Automatic image sharpening in an electronic imaging system

Digital reproductions of any size made from images scanned from film or taken by an electronic camera are optimally sharpened in a system-wide operation. The images are captured by an imaging device having an electronic imager and subsequently reproduced in a reproduction device, both imaging and reproduction devices having respective modulation transfer functions. Initially, the imaging device modulation transfer function and the reproduction device modulation transfer function are stored in separate devices. After an image is captured with the imaging device, a system modulation transfer function is generated from the imaging device modulation transfer function and the reproduction device modulation transfer function. Then a sharpening function is generated from the system modulation transfer function and the sharpening filter function is applied to the image captured by the imaging device in order to sharpen the image.

FIELD OF THE INVENTION 
The invention pertains to optimal operation of an imaging system comprising 
an electronic imaging device and a reproduction device. 
BACKGROUND OF THE INVENTION 
As understood in the prior art, a digital imaging device is a device which 
uses an electronic sensor to capture an image either directly from an 
object or indirectly from a medium, such as film; signal processing to 
represent the captured signal numerically; and some storage device to 
preserve the numerical image data. It is further known for a digital 
imaging device to use a removable storage device, such as an integrated 
circuit memory card, to store images. For instance, U.S. Pat. No. 
5,016,107 describes an electronic still camera utilizing image compression 
and providing digital storage in a removable memory card having a static 
random access memory. In this camera, the integrated circuits in the 
removable memory card store image data and a directory locating the data. 
The image data provided by the digital imaging device and stored in a 
memory card is ordinarily used to produce some type of display or print, 
for example, a digital print of optional size made from images scanned 
from film or taken by an electronic camera. 
The image quality of digital prints can be improved by using an appropriate 
sharpening or "edge enhancement" filter. The appropriate filter depends on 
the characteristics of the image input source (digital camera or film plus 
scanner), the output printer, and the print size. In a prior art system, 
such as shown in FIG. 1, some amount of sharpening is normally performed 
in an input imaging device, such as a digital camera 10 or a film scanner 
11, and in an output reproduction device, such as a display 12 or a 
printer 14. In this type of known electronic still photography system, the 
camera 10, or the scanner 11, includes a slot for receiving a removable 
memory card 16. The memory card 16 also interfaces with a memory card slot 
in a host computer 18, which also performs some amount of sharpening on 
the images downloaded from the memory card 16. Consequently, it is known 
for the different parts of the system to have their own sharpening 
algorithms; in particular, the camera 10 has a hardware sharpening filter 
20, the host computer 18 has user selectable sharpening software 22, and 
the printer has a firmware sharpening filter 24. 
The image quality of captured images can be improved by the selection of 
appropriate filters for the input imaging device and subsequent devices 
that process the captured images. For instance, in U.S. Pat. No. 4,970,593 
(Cantrell), the modulation transfer function (MTF) of the uncorrected 
optical system is measured and an aperture correction function is created 
from an inverse of the MTF function to correct an image captured through 
the optical system. Some software packages, such as Adobe Photoshop.TM., 
used with the host computer 18 allow the user to select different levels 
of image sharpening as part of their image processing routines. The amount 
of sharpening in the printer 14 can sometimes be selected by the user, as 
can be done in the driver for the Kodak XL7700.TM. printer, which allows 
five preset choices of sharpening. 
Unfortunately, in the case of the printer, the preset printer sharpening 
may under or over correct images, depending on the print size and image 
source, and user selection of the sharpening level is a trial and error 
process. In addition, providing sharpening processing in two or three 
different places (camera, host computer software, and printer) takes extra 
time and may cause an increase in noise or artifacts. What is needed is an 
automatic method for providing the optimum sharpening level, without user 
intervention, for the output print or display. Preferably this would be 
accomplished using only a single sharpening operation. 
SUMMARY OF THE INVENTION 
The aforementioned problems are solved with a technique for optimally 
sharpening digital prints of any size made from images scanned from film 
or taken by an electronic camera. The invention comprises a method, and 
apparatus and system for use therewith, for sharpening images captured by 
an imaging device having an electronic imager and subsequently reproduced 
in a reproduction device, both imaging and reproduction devices having 
respective spatial response functions. Initially, the imaging device 
spatial response function and the reproduction device spatial response 
function are stored. After an image is captured with the imaging device, a 
system spatial response function is generated from the imaging device 
spatial response function and the reproduction device spatial response 
function. Then a sharpening filter function is generated from the system 
spatial response function and the sharpening filter function is applied to 
the image captured by the imaging device in order to sharpen the image. 
The advantage of the invention lies in its straightforward approach to 
automating the sharpening process. The preferred spatial response 
function, the MTF (modulation transfer function) of the image source, is 
measured and preferably stored in an image header file along with each 
image. The MTF of the printer is likewise measured, and preferably stored 
in a printer characterization file. Then, in a single sharpening 
operation, when the user requests a print from one of the digital images, 
the MTF of the source image is scaled appropriately for the output print 
size, and a "composite" MTF is calculated. An optimum sharpening filter is 
then calculated to equalize the system MTF, and used to sharpen the image. 
This avoids a plurality of separate sharpening algorithms, while providing 
an optimal sharpening of the image produced in the print or display. 
These and other aspects, objects, features and advantages of the present 
invention will be more clearly understood and appreciated from a review of 
the following detailed description of the preferred embodiments and 
appended claims, and by reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Because electronic imaging devices employing electronic sensors are well 
known, the present description will be directed in particular to elements 
forming part of, or cooperating more directly with, apparatus in 
accordance with the present invention. Elements not specifically shown or 
described herein may be selected from those known in the art. Certain 
aspects of the embodiments to be described are provided in software. Given 
the system description as described in the following materials, all such 
software implementation is conventional and within the ordinary skill in 
such arts. 
One of the most important characteristics of an electronic imaging system 
is the ability of its imaging device to capture fine detail found in an 
original scene. This ability to resolve detail is determined by a number 
of factors, including the performance of the optical system, the number of 
addressable photo elements in the optical imaging device, and the 
electrical circuits in the camera, which may include image compression and 
gamma correction functions. Different measurement methods can provide 
different metrics to quantify the resolution of an imaging system, or a 
component of an imaging system, such as a lens. Resolution measurement 
metrics include resolving power, limiting resolution (at some specified 
contrast), modulation transfer function (MTF), and optical transfer 
function (OTF). Mathematically, the modulation transfer function is the 
modulus of the optical transfer function, which is the two-dimensional 
Fourier transform of the point spread function of the imaging system under 
consideration. The OTF is a complex function whose modulus (MTF) has the 
value unity at zero spatial frequency. Although the focus in this 
application is on use of the modulation transfer function to characterize 
the resolution of the capture and output devices, other metrics could be 
used, for example the OTF, spatial frequency response or depth of 
modulation level at various spatial frequencies. These are all various 
forms of spatial transfer functions that can be used to characterize the 
sharpness of an image from an imaging device. 
The advantage of the spatial transfer functions is that they provide 
information about image quality over a range of frequencies rather than 
just at the limiting frequency as does resolving power. More particularly, 
the modulation transfer function is a graph (i.e., a set of discrete 
modulation factors) that represents the image contrast relative to the 
object contrast on the vertical axis over a range of spatial frequencies 
on the horizontal axis, where high frequency corresponds to small detail 
in an object. If it were possible to produce a facsimile image, the 
contrast of the image would be the same as the contrast of the object at 
all frequencies, and the MTF would be a straight horizontal line at a 
level of 1.0. In practice, the lines always slope downward to the right, 
since image contrast decreases as the spatial frequency increases. 
Eventually the lines reach the baseline, representing zero contrast, when 
the image-forming system is no longer able to detect the luminance 
variations in the object. The MTF can be determined for each component in 
an image-forming system or for combinations of components. The MTF for a 
system can be calculated by multiplying the modulation factors of the 
components at each spatial frequency. Since the MTF curves of all of the 
devices in a system are multiplied together point by point to provide the 
system MTF curve, the system curve is also a downwardly sloping function 
diminishing to zero resolution as the spatial frequency increases. 
This downwardly sloping characteristic results in a gradual loss of 
contrast in the detail of the image as the detail becomes finer and finer. 
For example, all optical devices have a non-ideal MTF response curve 
because of the finite size of the optical aperture associated therewith. 
The MTF curve of such optical devices is normally a monotonically 
decreasing function such as a downwardly sloping diagonal line, i.e., a 
set of diminishing modulation factors, that intersects the spatial 
frequency axis at a point of frequency less than or equal to the 
diffraction limit-the point at which contrast or resolution diminishes to 
zero. A filter can be designed with a transfer function to compensate for 
the diffraction effects of the finite size of the optical aperture of the 
system. If the filter curve is the inverse of the system MTF curve, the 
composite curve will be substantially flat out to the diffraction limit. 
The filter thus boosts the high spatial frequency contrast to compensate 
for the downwardly sloping characteristic of the system MTF. 
A diagram of a system utilizing our invention is shown in FIG. 2. Optical 
devices and systems such as lenses, electronic image sensors, video 
monitors, digital printers, filters, and the like all have a response to 
spatial frequency denoted by their spatial response functions, such as 
their MTF curves. In particular, the camera or scanner manufacturer 
characterizes the MTF of the camera 10 or the scanner 11 (relative to the 
camera or scanner sampling frequency), and records this information as a 
camera (or scanner) MTF calibration file 26 in a PROM (programmable 
read-only memory) 28 in the camera (or scanner), or in a computer file 30 
supplied with the camera 10 (or scanner 11). Likewise, the printer MTF is 
similarly characterized (relative to the printer sampling frequency), and 
recorded as a printer MTF calibration file 32 in a PROM 34 in the printer 
14 or in a computer file 36 supplied with the printer. A typical computer 
file 30 or 36 would be a floppy magnetic disk with the calibration file 
stored thereon along with program code for enabling an external device to 
access the calibration file. Both the camera (or scanner) and disk, or the 
printer and disk, would be supplied together as a kit by the manufacturer 
to the purchaser. 
The host computer 18 contains automatic sharpening filter software 38, 
stored in conventional program memory, which generates a system MTF from 
the camera MTF calibration file 26 and the printer MTF calibration file 
32. In addition, the selected print size is used to adjust the spatial 
frequency axis scale of the camera or scanner MTF relative to the printer 
MTF. This is desirable because different sized prints from the same 
digital image source are optimally reproduced with different sharpening 
levels. The user decides what size print to make from the camera image. 
The system MTF without sharpening is calculated by adjusting the spatial 
frequency axis of the camera to correspond to that of the printer (if the 
pixels of the camera are not mapped one-to-one to the pixels of the 
printer) and multiplying the two MTF curves. A sharpening filter is 
calculated to normalize this response either to unity (in which case the 
sharpening filter is an inverse filter), or to any other desired system 
curve shape. The filter is then applied to the image data to sharpen the 
image produced by the printer 14 (or the display 12). 
FIGS. 3 and 4 show block diagrams of exemplary embodiments of the camera 10 
and the printer 14, respectively. While not shown specifically in the 
figures, an exemplary scanner would contain elements similar to those 
shown in FIG. 3 for a camera with an added mechanism in the optical stage 
for positioning a film strip in the optical axis. Likewise, a display 12 
would contain similar blocks to those shown in FIG. 4 for a printer, 
except the output would be to a cathode ray tube, or the like, and 
associated scanning electronics. 
Referring first to FIG. 3, a lens 40 directs image light from a subject 
(not shown) through an aperture/shutter controller 41 and a blur filter 42 
upon an image sensor, which is preferably a charge coupled device (CCD) 
sensor 44. The sensor 44 generates an image signal that is processed by an 
analog video processor 46 before being converted into a digital image 
signal by an analog to digital (A/D) converter 48. The digitized image 
signal is temporarily stored in a frame memory 50, and then compressed by 
a digital signal processor 52. The compressed image signal is then stored 
in a data memory 54 or, if a memory card 56 is present in a memory card 
slot of the camera, transferred through a memory card interface 58 to the 
memory card 56. In this embodiment, the memory card is adapted to the 
PCMCIA card interface standard, such as described in the PC Card Standard, 
Release 2.0, published by the Personal Computer Memory Card International 
Association, Sunnyvale, Calif., September, 1991. 
Electrical connection between the memory card 56 and the camera 10 is 
maintained through a card connector 59 positioned in the memory card slot. 
The card interface 58 and the card connector 59 provide, e.g., an 
interface according to the aforementioned PCMCIA card interface standard. 
The compressed image signal may also be sent to the host computer 18 (see 
FIG. 2), which is connected to the camera 10 through a host computer 
interface 60. A camera microprocessor 62 receives user inputs 64, such as 
from a shutter release, and initiates a capture sequence by triggering a 
flash unit 66 (if needed) and signaling a timing generator 68. The timing 
generator 68 is connected generally to the elements of the camera 10, as 
shown in FIG. 3, for controlling the digital conversion, compression, and 
storage of the image signal. The microprocessor 62 also processes a signal 
from a photodiode 70 for determining a proper exposure, and accordingly 
signals an exposure driver 72 for setting the aperture and shutter speed 
via the aperture/shutter controller 41. The CCD sensor 44 is then driven 
from the timing generator 68 via a sensor driver 74 to produce the image 
signal. 
The MTF calibration file 26 is contained either in the camera PROM 28, 
which is connected to the digital signal processor 52, or in the computer 
file 30. Typically, the MTF would characterize the optical system 
comprising the lens 40, the blur filter 42, the CCD 44, and any other 
elements in the optical chain, such as infrared filters, color filter 
arrays, or the like. If the MTF is in the camera, the PROM 28 contains a 
table of calibration coefficient data, i.e., modulation factors, which are 
supplied with the image data to the host computer. FIGS. 5A and 5B show 
two examples of data structures that may be used to transfer the MTF 
coefficient data and the image data. In FIG. 5A, the digital signal 
processor 52 writes the MTF data into a camera header 76, followed by 
individual image trailer records 78. In FIG. 5B, the MTF data is written 
into individual camera headers 80 together with individual image trailer 
records 78. Alternatively, the camera MTF data may be contained in the 
computer file 30 (instead of in the PROM 28), which is provided as a 
floppy disk or the like in combination with the camera 10 (or the scanner 
11). The MTF data is then accessed by the host computer 18 through a 
conventional disk drive interface(not shown) when the user loads the disk 
into the interface. 
FIG. 4 shows a block diagram of the printer 14. A host computer interface 
78 receives digital image data from the host computer 18 and stores the 
image data in a printer data memory 80. A digital signal processor 82 
processes the digital image data, e.g., decompressing the image data, and 
stores the processed digital image data in a printer frame memory 84. The 
digital image data is then converted into an analog signal by a 
digital-to-analog (D/A) converter 86 and applied to a printer engine 88, 
which produces the printed output. Depending on the type of printing 
technology involved, the printing engine 88 could be a thermal printer, an 
inkjet printer, an electrophotographic printer, or the like. A printer 
microprocessor 90 receives user inputs 92, such as from a printer start 
switch, and initiates a printing sequence by signaling a timing generator 
94. The timing generator 94 is connected generally to the elements of the 
printer 14 for controlling the reception, processing, and conversion of 
digital image data. Alternately, the printing sequence could be initiated 
by digital command codes received from computer 18 via host computer 
interface 78. 
The printer MTF calibration file is stored either in the printer PROM 34 or 
in the computer file 36. In one embodiment, in which the printer contains 
the MTF data, the printer 14 also contains the automatic sharpening filter 
software 38 in a filter memory 96 (shown in broken line to indicate that 
it is optional to this embodiment). The digital signal processor 82 
recovers the camera MTF calibration file 26 from the header 76 (or 80) 
accompanying the digital image data, which is read through a card 
interface (not shown), retrieves the printer MTF calibration file 32 from 
the PROM 34, and performs the sharpening algorithm stored in the filter 
memory 96. In another embodiment, the host computer 18 contains the filter 
software 38 and performs the sharpening algorithm. In the latter 
embodiment, the host computer interface 78 is a two-way interface and 
communicates the printer MTF calibration file 32 to the host computer 18 
for use in the automatic sharpening algorithm. Alternatively, the printer 
MTF data may be contained in the computer file 36 (instead of the PROM 
34), which is provided to the user as a floppy disk or the like in 
combination with the printer 14. The MTF data is then accessed by the host 
computer 18 (or the printer 14) through a conventional disk drive 
interface(not shown) when the user loads the disk into the interface. 
A flowchart of the sharpening process is shown in FIG. 6 for an electronic 
camera and a printer. Initially, in steps 100 and 102, the manufacturer of 
the imaging device and the reproduction device characterizes the 
respective modulation transfer functions with respect to the sampling 
frequencies of the respective devices. A conventional method of measuring 
MTF is to utilize a graduated series of bar charts, either square wave or 
sinusoidal, and to measure the difference in the video between the black 
areas and the white background with, e.g., an oscilloscope. Three steps 
are basically involved in the preparation of the modulation transfer 
function: first, determine the modulation of the bar charts and the 
modulation of the image at each frequency; second, determine the 
modulation factor at each frequency by dividing the image modulation by 
the chart modulation; and third, prepare the modulation transfer function 
by plotting modulation factors against spatial frequencies. 
Next, the user operates the camera 10 or scanner 11 to record an image 
(step 104) and, in the printing stage, selects an output print size (step 
106). In step 108, the respective calibration files are retrieved from the 
PROMS 28 and 34, or computer files 30 and 36. The system MTF is then 
calculated in step 110 by scaling the camera MTF function to the selected 
print size (by adjusting the frequency axis of the camera to correspond to 
that of the printer) and by multiplying the scaled camera MTF 
point-by-point with the printer MTF. The scaling operation effectively 
adjusts the number of camera pixels per image height to match the number 
of printer pixels per image height for the print size selected by the 
user. The coefficients of the inverse filter are calculated in step 112 by 
inverting the system MTF curve on a point by point basis to provide the 
spatial frequency domain of the desired inverse filter. Inverse filter 
coefficient values that provide the desired inverse filter spatial 
frequency characteristic can then be calculated using conventional filter 
design techniques, for example as described in Digital Signal Processing 
by Alan Oppenheim and Ronald Schafer, .COPYRGT.1975, Prentice-Hall. 
Inverse filtering is then performed in step 114 by applying the inverse 
filter to the camera image data. The filtered image data is then used to 
print an inverse filtered image (step 116). 
FIGS. 7-10 show example MTF curves for the various steps just described. 
FIG. 7 shows the MTF of the camera 10, which has a CCD sensor 44 with 9 
micron pixels. The x-axis shows cycles per mm on the CCD sensor. FIG. 8 
shows the MTF of the thermal printer, which has a 200 pixels per inch 
print head in the printer engine 88. The x-axis shows cycles per inch on 
the print. FIG. 9 shows two different system MTF curves for two different 
size prints using the camera MTF of FIG. 7 and the printer MTF of FIG. 8. 
The x-axis shows cycles per inch on the print. 
The difference in the two curves in FIG. 9 is due to the difference in 
printer magnification, which determines the spatial frequency axis scaling 
between the camera MTF and the printer MTF. For example, if the image 
sensor in the camera has 1000 rows of pixels with a 9 micron spacing 
between rows, the sensor sampling frequency equals approximately 110 
cycles per mm. A printer with a 200 pixels per inch head has a sample 
frequency of 200 cycles per inch. If the user chooses to produce a 5 inch 
tall print from the camera, the camera pixels will be printed "one for 
one", since there are 1000 rows of camera pixels which will be used to 
produce a print having 1000 rows of pixels within the 5 inch print height. 
Therefore, the camera MTF curve will be scaled in the horizontal direction 
so that a frequency of 55 cycles per mm corresponds to 100 cycles per inch 
on the print. After this scaling, the camera MTF curve, now expressed as a 
function of cycles per inch on the print, is multiplied point by point 
with the printer MTF curve shown in FIG. 8 to produce the "large print" 
uncorrected system MTF curve shown in FIG. 9. 
If the user instead chooses to print a 2.5 inch tall print from the camera, 
the camera pixels will be printed "two for one" (two camera pixels for 
each printer pixel), since there are 1000 rows of camera pixels which will 
be used to produce a print having 500 rows of pixels within the 2.5 inch 
print height. Therefore, the camera MTF curve will be scaled in the 
horizontal direction so that a frequency of 55 cycles per mm corresponds 
to 200 cycles per inch on the print. After this scaling, the camera MTF 
curve, in cycles per inch on the print, is multiplied point by point with 
the printer MTF curve shown in FIG. 8 to produce the "small print" 
uncorrected system MTF curve shown in FIG. 9. 
In all cases, the camera MTF curve is scaled relative to the printer MTF 
curve by an amount proportional to the print magnification. For example, 
the user may decide to make a 2.5 inch tall print using only a portion of 
the camera image, for example only 500 out of the 1000 rows, by cropping 
the camera image prior to printing. In this case, the camera pixels will 
again be printed "one for one", since the 500 rows of camera pixels will 
be used to produce a print having 500 rows of pixels within the 2.5 inch 
print height. Therefore, the camera MTF curve will be scaled in the 
horizontal direction so that a frequency of 55 cycles per mm corresponds 
to 100 cycles per inch on the print, as in the case of the "large print" 
described above. After this scaling, the camera MTF curve is multiplied 
point by point with the printer MTF curve to produce a curve similar to 
the "large print" curve shown in FIG. 9. 
FIG. 10 shows the inverse filter MTF, the system MTF without inverse 
filtering (same as the small print system MTF from FIG. 9), and the final 
system MTF, for the smaller print size curve from FIG. 9. The inverse 
filter frequency response characteristic shown in FIG. 10 can be 
approximated using a conventional seven-tap symmetric finite impulse 
response (FIR) filter having coefficients equal to -0.03, +0.06, -0.17, 
+1.28, -0.17, +0.06, -0.03. To provide sharpening in the horizontal 
direction, the filter output for any pixel is set equal to 1.28 times the 
pixel input value, minus 0.17 times the sum of the horizontally adjacent 
pixels, plus 0.06 times the sum of the pixel values two pixel positions to 
the left and right of the input pixel, minus 0.03 times the sum of the 
pixel values two pixel positions to the left and right of the input 
pixels. To provide vertical sharpening, the filter is applied using the 
pixels immediately above and below the input pixel. This FIR filter is 
applied to the camera pixel data in both the horizontal and vertical 
direction, prior to printing, in order to provide a "flat" (unity) 
corrected system MTF after inverse filtering as shown in FIG. 10, for the 
small print size. To provide an appropriate inverse filter for the 
uncorrected large print system MTF shown in FIG. 9, the seven-tap filter 
coefficients should equal -0.08, +0.17, -0.38, +1.58, -0.38, +0.17, -0.08. 
Therefore, different filter coefficients, providing different amounts of 
sharpening designed to produce similar final system MTF responses, are 
used for different size prints made from the same digital camera on the 
same digital printer. 
For any print sizes except those providing "one for one" printing, an 
interpolation or decimation filter is used to provide, using the camera 
pixels as the input, a greater or smaller number of pixels as required by 
the printer to produce the desired print size. While such interpolation or 
decimation filtering is normally performed in the printer, it is possible 
to perform this operation in combination with the sharpening filter just 
described. 
In some applications, it may be desirable to limit the maximum value of the 
sharpening filter MTF to limit the increase in the visible noise resulting 
from the large gain of the inverse filter at high spatial frequencies. For 
example, the gain may be limited to a maximum value of 2 or 4, depending 
on the noise level of the camera. In this case, the sharpening filter 
coefficients are calculated so that the filter response does not exceed 
the desired maximum gain level at any point within the spatial frequency 
bandwidth of the printed image, that is for any frequency up to 100 
cycles/mm for a 200 pixels per inch printer. By measuring the noise level 
of the digital camera, the desired maximum gain level can be set 
appropriately for a given camera. Noisier cameras are given proportionally 
lower maximum gain values. 
In some applications, it may be desirable to produce a non-unity system MTF 
after the sharpening filter operation, for example one where the middle 
frequencies have a system MTF somewhat greater than unity, in order to 
provide a subjectively "crisper" image. In such a situation, the 
sharpening filter coefficients are calculated so as to compensate for the 
camera and printer MTFs and provide this desired non-unity response curve. 
In the embodiment shown in the figures, the camera and printer MTFs are the 
same in both the vertical and horizontal directions, so only a single 
curve is shown. In this case, the same sharpening filter is applied in 
both the horizontal and vertical directions, either using cascaded 
separable horizontal and vertical filters, or a single two-dimensional 
filter. In some cases, the camera MTF or the printer MTF may be different 
for the vertical and horizontal directions. In such cases, the sharpening 
filters will be different in the horizontal and vertical directions. 
The technique can also be used for optimally sharpening images for soft 
display on, e.g., the display 12. In this case, the soft display MTF 
characteristics are stored in a PROM in the display, or in a computer file 
supplied with the display. The display is then treated the same as the 
printer as described heretofore. The MTF of the image data supplied to the 
display is adjusted by an inverse filter appropriate to correct the camera 
image data for the size of the picture on the display. 
The measurement of the modulation transfer function for imaging devices and 
reproduction devices is a well-established technique and well-known to 
those of ordinary skill in this art. For example, FIG. 11 shows an 
arrangement for automatically measuring the MTF and noise level of a 
digital camera. The camera 10 captures a picture of a reflection test 
chart 120 which is illuminated by lamps 122. An image of the chart is 
captured by the camera 10 and transferred to the host computer 18 via a 
cable or memory card 16. The computer analyzes the camera data of the 
chart image in order to automatically calculate the camera MTF and noise 
level. The chart 120 includes four registration marks 124, a slightly 
slanted vertical bar 126, a slightly slanted horizontal bar 128, and a 
uniform grey patch 130. The registration marks 124 are used by the 
software on the computer 18 to determine the exact location of the other 
chart features, so that minor errors in the framing of the camera 10 
relative to the chart 120 do not affect the camera measurement. The camera 
pixel data along one edge of the slightly slanted vertical black bar 126 
is used to measure the horizontal camera MTF, using the method described 
by S. E. Reichenbach, et al, in "Characterizing digital image acquisition 
devices", Optical Engineering, Vol. 30, No. 2, February 1991, pp. 170-176. 
Similarly, the camera pixel data along one edge of the slightly slanted 
horizontal black bar 128 is used to measure the vertical camera MTF. The 
rms noise level of a subset of the pixels within the uniform grey patch 
130 are used to measure the camera noise level. The horizontal, vertical, 
and noise data is then stored in the calibration file 26 in the PROM 28 
(FIG. 3) within camera 10. 
The invention has been described with reference to a preferred embodiment. 
However, it will be appreciated that variations and modifications can be 
effected by a person of ordinary skill in the art without departing from 
the scope of the invention. For example, the camera, scanner, printer or 
display may include some nominal level of image sharpening, and the MTF 
calibration files provided for these components would include the effect 
of such sharpening operations. Additionally, the MTF calibration files for 
the camera, scanner, printer, or display could be supplied for existing 
printers after they are delivered to customers, by providing estimated MTF 
data for typical devices. These calibration files could be delivered by 
any digital data storage media, or could be communicated by paper, phone, 
fax, and electronic communications using modem or other connections to the 
Internet or other computer-based electronic communications. 
TS LIST 
10 digital camera 
12 display 
14 printer 
16 memory card 
18 host computer 
20 hardware sharpening filter 
22 user selectable sharpening software 
24 firmware sharpening filter 
26 camera MTF calibration file 
28 camera PROM 
30 camera computer file 
32 printer MTF calibration file 
34 printer PROM 
36 printer computer file 
38 automatic sharpening filter software 
40 lens 
41 aperture/shutter controller 
42 blur filter 
44 CCD 
46 analog video processor 
48 A/D converter 
50 frame memory 
52 digital signal processor 
54 data memory 
56 memory card 
58 memory card interface 
59 card connector 
60 host computer interface 
62 camera microprocessor 
64 user inputs 
66 flash unit 
68 timing generator 
70 photodiode 
72 exposure driver 
74 sensor driver 
78 host computer interface 
80 data memory 
82 digital signal processor 
84 frame memory 
86 D/A converter 
88 printer engine 
90 printer microprocessor 
92 user inputs 
94 timing generator 
96 filter memory 
107-116 process steps 
120 reflection test target 
122 lamps 
124 registration mark 
126 slanted vertical bar 
128 planted horizontal bar 
130 grey patch