlaps the light-emitting diodes of the backlight structures (e.g., the flexible printed circuit may extend along the left edge of the thin-film transistor layer). This example is merely illustrative, and the flexible printed circuit (and stiffener) may extend along any desired edge of the thin-film transistor layer. 
       FIG. 35  is a cross-sectional side view of attachment region  408  of the flexible printed circuit showing how flexible printed circuit  64  is attached to rigid printed circuit board  404 .  FIG. 35  shows how printed circuit board  404  includes a conductive layer  420  that is coupled to anisotropic conductive film  418 . The anisotropic conductive film is also coupled to trace  416  of flexible printed circuit  64 . 
     As shown in  FIG. 35 , a portion  432  of trace  416  may not directly contact anisotropic conductive film  418 . Portion  432  of trace  416  is therefore separated from the upper surface of rigid printed circuit board  404 . To improve reliability of the trace, a solder mask layer  434  may be included between portion  432  of trace  416  and rigid printed circuit board  404 . Without solder mask layer  434  present, a gap  438  may be present between trace  416  and the upper surface of rigid printed circuit board  404 . With this large gap, traces  416  may dent or crack after bonding (due to compression on portion  432  during the bonding process and portion  432  bending towards rigid printed circuit board  404 ). To prevent these issues, solder mask layer  434  may be included below portion  432  of trace  416 . With the presence of solder mask layer  434 , the gap  436  below portion  432  of trace  416  is much smaller (e.g., than gap  438 ). Reducing the size of the gap below portion  432  of trace  416  improves reliability of the trace by preventing portion  432  from bending and cracking. 
     Gap  436  may be less than 1 millimeter, less than 0.1 millimeters, less than 0.01 millimeters, less than 0.001 millimeters, etc. In some cases, the gap may be removed entirely (and portion  432  of trace  416  directly contacts solder mask layer  434 ). The example of including solder mask layer  434  below portion  432  of trace  416  is merely illustrative. In general, any desired type of material may be incorporated below portion  432  of trace  416  to prevent damage to the trace. Solder mask layer  434  may therefore instead sometimes be referred to as a layer, dielectric layer, gap-filling layer, filler, etc. Using the solder mask material as the gap-filling layer may be advantageous for manufacturing as the solder mask material may already be deposited on the rigid printed circuit board during the manufacturing process. Therefore, no additional manufacturing steps are required to use the solder mask material as gap-filling layer  434 . 
     When flexible printed circuit  64  is bonded to rigid printed circuit board  404  in region  408 , excess material that forms anisotropic conductive film (ACF)  418  may flow over the edge of the printed circuit board. This process may be referred to as squeeze-out, for example. If gap-filling layer  434  extended as a strip across the entire printed circuit board  404 , the paths for the excess ACF material may be blocked. This may result in poor anisotropic conductive film compression (and a poor electrical connection between the flexible printed circuit and rigid printed circuit board). 
     To fill the gap between trace  416  and rigid printed circuit board  404  while still providing channels for excess ACF material to flow during compression, a number of discrete gap-filling layers  434  may be formed along the edge of the rigid printed circuit board.  FIG. 36  is a top view of the rigid printed circuit board showing a number of discrete gap-filling layers  434  along the edge of rigid printed circuit board  404 . Each gap-filling layer  434  may be completely laterally surrounded by portions of rigid printed circuit board  404  that are not attached to the gap-filling layer. The discrete gap-filling layers  434  may be referred to as island-type or island-shaped gap-filling layers. 
     Returning to  FIG. 35 , encapsulant such as encapsulant  442  and  444  may be incorporated in attachment region  408 . The encapsulant may provide strain relief to improve reliability and robustness of the attachment between flexible printed circuit  64  and rigid printed circuit board  404 . 
     Encapsulant  444  is attached to a lower surface of flexible printed circuit  64  and an edge surface of rigid printed circuit board  404 . Encapsulant  444  may protect exposed metal traces on flexible printed circuit  64  from touching the edge of printed circuit board  404 . Encapsulant  444  may also absorb heat generated by the electronic device. Encapsulant  444  may also prevent moisture or other contaminants from reaching the conductive components that form the attachment between flexible printed circuit  64  and rigid printed circuit board  404 . Encapsulant  444  may conform to the edge of printed circuit board  404  if desired. 
     Encapsulant  442  is attached to an upper surface of flexible printed circuit  64 , an edge surface of flexible printed circuit  64 , and an upper surface of rigid printed circuit board  404 . Encapsulant  442  may also absorb heat generated by the electronic device. Encapsulant  442  may also prevent moisture or other contaminants from reaching the conductive components that form the attachment between flexible printed circuit  64  and rigid printed circuit board  404 . Encapsulant  442  may be formed from a conformal material that conforms to the edge of the flexible printed circuit the edge of anisotropic conductive film  418 , and the edges of traces  416  and  420 . Encapsulant  442  and  444  may be formed from the same material or different materials. The encapsulation may be formed from an elastic material (e.g., a material with a low Young&#39;s modulus). 
       FIG. 37  is a top view of flexible printed circuit  64  in an unbent state showing how the pitch of traces  410  may vary across the flexible printed circuit. As shown, traces  410  may be distributed across the flexible printed circuit in attachment region  406  (e.g., that attaches the flexible printed circuit to thin-film transistor layer  58 ). In general, it is desirable to position the traces close together (as this may allow more traces to be included on the flexible printed circuit in the same amount of area). However, the traces must be separated by a sufficient distance to ensure that the traces do not touch (thus shorting the traces and preventing them from functioning correctly). 
     Based on the manufacturing methods in producing flexible printed circuit  64 , the tolerance in producing the traces may vary dependent upon the position of the trace within the flexible printed circuit. For example, the position of traces in the center of the flexible printed circuit may be more controllable (e.g., have a lower tolerance) than the traces at the edges of the flexible printed circuit. Therefore, the pitch of the traces may vary across the flexible printed circuit to match the manufacturing tolerance. 
     As shown in  FIG. 37 , traces  410 - 1  in the center of the flexible printed circuit may have a corresponding pitch  452 - 1 . Because manufacturing tolerance is smallest in the center of the flexible printed circuit (and therefore the position of the traces is best controlled in the center of the flexible printed circuit), pitch  452 - 1  may be small. Traces  410 - 2  are closer to the edge of the flexible printed circuit than traces  410 - 1 . The manufacturing tolerance for traces  410 - 2  may be larger than the manufacturing tolerance for traces  410 - 1 . Therefore, the position of traces  410 - 2  cannot be controlled as well as the position of traces  410 - 1 . Accordingly, pitch  452 - 2  is greater than pitch  452 - 1 . Manufacturing tolerance may be at a maximum at the edges of the flexible printed circuit (and therefore the position of the traces is worst controlled in the edges of the flexible printed circuit). Therefore, pitch  452 - 3  may be larger than pitches  452 - 2  and  452 - 1 . 
       FIG. 38  is a graph showing pitch and density of traces  410  as a function of position along the flexible printed circuit. As shown, the traces may have a pitch that follows an illustrative profile  454 . At the edges of the flexible printed circuit, the pitch is at its highest levels. At the center of the flexible printed circuit, the pitch is at its minimum level. The traces may have a uniform width. Therefore, the density of the traces is inversely proportional to the pitch of the traces. Profile  456  shows an illustrative profile for the density of the traces. At the edges of the flexible printed circuit, the density is at its lowest levels. At the center of the flexible printed circuit, the density is at its highest level. The illustrative profiles shown in  FIG. 38  are merely illustrative. The profiles may have sloped portions (as in  FIG. 38 ) or may follow a step-function that follows a similar shape as profiles  454  and  456 . 
     The pitch and density profiles may have a similar shape as a profile of the manufacturing tolerance of the traces.  FIG. 39  is a graph showing the pitch profile of the traces relative to the tolerance profile for the traces. Profile  458  shows the tolerance of the traces as a function of position within the flexible printed circuit. As shown, the tolerance increases as the traces get further from the center of the flexible printed circuit and closer to the edges of the flexible printed circuit. 
     Profile  460  shows a profile for traces having a constant pitch across the flexible printed circuit. As shown, when the pitch of the traces does not change, margin  464  between the pitch and the tolerances at the center of the flexible printed circuit may be larger than margin  466  between the pitch and the tolerances at the edges of the flexible printed circuit. At the center of the flexible printed circuit, margin  464  may be too large (meaning that a lower-than-necessary number of traces are fit at the center of the flexible printed circuit). At the edges of the flexible printed circuit, margin  466  may be small (meaning that the traces may be susceptible to being shorted together). 
     To avoid these issues and have a consistent margin across the flexible printed circuit, the pitch may instead vary across the flexible printed circuit as shown by profile  462 . In profile  462 , the pitch increases stepwise from a minimum at the center of the flexible printed circuit to a maximum at the edge of the flexible printed circuit. This allows the pitch to follow the same profile as tolerance profile  458  and ensures that the margin remains consistent regardless of position (e.g., margin  468  close to the center is the same as margin  470  close to the edge). 
     The examples of profile shapes in  FIGS. 38 and 39  are merely illustrative. In general, the pitch of the traces may follow any desired profile. For example, the pitch profiles may be asymmetrical (instead of symmetrical as in  FIG. 38 ). If manufacturing tolerance was instead highest in the center of the flexible printed circuit, the pitch may be highest in the center of the flexible printed circuit as well. The pitch of the traces at any given positon may be based on the manufacturing tolerance for the traces at that position. 
       FIG. 40  is a cross-sectional side view of flexible printed circuit  64  and thin-film transistor layer  58  in attachment region  406 .  FIG. 40  shows how thin-film transistor layer  58  includes a conductive layer  414  that is coupled to anisotropic conductive film  412 . The anisotropic conductive film is also coupled to trace  410  of flexible printed circuit  64 . To prevent damage to traces on flexible printed circuit layer  64  (e.g., caused by flexible printed circuit  64  contacting the edge of thin-film transistor layer  58  when bent), a protective layer  472  may be included. Protective layer  472  overlaps the edge of thin-film transistor layer  58 . The protective layer may be formed from a portion of a solder resist layer (e.g., a solder mask layer) or any other desired dielectric material. Protective layer  472  may directly contact the edge of thin-film transistor layer  58  when flexible printed circuit  64  is bent. 
     The traces of flexible printed circuit  64  may include both copper and tin. However, areas of the flexible printed circuit with tin may be more brittle and may crack if bent (due to metallic bonding stresses). Therefore, tin may be omitted from the flexible printed circuit in the bent region of the flexible printed circuit board.  FIG. 41  is a cross-sectional side view of an illustrative flexible printed circuit board. As shown, the flexible printed circuit board may include a base layer  482  (sometimes referred to as substrate layer  482  or polyimide layer  482 ). The base layer  482  may be formed from any desired dielectric material (e.g., polyimide). A copper trace  484  may be formed over the base layer and a tin trace may be formed over the copper trace. To prevent the tin trace from being present in bending region  407 , a solder resist  490  (sometimes referred to as masking layer  490 , solder mask  490 , etc.) may be included in bending region  407 . An additional solder resist  488  may be included over the tin layer  486  (in the non-bending regions) and over the solder resist  490  (in bending region  407 ). 
     The arrangement of  FIG. 41  may ensure that no tin is present in bending region  407 . However, the presence of two solder resist layers (e.g., solder resist  488  and solder resist  490 ) may result in a higher stiffness than desired in bending region  407 . To ensure that no tin is present in bending region  407  and reduce stiffness in the bending region, an arrangement of the type shown in  FIG. 42  may be used. 
     As shown in  FIG. 42 , a removable masking layer may be used to omit tin in bending region  407 . Solder resist  488  may then fill the gap between tin  486  and cover tin  486  in regions where the tin is present. In other words, solder resist  488  may directly contact both the upper surface of copper trace  484  (e.g., in bending region  407 ) and tin trace  486  (e.g., outside of bending region  407 ). Removing the first masking layer used to omit tin in bending region  407  reduces the thickness of the flexible printed circuit board in bending region  407 . The reduced thickness in bending region  407  reduces stiffness compared to  FIG. 41 . 
     Display  14  may be characterized by color performance statistics such as white point. The white point of a given display is commonly defined by a set of chromaticity values that represent the color produced by the display when the display is generating all available display colors at full power. Prior to any corrections during calibration, the white point of the display may be referred to as the “native white point” of that display. For example, the native white point may be associated with operating all of the red, green, and blue subpixels in the display at RGB levels of 255 (on a scale from 0-255). 
     Due to manufacturing differences between displays, the native white point of a display may differ, prior to calibration of the display, from the desired (target) white point of the display. The target white point may be defined by a set of chromaticity values associated with a reference white (e.g., a white produced by a standard display, a white associated with a standard illuminant such as the D65 illuminant of the International Commission on Illumination (CIE), a white produced at the center of a display). In general, any suitable white point may be used as a target white point for the display. 
     In some cases, pixel values may be reduced (truncated) in order to correct the white point of displayed light. For example, during display calibration, RGB values may be determined that correspond to various reference white points (e.g., a first set of RGB values may be identified that correspond to the D65 illuminant of the International Commission on Illumination (CIE), a second set of RGB values may be identified that correspond to the D110 illuminant of the CIE, etc.).  FIGS. 43A and 43B  shows illustrative tables of RGB values associated with different white points.  FIG. 43A  shows RGB values (sometimes referred to as gray levels) of 255 for red, 255 for green, and 255 for blue (e.g., (255, 255, 255)). This may correspond to the native white point of the display.  FIG. 43B , meanwhile, shows RGB values resulting in a white point that matches the D65 illuminant of the International Commission on Illumination (CIE). As shown in  FIG. 43B , to produce the desired white point, red has a gray level of 241, green has a gray level of 254, and blue has a gray level of 234. In other words, the native white point of the display may, in this example, be more blue than the target D65 illuminant. Therefore, the blue value is reduced (truncated) to match the D65 illuminant. 
     Truncating the gray levels of some of the subpixels allows for the display to match a desired white point but may present other challenges. In particular, the length of time to update each subpixel between frames may vary. For example, consider a scenario in which a user is scrolling through content on the display. In this use-case, the displayed content moves rapidly across the display. In one common scenario, the display may present content with a white background and black text. As the user scrolls, the black text moves across the white background. In these types of scenarios, the pixels may frequently transition from white to black. 
     As shown in  FIGS. 43A and 43B , the truncated pixel values used to provide the desired white point may result in differences in the white-to-black transitions of the different colored subpixels. White-to-black transition time may refer to the length of time it takes for the pixel to transition from its white point gray level to within 10% its black gray level (e.g.,  0 ). As depicted in  FIG. 43A , when the display has gray level values of 255 for red, green, and blue subpixels, the white-to-black transition time of each pixel-type may be the same (e.g., the red pixel transition time is 10.0 seconds, the green pixel transition time is 10.0 seconds, and the blue pixel transition time is 10.0 seconds). It should be understood that these times are merely illustrative. There may be some variance (e.g., ±0.1 milliseconds, ±0.2 milliseconds, ±0.3 milliseconds, etc.) between the transition times even when a gray level of 255 is used for each color pixel. However, because each pixel is changing from a gray level of 255 to a gray level of 0, the transition times are generally similar. 
     As shown in  FIG. 43B , when the truncated subpixel values are used (for a corrected white point), the white-to-black transitions of the different colored subpixels may vary. In  FIG. 43B , the red subpixel transition time (e.g., from 241 to 0) is 9.5 seconds, the green subpixel transition time (e.g., from 254 to 0) is 10.0 seconds, and the blue subpixel transition time (e.g., from 234 to 0) is 9.4 seconds. As a result of these differences in transition time, the display may have a greenish hue during white-to-black transitions (because the green brightness level falls more slowly than the red and blue brightness levels). 
       FIG. 44  is a graph showing how the green brightness level may fall slowly during white-to-black transitions, resulting in a greenish hue on the display during white-to-black transitions. A gray level response curve G is depicted for a green subpixel, a gray level response curve R is depicted for a red subpixel, and gray level response curve B is depicted for a blue subpixel. As shown, at to the pixel may have RGB values corresponding to the white point of  FIG. 43B  for matching the D65 illuminant. In other words, at to, the red subpixel has a value of 241, the green subpixel has a value of 254, and the blue subpixel has a value of 234. At t 1 , the pixel may begin to transition to black. Ultimately, at t 2 , the red, blue, and green subpixels will all have a gray level of 0. However, as shown in  FIG. 44 , the green pixel has a higher starting point and therefore has a response curve that is generally higher than the response curves of the red and blue pixels. The shapes of the response curves shown in  FIG. 44  are merely illustrative. In general, each of the response curves may have any desired shape. However, the green response curve may be higher than the blue and red response curves at one or more points during the transition, contributing to the apparent green hue during white-to-black transitions. 
       FIG. 45  illustrates how pixels may be provided with intermediate target values to suppress color motion blur (e.g., an undesirable green appearance during a white-to-black transition). Initially, a pixel in image frame F 1  may have red, green, and blue subpixel values  502  of (241, 254, 234) corresponding to the white point of  FIG. 43B . Then, the pixels may need to transition from white to black (e.g., during scrolling of a black text on a white background). Therefore, the final target values for the red, green, and blue subpixels are (0, 0, 0). This target pixel state  506  will be reached when display  14  displays frame F 3 . 
     However, to suppress the aforementioned green shadow, an intermediate set of target values is temporarily imposed on the pixels. The temporary target values for these pixel include increased (overdriven) red and blue subpixel values. In the example of  FIG. 45 , intermediate frame F 2  has been provided with temporary target values  504  for the red, green, and blue subpixels of (20, 0, 20)—i.e., the red subpixel value has been temporarily increased and the blue subpixel value has been temporarily increased. Temporarily increasing the red and blue subpixel values slows down the red and blue transition times to more closely match the green transition time. This therefore mitigates the undesired green appearance during white-to-black transitions. 
     The values provided in  FIG. 45  are merely illustrative. In general, any desired intermediate values may be used to mitigate artifacts associated with white-to-black transitions or other transitions in the display. When a pixel transitions from white to black, the red and blue subpixels may be temporarily overdriven (e.g., operated at a gray level that is higher than the ultimate target gray level) while the green subpixels may not be overdriven or may be overdriven to a lesser degree than the red and blue subpixels (e.g., operated at a gray level that is at or close to the ultimate target gray level). 
     A diagram of illustrative resources that may be used in device  10  to reduce color-artifacts such as the aforementioned green appearance during white-to-black transitions is shown in  FIG. 46 . As shown in  FIG. 46 , device  10  may include control circuitry such as control circuitry  508 . Control may be part of control circuitry  16  in  FIG. 1  if desired. Control circuitry  508  may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  508  may be used to control the operation of device  10 . This processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, etc. 
     Control circuitry  508  may be used to run software on device  10 , such as operating system software, application software, firmware, etc. As shown in  FIG. 46 , the software running on control circuitry  508  may include code that generates content that is to be presented on display  14  (see, e.g., content generator  510 , which may be an operating system function, an e-book reader or other software application, or other code that is running on control circuitry  508 ). Content generator  510  may generate content that has not been corrected to reduce color-based transition artifacts (uncorrected content) and this content may be supplied to graphics processing unit  512 . 
     Graphics processing unit  512  may include an input frame buffer such as buffer  514  or other storage to maintain information on a current image frame  516  and one or more earlier frames such as previous image frame  518 . Graphics processing unit  512  may also include an output frame buffer such as output frame buffer  524  that stores content in which one or more pixels may have been modified to reduce transition artifacts such as a green hue on white-to-black transitions. Content processor  520  may use one or more look-up tables  522  to process uncorrected content and produce corresponding updated content in which pixels have been modified to mitigate transition based artifacts. 
     Look-up tables may have associated overdrive gray levels associated with a starting gray level and an ending gray level. For example, in the example provided in  FIG. 45 , a blue subpixel transitions from an initial value of 234 to a target value of 0. The look-up table may have an entry associated with this scenario for a corresponding modified value of 20, as is reflected in  FIG. 45 . Any desired number of look-up tables may be used to generated the modified image data that is provided to output frame buffer  524 . 
     The updated content with decreased transition artifacts may be supplied to display driver circuitry  526  of display  14 . Display driver circuitry  526  may include integrated circuit(s) and/or thin-film transistor circuitry on display  14  for displaying the content that is received on the pixels of the display. 
     Illustrative operations involved in using resources of the type shown in  FIG. 46  in displaying content with reduced artifacts are shown in  FIG. 47 . Initially, content generator  510  may generate content to be displayed on display  14  and graphics processing unit  512  may receive the content. The content may include frames of image data. Content processor  520 , which may be implemented using software and/or hardware resources associated with graphics processing unit  512 , may receive a frame of image data (sometimes referred to as an image frame or content frame) from content generator  510  at step  532 . 
     During the operations of step  534 , content processor  520  may use frame buffer  514  to store frames of image data including current frame  516  and previous frame  518 . Content processor  520  may compare the pixel values in current frame  516  and previous frame  518  to identify gray level transitions that may be susceptible to artifacts (e.g., an undesired green hue). After each current frame is processed, processor  520  may store the data of the current frame as previous frame  518 . 
     At step  536 , content processor  520  may use one or more look-up tables  522  to determine intermediate values for one or more pixels. The intermediate values may be used to mitigate artifacts associated with white-to-black transitions, for example. Frames of data that have been processed by content processor  520  may be stored in output frame buffer  524 . The modified frame (i.e., a frame such as frame F 2  of  FIG. 45 ) can be output at step  538  and subsequently displayed by display driver circuitry  526 . As indicated by line  540 , processing can then loop back to step  532  so that additional content from content generator  510  can be processed. 
     If desired, the image processing operations involved in implementing the transition artifact mitigation process of processor  520  may be implemented in full or in part in control circuitry  508  (e.g., as part of an operating system or application or both an operating system and application), may be implemented in full or in part in display  14  (e.g., using resources in a timing controller integrated circuit or other circuitry in display drier circuitry  526 ), may be implemented in full or in part on graphics processing unit  512  as described in connection with  FIG. 46 , and/or may be implemented using other resources in device  10  or any combination of two or three or more of these sets of resources. The use of a scenario in which blur abatement image processor  520  is implemented on graphics processing unit  512  is merely illustrative. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20190612
Publication Date: 20201215
Grant Date: 20201215
Priority Date: 20181026
Inventors: LU, SHIN-YING
Xu, Daming
GORKHALI, SURAJ P.
HERRANZ, ADRIA FORES
SON, Mookyung
SUH, HEESANG
YIN, VICTOR H.
HOLSTEEN, AARON L.
LI, XIAOKAI
GE, ZHIBING
Assignee: APPLE INC
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Family ID: 67730717