Error diffusion filter for DMD display

A DMD display system includes an inverse gamma look-up-table (50) for converting raster scanned, gamma corrected video data of 8 bits to 12 bits inverse gamma data with 8 most significant bits (msb) and 4 least significant bits (lsb). The 8 msb are coupled to the micromirror of the DMD display (10) and the four lsb are delayed and halved such that one half of the lsb is added to the next pixel in the horizontal scan and one-half of the lsb is added to the next vertical pixel one line length delayed.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to digital imaging and more particularly to an error 
diffusion filter for a digital micromirror device (DMD) display. 
BACKGROUND OF THE INVENTION 
A new projection display that utilizes reflections from hundreds of 
thousands of micromirrors, each mounted above its own semi conductor 
memory cell is described in IEEE Spectrum, November 1993, vol. 30, no. 11, 
written by Jack M. Younse of Texas Instruments Incorporated. The digital 
micromirror device (DMD) comprises a special light modulator that was 
invented in 1987 by Larry J. Hornbeck, a Texas Instruments Incorporated 
scientist. The DMD, or digital micromirror device covers each memory cell 
of a CMOS static RAM with a movable micromirror. Electrostatic forces 
based on the data in this cell tilt the mirror either plus or minus 10 
degrees, modulating the light incident on the surface. The light reflected 
from any of the mirrors passes through a projection lens and creates an 
image on a large screen. Light from the remaining off mirrors is reflected 
away from the projection lens and trapped. The portion of the time during 
each video frame that the mirror remains in the on state determines the 
shades of grey- from black for zero on time to white for 100 percent on 
time. Color may be added in two ways, by a color wheel or a 3-DMD set up. 
Some DMD devices may have the capability to display only a low number of 
bits representing the on and off times and, therefore, the shades of grey 
or shades of color, leading to degradation of the video quality. Also, the 
use of digital degamma in the DMD display systems entails some loss of 
resolution (blockiness) in the low intensity regions. Finally, even the 
best of DMDs can have some defects (pixels stuck on, off, or flat). It is 
desirable to find some method to provide a correction for these display 
errors and to provide a more pleasing picture without significantly 
increasing the time for processing by increasing the number of bits for 
each on or off time. 
SUMMARY OF THE INVENTION 
In accordance with one preferred embodiment of the present invention, an 
error diffusion filter for a DMD display comprises means for determining 
an error between a desired intensity of a first pixel and a closest 
achievable intensity of a micromirror at said first pixel location and 
means for propagating that error to neighboring pixels to said first pixel 
.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
Referring to FIG. 1, there is illustrated an overall block diagram of a 
digital micromirror device (DMD) display system. Referring to FIG. 1, an 
example of a DMD system 10 is illustrated, wherein the light from a light 
source 11 is applied through a first condenser lens 13 and through a color 
wheel 15, which is rotating at about sixty cycles or hertz or 60 frames 
per second. The light passing through the color wheel 15 passes through a 
second condenser lens 17 onto a DMD chip 19. The DMD chip includes an 
array of tiny mirror elements, or micromirrors, where each mirror element 
is hinged by a torsion hinge and support post above a memory cell of a 
CMOS static RAM as shown in FIG. 2. The movable micromirror tilts into the 
on or off states by electrostatic forces based on data in the cell. The 
tilt of the mirror is either plus 10 degrees (on) or minus 10 degrees 
(off) to modulate the light that is incident on the surface. For 
additional details, see U.S. Pat. No. 5,061,049 entitled, "Spatial Light 
Modulator" and U.S. Pat. No. 5,280,277 entitled, "Field Updated Deformable 
Mirror Device," both of Larry J. Hornbeck. As shown, the light reflected 
from any of the mirrors may pass through a projection lens 20 and create 
images on a large screen 21. As stated previously, the portion of time 
that a mirror remains in the on state determines the shade of grey. The 
time duration in which the cell is in the positive direction, or on, is 
represented by 8 bits of data sent to that cell. The color wheel 15 is 
divided into red, green, and blue sectors. In the color wheel example, the 
maximum red would be when the red for example would be reflecting the 
maximum period of time as when the light is on the longest period of time 
in the red sector. The same would go for the other two colors. The minimum 
would be where the micromirror would not be reflecting through the color 
wheel and the lens, etc. at all during the color cycle. The intensity 
resolution in this pulse width modulation (PWM) is limited by the response 
time of the DMD mirrors. The total time available to display a color frame 
and the least time required to turn a mirror to the "on" state and back to 
"off" state defines the resolution of present systems. In the arrangement 
for the 8 bits, the most significant bit, as illustrated in FIG. 3, is the 
7th bit with that bit representing the longest "on" time, the 6th bit 
being then the next longest "on" time, and the 5th bit representing the 
third longest "on" time, etc., all the way down to the least significant 0 
bit, which is represented by the shortest time period. For example, a 
sequential color DMD system might have 5 (five) msec (milliseconds) 
available for a color frame. For 8-bit binary PWM, the least significant 
bit (0 bit on only), the shortest period would be "on" for about 19.6 
"Msec" (microseconds). The mirror on/off time would have to be less than 
19.6 Msec to implement this scheme with the current method. In a system 
where the DMD device has the capability of only 6 bits, or even those that 
have the full eight bits, the system would have too few number of grades 
of grey or shades of color and therefore tend to show blockiness between 
portions of the picture. This would represent one of the errors that the 
present invention is to overcome. 
Another error is due to the degamma effect in the display. On a typical CRT 
television display system, the intensity of the picture is a function of 
the voltage, which is represented by the CRT response in Curve A of FIG. 
4. Note that the intensity for the lower voltage is nearly flat in the low 
voltage region, but increases rapidly at middle to highest voltage inputs. 
In order to correct for this, the transmission that is sent to the display 
has a gamma characteristic of curve B so that the overall response is 
linear as represented by the linear solid line C. In order to duplicate 
the CRT response for the digital micromirror device, a digital degamma 
characteristic is made to follow curve A of FIG. 4. FIG. 4a illustrates 
conventional degamma, SMPTE degamma, and Texas Instruments' degamma 
curves. This is done, for example, by each color providing the raster 
scanned gamma corrected red, green, or blue video data as shown in FIG. 1 
using a gamma lookup table (LUT) 50 where for given input threshold levels 
the mirrors are turned on for given durations. However, due to the gamma 
LUT 50 being digital in nature the output is stepped as shown in FIG. 5 
rather than smooth between the thresholds of the bits and, therefore, the 
grade levels again take on a blockiness particularly in the low intensity 
regions. 
In accordance with the present invention, Applicants solve the problems of 
the low number of bits in the blockiness by an error diffusion filter as 
shown in FIG. 6 for each color path (red, green, and blue) on the raster 
scanned video output that would normally be written into the frame RAM 
buffer 53. The filter 70 computes an error between the desired intensity 
of a pixel and the immediately lower achievable intensity of the DMD 
display. This error is then propagated into pixels to the right and below 
the first pixel as shown in FIG. 
In accordance with one embodiment shown in FIG. 6, the implementation of 
this filter for one color is illustrated. The desired output for a given 
color is generated by a degamma lookup table (LUT) 50 at N bits of 
resolution and combined with errors for earlier pixels. The degamma LUT 
selected matches the appropriate curve in FIG. 4a. In the embodiment N is 
12 bits addressed by 8-bits video data into the LUT 50. The M MSB (most 
significant bits) are then sent to the DMD frame buffer 53, and DMD 
display hardware while the N-M LSB (least significant bits) representing 
error are delayed for combination with later pixels. In this embodiment, M 
equals 8-bits and N-M=4 lsb (least significant bits). The error is 
distributed to the right and below using the four lsbs (least significant 
bits). The horizontal delay element 55 is implemented as a single N-M bit 
latch, while the vertical delay 57 is accomplished with an L word by N-M 
bit FIFO (first in first out) memory where L is the number of pixels in a 
video line. Both delay elements must have appropriate initialization 
circuitry. The vertical and horizontal error is divided by 2 at divider 59 
and summed to the following row and column at summer 52. This filter may 
be accomplished with a video processor and with memory to accomplish the 
horizontal and vertical delays. As illustrated in FIG. 7, one-half of the 
error from the previous row (r-1) is provided to the row r and half of the 
error from the previous column (c-1) is provided to the next column (c). 
In this manner the error added enhances the apparent intensity resolution 
of the video display system. 
The FIFO memory for a video line described above can add a significant cost 
to the system, particularly when integrated with multiple functions in 
custom integrated circuits as might be desirable in high volume 
applications. In accordance with an improved embodiment of the present 
invention, only half of the pixels require half the storage that the 
filter mentioned above requires if the error terms least significant bits 
(lsbs) are shifted to the next video line from only half of the pixels. 
One such filter design is illustrated in FIG. 8. The error terms from odd 
numbered pixel columns are shifted only horizontally, while errors from 
even columns shift to the next line. As a result, the FIFO vertical delay 
memory can be reduced to a length of half the number of pixels in a line. 
FIG. 9 illustrates a hardware block diagram for the filter to achieve the 
operation of FIG. 8. The pixel clock rate is divided by 2 so that every 
other error term is written into the vertical delay memory. The 12-bit 
output from the lookup table 51 is summed at summer 62 with the 4 least 
significant bits (lsbs) from the error diffusion. The 8 most significant 
bits (msb) are applied to the frame buffer. The lsbs (4 bits) are applied 
directly to multiplexer 67 and to delay 60, which are clocked by half the 
pixel rate by divider 61 to clockout from delay 60 the pixel data two 
pixels short of a full line delay. The output from delay 60 is applied to 
delay 63 and summer 65. Delay 63 is also clocked at the half pixel rate to 
add a two pixel delay to the output from delay 60. The output from each of 
the delays 60 and 63 is divided into half and applied to the other input 
of multiplexer 67, which alternately clocks the output from the two inputs 
at half the clock rate of the picture clock. The output from the 
multiplexer 67 is delayed one picture element at delay 69 that is clocked 
by the picture clock. The output from delay 69 is applied as 4 lsbs to 
summer 62. Another embodiment is illustrated in FIG. 10, wherein error 
added to a given or next pixel is from the error from the pixel delayed by 
one line delay and one pixel delay to the given pixel. The delays used are 
clocked at half the pixel clock rate to again reduce the size of the FIFO. 
OTHER EMBODIMENTS 
Although the DMD system described uses a color wheel the claimed filter is 
equally applicable to a three DMD system in place of the color wheel. The 
degamma lookup table may be different for the three primary colors. 
Although the present invention and its advantages have been described in 
detail, it should be understood that various changes, substitutions and 
alterations can be made herein without departing from the spirit and scope 
of the invention as defined by the appended claims.