# Patent

## Title
Noise reduction of panchromatic and color image

## Abstract

A method for producing a noise-reduced digital color image, includes providing an image having panchromatic pixels and color pixels corresponding to at least two color photoresponses; providing from the image a panchromatic image and at least one color image; and using the panchromatic image and the color image to produce the noise-reduced digital color image.

## Background
CROSS REFERENCE TO RELATED APPLICATION
The present application is related to U.S. patent application Ser. No. 11/191,538, filed Jul. 28, 2005, entitled “Processing Color and Panchromatic Pixels” by Compton et al, the disclosure of which is incorporated herein.
FIELD OF THE INVENTION
The invention relates generally to the field of digital image processing operations that produce a full-color noise-reduced full-resolution image from an image having panchromatic and color pixels.
BACKGROUND OF THE INVENTION
One of the most common and frequently essential image processing operations is noise reduction. This is especially true for digital still and video camera images that may have been captured under insufficient lighting conditions. One way to address digital image capture under less than optimum lighting conditions is to either acquire or synthesize one or more color channels that are particularly sensitive to low or insufficient scene illumination. The data from the channels with increased light sensitivity are generally used to guide the subsequent image processing of the data from the accompanying standard color channels. Noise reduction is a prime candidate for benefiting from this additional image data. A number of examples exist in the literature. U.S. Pat. No. 6,646,246 (Gindele, et al.) teaches using an extended dynamic range color filter array (CFA) pattern with slow and fast pixels, noise-cleaning the slow pixel data using only slow pixel data and noise-cleaning the fast pixel data using only fast pixel data. This approach achieved noise reduction at the expense of image resolution as each color channel is now subdivided into a fast channel and a slow channel and the subsequent merger can produce image processing artifacts more troublesome than the original noise being addressed. U.S. Pat. No. 7,065,246 (Xiaomang, et al.) is representative of a fair number of similarly disclosed inventions in that it reveals constructing a luminance signal from directly sensed color channel data, in this case cyan, magenta, yellow, and green. The high-frequency component of the constructed luminance is used to replace the high-frequency component of the original color channel signals to affect a net noise reduction of the image data. While somewhat effective, the major liability of this approach is that the synthesized luminance channel is constructed from noisy color channel components resulting in an essentially equally noisy synthetic channel.
A suggestion of a better approach can be found in U.S. Pat. No. 5,264,924 (Cok). Cok discloses direct measurement of red, green, blue, and luminance values at each pixel location. The high-frequency luminance data which is designed to be inherently less noisy than the corresponding high-frequency red, green, and blue data is used to replace said high-frequency red, green, and blue data to produce noise-cleaned red, green, and blue signals. Since the vast majority of digital still and video cameras use a single sensor equipped with a CFA that only senses one color channel per pixel, Cok cannot be directly practiced in such systems.
While Xiaomang and Cok describe luminance signals, a color channel with photometric sensitivity conforming to the luminance channel of the human visual system is unnecessarily restrictive. A more useful signal can be captured with a more general panchromatic channel, which has higher sensitivity at all wavelengths over the luminance channel of the human visual system.
SUMMARY OF THE INVENTION
It is an object of the present invention to produce a noise-reduced full-resolution full-color image from a digital image having panchromatic and color pixels.
This object is achieved by a method for producing a noise-reduced digital color image, comprising:
(a) providing an image having panchromatic pixels and color pixels corresponding to at least two color photoresponses;
(b) providing from the image a panchromatic image and at least one color image; and
(c) using the panchromatic image and the color image to produce the noise-reduced digital color image.
It is a feature of the present invention that images can be captured under low-light conditions with a sensor having panchromatic and color pixels and processing reduces noise in a full-color image produced from the panchromatic and colored pixels.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective of a computer system including a digital camera for implementing the present invention;
FIG. 2 is a block diagram of a preferred embodiment of the present invention;
FIG. 3 is a block diagram of an alternate embodiment of the present invention;
FIG. 4 is a block diagram of an alternate embodiment of the present invention;
FIG. 5 is a block diagram of an alternate embodiment of the present invention;
FIG. 6 is a region of pixels used in block 202 in FIG. 2;
FIG. 7 is a region of pixels used in block 206 in FIG. 2;
FIG. 8 is a region of pixels used in block 210 in FIG. 3; and
FIG. 9 is a region of pixels used in block 204 in FIG. 4.



DETAILED DESCRIPTION OF THE INVENTION
In the following description, a preferred embodiment of the present invention will be described in terms that would ordinarily be implemented as a software program. Those skilled in the art will readily recognize that the equivalent of such software can also be constructed in hardware. Because image manipulation algorithms and systems are well known, the present description will be directed in particular to algorithms and systems forming part of, or cooperating more directly with, the system and method in accordance with the present invention. Other aspects of such algorithms and systems, and hardware or software for producing and otherwise processing the image signals involved therewith, not specifically shown or described herein, can be selected from such systems, algorithms, components and elements known in the art. Given the system as described according to the invention in the following materials, software not specifically shown, suggested or described herein that is useful for implementation of the invention is conventional and within the ordinary skill in such arts.
Still further, as used herein, the computer program can be stored in a computer readable storage medium, which can comprise, for example; magnetic storage media such as a magnetic disk (such as a hard drive or a floppy disk) or magnetic tape; optical storage media such as an optical disc, optical tape, or machine readable bar code; solid state electronic storage devices such as random access memory (RAM), or read only memory (ROM); or any other physical device or medium employed to store a computer program.
Before describing the present invention, it facilitates understanding to note that the present invention is preferably utilized on any well-known computer system, such a personal computer. Consequently, the computer system will not be discussed in detail herein. It is also instructive to note that the images are either directly input into the computer system (for example by a digital camera) or digitized before input into the computer system (for example by scanning an original, such as a silver halide film).
Referring to FIG. 1, there is illustrated a computer system 110 for implementing the present invention. Although the computer system 110 is shown for the purpose of illustrating a preferred embodiment, the present invention is not limited to the computer system 110 shown, but can be used on any electronic processing system such as found in home computers, kiosks, retail or wholesale photofinishing, or any other system for the processing of digital images. The computer system 110 includes a microprocessor-based unit 112 for receiving and processing software programs and for performing other processing functions. A display 114 is electrically connected to the microprocessor-based unit 112 for displaying user-related information associated with the software, e.g., by a graphical user interface. A keyboard 116 is also connected to the microprocessor based unit 112 for permitting a user to input information to the software. As an alternative to using the keyboard 116 for input, a mouse 118 can be used for moving a selector 120 on the display 114 and for selecting an item on which the selector 120 overlays, as is well known in the art.
A compact disk-read only memory (CD-ROM) 124, which typically includes software programs, is inserted into the microprocessor based unit for providing a way of inputting the software programs and other information to the microprocessor based unit 112. In addition, a floppy disk 126 can also include a software program, and is inserted into the microprocessor-based unit 112 for inputting the software program. The compact disk-read only memory (CD-ROM) 124 or a floppy disk 126 can alternatively be inserted into an externally located disk drive unit 122 which is connected to the microprocessor-based unit 112. Still further, the microprocessor-based unit 112 can be programmed, as is well known in the art, for storing the software program internally. The microprocessor-based unit 112 can also have a network connection 127, such as a telephone line, to an external network, such as a local area network or the Internet. A printer 128 can also be connected to the microprocessor-based unit 112 for printing a hardcopy of the output from the computer system 110.
Images can also be displayed on the display 114 via a personal computer card (PC card) 130, such as, as it was formerly known, a PCMCIA card (based on the specifications of the Personal Computer Memory Card International Association) which contains digitized images electronically embodied in the PC card 130. The PC card 130 is ultimately inserted into the microprocessor based unit 112 for permitting visual display of the image on the display 114. Alternatively, the PC card 130 can be inserted into an externally located PC card reader 132 connected to the microprocessor-based unit 112. Images can also be input via the compact disk 124, the floppy disk 126, or the network connection 127. Any images stored in the PC card 130, the floppy disk 126 or the compact disk 124, or input through the network connection 127, can have been obtained from a variety of sources, such as a digital camera (not shown) or a scanner (not shown). Images can also be input directly from a digital camera 134 via a camera docking port 136 connected to the microprocessor-based unit 112 or directly from the digital camera 134 via a cable connection 138 to the microprocessor-based unit 112 or via a wireless connection 140 to the microprocessor-based unit 112.
In accordance with the invention, the algorithm can be stored in any of the storage devices heretofore mentioned and applied to images in order to interpolate sparsely populated images.
FIG. 2 is a high level diagram of a preferred embodiment. The digital camera 134 is responsible for creating an original digital red-green-blue-panchromatic (RGBP) color filter array (CFA) image 200, also referred to as the digital RGBP CFA image or the RGBP CFA image. It is noted at this point that other color channel combinations, such as cyan-magenta-yellow-panchromatic, can be used in place of red-green-blue-panchromatic in the following description. The key item is the inclusion of a panchromatic channel. This image is considered to be a sparsely sampled image because each pixel in the image contains only one pixel value of red, green, blue, or panchromatic data. A panchromatic image interpolation block 202 produces a full-resolution panchromatic image 204. At this point in the image processing chain each color pixel location has an associated panchromatic value and either a red, green, or blue value. With the help of the full-resolution panchromatic image the noise associated with the red, green, and blue pixel values is now reduced in an RGB CFA image noise reduction block 210 to produce a noise-reduced RGB CFA image 212. An RGB CFA image interpolation block 214 subsequently produces a noise-reduced full-resolution full-color image 226.
FIG. 3 is a high level diagram of a second preferred embodiment. The digital camera 134 is responsible for creating an original digital red-green-blue-panchromatic (RGBP) color filter array (CFA) image 200, also referred to as the digital RGBP CFA image or the RGBP CFA image. It is noted at this point that other color channel combinations, such as cyan-magenta-yellow-panchromatic, can be used in place of red-green-blue-panchromatic in the following description. The key item is the inclusion of a panchromatic channel. This image is considered to be a sparsely sampled image because each pixel in the image contains only one pixel value of red, green, blue, or panchromatic data. A panchromatic image interpolation block 202 produces a full-resolution panchromatic image 204. At this point in the image processing chain each color pixel location has an associated panchromatic value and either a red, green, or blue value. With the help of the full-resolution panchromatic image the noise associated with the red, green, and blue pixel values is now reduced in an RGB CFA image noise reduction block 210 to produce a noise-reduced RGB CFA image 212. An RGB CFA image interpolation block 214 subsequently produces a noise-reduced full-resolution full-color image 216. Finally, a full-resolution full-color noise reduction block 218 produces a final noise-reduced full-resolution full-color image block 224.
FIG. 4 is a high level diagram of a preferred embodiment. The digital camera 134 is responsible for creating an original digital red-green-blue-panchromatic (RGBP) color filter array (CFA) image 200, also referred to as the digital RGBP CFA image or the RGBP CFA image. It is noted at this point that other color channel combinations, such as cyan-magenta-yellow-panchromatic, can be used in place of red-green-blue-panchromatic in the following description. The key item is the inclusion of a panchromatic channel. This image is considered to be a sparsely sampled image because each pixel in the image contains only one pixel value of red, green, blue, or panchromatic data. A panchromatic image interpolation block 202 produces a full-resolution panchromatic image 204. At this point in the image processing chain each color pixel location has an associated panchromatic value and either a red, green, or blue value. Next a full-resolution panchromatic image noise reduction block 206 produces a noise-reduced full-resolution panchromatic image 208. With the help of the noise-reduced full-resolution panchromatic image the noise associated with the red, green, and blue pixel values is now reduced in an RGB CFA image noise reduction block 210 to produce a noise-reduced RGB CFA image 212. An RGB CFA image interpolation block 214 subsequently produces a noise-reduced full-resolution full-color image 222.
FIG. 5 is a high level diagram of a preferred embodiment. The digital camera 134 is responsible for creating an original digital red-green-blue-panchromatic (RGBP) color filter array (CFA) image 200, also referred to as the digital RGBP CFA image or the RGBP CFA image. It is noted at this point that other color channel combinations, such as cyan-magenta-yellow-panchromatic, can be used in place of red-green-blue-panchromatic in the following description. The key item is the inclusion of a panchromatic channel. This image is considered to be a sparsely sampled image because each pixel in the image contains only one pixel value of red, green, blue, or panchromatic data. A panchromatic image interpolation block 202 produces a full-resolution panchromatic image 204. At this point in the image processing chain each color pixel location has an associated panchromatic value and either a red, green, or blue value. Next a full-resolution panchromatic image noise reduction block 206 produces a noise-reduced full-resolution panchromatic image 208. With the help of the noise-reduced full-resolution panchromatic image the noise associated with the red, green, and blue pixel values is now reduced in an RGB CFA image noise reduction block 210 to produce a noise-reduced RGB CFA image 212. An RGB CFA image interpolation block 214 subsequently produces a noise-reduced full-resolution full-color image 216. Finally, a full-resolution full-color noise reduction block 218 produces a final noise-reduced full-resolution full-color image block 220.
Returning to FIG. 2, panchromatic image interpolation block 202 can be performed in any appropriate way known to those skilled in the art. Two examples are now given. Referring to FIG. 6, one way to estimate a panchromatic value for pixel X5 is to simply average the surrounding six panchromatic values, i.e.:

X5=(P1+P2+P3+P7+P8+P9)/6

Alternate weighting to the pixel value in this approach are also well known to those skilled in the art. As an example,

X5=(P1+2P2+P3+P7+2P8+P9)/8

Alternately, an adaptive approach can be used by first computing the absolute values of directional gradients (absolute directional gradients).

B5=|P1−P9|

V5=|P2−P8|

S5=|P3−P7|

The value of X5 is now determined by one of three two-point averages.

BX5=(P1+P9)/2

VX5=(P2+P8)/2

SX5=(P3+P7)/2

The two-point average associated with the smallest value of the set of absolute direction gradients is used for computing X5, e.g., if V5≦B5 and V5≦S5, then X5=VX5.

Returning to FIG. 2, RGB CFA image noise reduction block 210 is preferably performed in an adaptive manner to take advantage of the full-resolution panchromatic image produced by block 202. Any number of such methods will be known to those skilled in the arts. Two examples are now given. Referring to FIG. 7, it is to be noted that each pixel location has an associated panchromatic value. It is also noted that the number and placement of intervening non-red pixels (i.e., blank pixels in FIG. 7) is irrelevant. All that is required is a central pixel value with at least one or more neighboring pixel values of the same color. Also, in the following discussion, green or blue would be substituted for red when cleaning the green or blue color channels. A sigma filter can be used to accomplish noise reduction of the red pixel R5. This is accomplished by computing the following weighted average.

R5=(c1R1 +c2R2+c3R3+c4R4+c5R5+c6R6+c7R7+c8R8+c9R9)/(c1+c2+c3+c4+c5+c6+c7+c8+c9)

The weighting coefficients c1 through cg are computed from differences in panchromatic values.

c1=1 if |P1−P5|≦t, otherwise c1=0

c2=1 if |P2−P5|≦t, otherwise c2=0

c3=1 if |P3−P5|≦t, otherwise c3=0

c4=1 if |P4−P5|≦t, otherwise c4=0

c5=1 if |P5−P5|≦t, otherwise c5=0

c6=1 if |P6−P5|≦t, otherwise c6=0

c7=1 if |P7−P5|≦t, otherwise c7=0

c8=1 if |P8−P5|≦t, otherwise c8=0

c9=1 if |P9−P5|≦t, otherwise c9=0

In these expressions t is a predetermined threshold value that is chosen to exclude pixel values that are separated from the central pixel (R5) by any edges in the pixel neighborhood shown in FIG. 7. It is noted that by these definition c5 is always 1; this is to assure that we always include at least one pixel in the summation. Alternate schemes for populating sigma filter weighting coefficients are well known in the art and can be similarly adapted to using full-resolution panchromatic data. An alternate method is to use an adaptive median filter. Again referring to FIG. 7, four panchromatic and four red 3-point median values are computed.

PH=median (P4, P5, P6)

PB=median (P1, P5, P9)

PV=median (P2, P5, P8)

PS=median (P3, P5, P7)

RH=median (R4, R5, R6)

RB=median (R1, R5, R9)

RV=median (R2, R5, R8)

RS=median (R3, R5, R7)

The noise-reduced value for R5 corresponds to the red median value that corresponds to the panchromatic median value that is closest to panchromatic value associated with R5, i.e., P5.

R5=RH if |PH−P5|≦{|PB−P5|, |PV−P5|, |PS−P5|}

R5=RB if |PB−P5|≦{|PH−P5|, |PV−P5|, |PS−P5|}

R5=RV if |PV−P5|≦{|PH−P5|, |PB−P5|, |PS−P5|}

R5=RS if |PS−P5|≦{|PH−P5|, |PB−P5|, |PV−P5|}

Alternate schemes for employing adaptive median filters include those using fewer than four median values within the neighborhood are well known in the art and can be similarly adapted to using full-resolution panchromatic data.

In addition to the methods described above, other well-known noise reduction methods such as, but not limited to, infinite impulse response (IIR) filtering and singular value decomposition (SVD) could be used.
It is also well known by those skilled in the art that pixel neighborhoods such as depicted in FIG. 7 can result from Laplacian or Gaussian pyramid decompositions or wavelet decompositions of the image data. By simultaneously applying the same decomposition methods to the full-resolution panchromatic data, each resulting pixel within each RGB image decomposition component will still have an associated panchromatic value. Therefore, the foregoing discussion and examples remain relevant and unaltered.
Returning to FIG. 2, RGB CFA image interpolation block 214 can be performed using any of the well-known CFA interpolation or demosaicking techniques described in the prior art. U.S. Pat. No. 5,852,468 (Okada) describes a typical nonadaptive method while U.S. Pat. No. 5,506,619 (Adams, et al.) teaches a representative adaptive approach.
In FIG. 3, full-resolution full-color noise reduction block 216 can be performed in a manner similar to the RGB CFA image noise reduction block 210, only in block 216 there are red, green, blue, and panchromatic values at each pixel location. Therefore, noise reduction can be performed using contiguous pixel neighborhoods, such as depicted in FIG. 8. When referenced to FIG. 8, the examples given above of the sigma filter and adaptive median filter are directly applicable.
In FIG. 4, full-resolution panchromatic image noise reduction block 206 would be performed using any of the well-known methods in the prior art for grayscale or single-channel image noise reduction. These would include the previously described examples of sigma filtering and adaptive median filtering. Referring to FIG. 9, a sigma filter can be used to accomplish noise reduction of the panchromatic pixel P5. This is accomplished by computing the following weighted average.

P5=(c1P1+c2P2+c3P3+c4P4+c5P5+c6P6+c7P7+c8P8+c9P9)/(c1+c2+c3+c4+c5+c6+c7+c8+c9)

The weighting coefficients c1 through c9 are computed from differences in panchromatic values.

c1=1 if |P1−P5|≦t, otherwise c1=0

c2=1 if |P2−P5|≦t, otherwise c2=0

c3=1 if |P3−P5|≦t, otherwise c3=0

c4=1 if |P4−P5|≦t, otherwise c4=0

c5=1 if |P5−P5|≦t, otherwise c5=0

c6=1 if |P6−P5|≦t, otherwise c6=0

c7=1 if |P7−P5|≦t, otherwise c7=0

c8=1 if |P8−P5|≦t, otherwise c8=0

c9=1 if |P9−P5|≦t, otherwise c9=0

In these expressions t is a predetermined threshold value that is chosen to exclude pixel values that are separated from the central pixel (P5) by any edges in the pixel neighborhood shown in FIG. 9. It is noted that by these definition c5 is always 1; this is to assure that we always include at least one pixel in the summation. Alternate schemes for populating sigma filter weighting coefficients are well known in the art. An alternate method is to use an adaptive median filter. Again referring to FIG. 9, four panchromatic are computed.

PH=median (P4, P5, P6)

PB=median (P1, P5, P9)

PV=median (P2, P5, P8)

PS=median (P3, P5, P7)

The noise-reduced value for P5 corresponds to the panchromatic median value that is closest to original panchromatic value associated with P5.

P5=PH if |PH−P5|≦{|PB−P5|, |PV−P5|, |PS−P5|}

P5=PB if |PB−P5|≦{|PH−P5|, |PV−P5|, |PS−P5|}

P5=PV if |PV−P5|≦{|PH−P5|, |PB−P5|, |PS−P5|}

P5=PS if |PS−P5|≦{|PH−P5|, |PB−P5|, |PV−P5|}

Alternate schemes for employing adaptive median filters are well known in the art and can be used.

In addition to the methods described above, other well-known noise reduction methods such as, but not limited to, infinite impulse response (IIR) filtering and singular value decomposition (SVD) could be used.
It is also well known by those skilled in the art that pixel neighborhoods such as depicted in FIG. 9 can result from Laplacian or Gaussian pyramid decompositions or wavelet decompositions of the image data. Therefore, the foregoing discussion and examples remain relevant and unaltered.
The noise reduction algorithms disclosed in the preferred embodiments of the present invention can be employed in a variety of user contexts and environments. Exemplary contexts and environments include, without limitation, wholesale digital photofinishing (which involves exemplary process steps or stages such as film in, digital processing, prints out), retail digital photofinishing (film in, digital processing, prints out), home printing (home scanned film or digital images, digital processing, prints out), desktop software (software that applies algorithms to digital prints to make them better—or even just to change them), digital fulfillment (digital images in—from media or over the web, digital processing, with images out—in digital form on media, digital form over the web, or printed on hard-copy prints), kiosks (digital or scanned input, digital processing, digital or scanned output), mobile devices (e.g., PDA or cell phone that can be used as a processing unit, a display unit, or a unit to give processing instructions), and as a service offered via the World Wide Web.
In each case, the noise reduction algorithms can stand alone or can be a component of a larger system solution. Furthermore, the interfaces with the algorithm, e.g., the scanning or input, the digital processing, the display to a user (if needed), the input of user requests or processing instructions (if needed), the output, can each be on the same or different devices and physical locations, and communication between the devices and locations can be via public or private network connections, or media based communication. Where consistent with the foregoing disclosure of the present invention, the algorithms themselves can be fully automatic, can have user input (be fully or partially manual), can have user or operator review to accept/reject the result, or can be assisted by metadata (metadata that can be user supplied, supplied by a measuring device (e.g. in a camera), or determined by an algorithm). Moreover, the algorithms can interface with a variety of workflow user interface schemes.
The noise reduction algorithms disclosed herein in accordance with the invention can have interior components that utilize various data detection and reduction techniques (e.g., face detection, eye detection, skin detection, flash detection)
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
PARTS LIST


    110 Computer System
    112 Microprocessor-based Unit
    114 Display
    116 Keyboard
    118 Mouse
    120 Selector on Display
    122 Disk Drive Unit
    124 Compact Disk—read Only Memory (CD-ROM)
    126 Floppy Disk
    127 Network Connection
    128 Printer
    130 Personal Computer Card (PC card)
    132 PC Card Reader
    134 Digital Camera
    136 Camera Docking Port
    138 Cable Connection
    140 Wireless Connection
    200 RGBP CFA Image
    202 Panchromatic Image Interpolation
    204 Full-Resolution Panchromatic Image
    206 Full-Resolution Panchromatic Image Noise Reduction
    208 Noise-Reduced Full-Resolution Panchromatic Image
    210 RGB CFA Image Noise Reduction
    212 Noise-Reduced RGB CFA Image
    213 RGB CFA Image Interpolation
    216 Noise-Reduced Full-Resolution Full-Color Image
    218 Full-Resolution Full-Color Noise Reduction
    220 Final Noise-Reduced Full-Resolution Full-Color Image
    222 Noise-Reduced Full-Resolution Full-Color Image
    224 Final Noise-Reduced Full-Resolution Full-Color Image
    226 Noise-Reduced Full-Resolution Full-Color Image

## Claims

1. A method for producing a noise-reduced digital color image, comprising:
(a) providing an image having panchromatic pixels and color pixels corresponding to at least three color photoresponses;
(b) providing from the image a panchromatic image and interpolating the panchromatic image to provide a full-resolution panchromatic image;
(c) using the full-resolution panchromatic image and neighboring same color pixel values to produce a noise-reduced color pixel and repeating this step for each color pixel; and
(d) interpolating the noise-reduced color pixels to produce interpolated color pixels to thereby provide a noise-reduced digital color image.

2. The method of claim 1 further includes providing further noise reduction on the noise-reduced digital color image to produce a final noise-reduced digital color image.

3. The method of claim 1 further including noise-reducing all panchromatic values in the full-resolution panchromatic image prior to step (c) to produce a noise-reduced full-resolution panchromatic image.

4. The method of claim 1 wherein step a) includes color pixels having the following photosensitivities of red, green, and blue.

5. The method of claim 1 wherein step a) includes color pixels having the following photosensitivities of cyan, magenta, and yellow.