Patent Application
20060158692
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1. Field of the Invention
The present invention is directed to systems and methods for implementing error diffusion when processing an image, such as for printing.
2. Description of the Related Art
When printing an image using an output device which places discreet units of colorants (ink drops, toner, etc.) on media, it is necessary to reduce the range of the image pixels to match the reproduction capabilities of the printing device. This typically means a reduction in the bit resolution of the image.
Most often, the reduction in bit resolution is accomplished by halftone transformation. Halftone transformation results, on a pixel-by-pixel basis for all image pixels, in the replacement of an original non-binary, or “gray-level” value of, e.g., 8 bits, with a binary value after comparison with some threshold. The threshold itself may vary dynamically depending on the non-binary pixel value, and other factors. The original 8-bit value at each pixel is thus substituted by either a “0” (representing an 8-bit value of 0) or a “1” (representing an 8-bit value of 255). The consequence of such a transformation at a pixel is that the overall “brightness” of the image is changed. To mitigate this, the change, or “error”, may be diffused to nearby, as yet untransformed pixels through a technique known as error diffusion. Error diffusion works by spreading the inaccuracy, or error, of the halftone decision at one pixel in the output image among nearby pixels, creating a visually superior transformation. Each original pixel value is adjusted based on the error contributed by adjacent and nearby pixels, and these contributions are taken into account in calculating the correct transformed value for the pixel.
There are a number of error diffusion techniques, each of which uses a different combination of thresholding approaches, collection of nearby pixels to which the error is spread, error weightings to each of these nearby pixels, and other factors. The Floyd-Steinberg algorithm, developed in 1975 and known to those skilled in the art, is one of the more well-known implementations of error diffusion. This algorithm generates a series of error values for each image element as an image line is transformed. These error values are calculated by taking a fraction of nearby pixel error values and adding them together to represent a pixel location.
In the Floyd-Steinberg algorithm, the error at a transformed pixel 420 is spread to a collection of four specific nearby pixels in the fashion shown in
From the foregoing description, it can be seen that in the Floyd-Steinberg algorithm the error created from transforming a pixel is spread to four adjacent pixels. Furthermore, prior to transformation, each pixel receives a portion of the error from each of the four adjacent pixels that have previously been transformed.
The Floyd-Steinberg algorithm typically operates in row order (sometimes called “line order”). That is, an entire row, or line, of an image is transformed before the next row or line is transformed. Transformation of a row results in the storage of a large number of error values. For instance, if an image has a resolution of 600 pixels per inch (PPI), and each row of the image is 9 inches wide, then 5400 pixels worth of error data, each error datum comprising anywhere from 1 color (for a black & white printer) to 3 or more colors (for a color printer), may need to be stored.
Originally, the Floyd-Steinberg algorithm was implemented in software with data being read from, and written to a main memory having ample space. More recently, however, high-speed ASIC-based hardware implementations using integer arithmetic have been realized. For cost reasons, it is best to minimize the amount of memory used in such implementations.
The target platform for a system in accordance with the present invention is a device, such as a printer, that is configured to perform error diffusion on image data pixels, each image data pixel comprising a non-binary pixel value.
In one aspect, the present invention is directed to an error diffusion system configured to perform halftoning of image pixel data. The system comprises an error diffusion processor configured to receive a current pixel from a current image line and output an error diffused current pixel in response thereto; a first decompressor connected to the error diffusion processor and configured to decompress a compressed previous running error of a pixel belonging to a previous line of the image for use in calculating an adjusted pixel value of said current pixel; and a first compressor connected to the error diffusion processor and configured to compress a current running error of said current pixel to thereby form a compressed current running error for the current pixel.
In another aspect, the present invention is also directed to a method for handling running error values during a halftoning process of an image. The method entails decompressing a compressed previous running error of a pixel belonging to a previous line of said image to form a truncated previous running error for use in calculating an adjusted pixel value of a current pixel in a current line of an image; and compressing a current running error of that same current pixel to thereby form a compressed running error for that pixel.
In still another aspect, the present invention is directed to an error diffusion system for halftoning image data pixels one image line at a time, the system configured to calculate an adjusted pixel value for a current pixel in a current image line, the adjusted pixel value including partial errors from pixels on a previous image line, wherein the partial errors from the pixels on the previous image line are calculated only after halftoning of entire previous line has been completed.
The invention is now described with reference to the attached drawings in which:
The system 100 includes a general purpose microprocessor 110 that is connected to a main memory 104. Main memory 104 typically stores the input pixel data 106 of an image whose pixels are to be transformed from a non-binary format to a binary format, using error diffusion. The microprocessor 110 is part of an Application Specific Integrated Circuit (ASIC) 102 (represented by the dashed) configured to implement error diffusion. The dotted arrows represent connections between the microprocessor 110 and the other components of the ASIC, through data buses, control buses and other structures known to those skilled in the art of integrated circuit design. While in this embodiment, the error diffusion is performed using hardware on the ASIC, it can instead be performed entirely in software by the microprocessor 110. It is further understood that in some embodiments, the main memory 104 can also be part of the ASIC 102, or the input pixel data can be stored in a local memory on-board the ASIC.
In addition to the microprocessor 110, the ASIC 102 includes an error diffusion processor 120, and error spread coefficient subsystem 130, threshold generation circuitry 140, and a running error compression/decompression subsystem, shown generally at 190.
The error diffusion processor 120 receives pixel data 106 from the main memory 104, error spread coefficients 132 from the error spread coefficient system 130, and threshold information 142 from threshold generation circuitry 140. The error diffusion processor 120 uses this information, along with the truncated previous line running error information 124 provided by the error compression/decompression subsystem 190, to transform the pixel data 106 into error diffused pixel data 126 which is stored in the main memory or in another memory location. Control signals 121 are sent from the error diffusion processor 120 to the error spread coefficient system 130 for requesting coefficients and performing other functions.
The error spread coefficient system 130 receives input 112 from the microprocessor 110 and pixel data 106 in the case of data-driven determinations of the error spread coefficients. The error spread coefficient system 130 provides the error diffusion processor 120 with the error spread coefficients 132 to be used in allocating the error from a transformed pixel. As discussed further below, the error spread coefficient system 130 may be implemented in a number of different ways.
The threshold generation circuitry 140 creates a threshold 142 that is used to compare with each adjusted non-binary (e.g., 8 bit) gray level pixel datum to determine whether the corresponding pixel is to be set to “0” or “1”. The threshold generation circuitry 140 is under the control of the microprocessor 110 and can include pseudo-random circuitry or the like to form a dynamic threshold, in a known manner.
As is known to those skilled in the art, the error diffusion processor 120 typically processes image pixel data in line order—each pixel belonging to one line of an image is error diffused, before pixels of the next line are processed. The running error compression/decompression subsystem 190, described in further detail below, is used to efficiently store the total error at each transformed pixel in an immediately preceding row of image data for use in adjusting a current pixel value of a pixel in a current row of image data.
The first step 301 is to receive the 8-bit value of a current pixel, from main memory 104. The next step 302 is to shift this pixel value left by four bits, which is equivalent to multiplying by 16. At this point, the original 8-bit pixel has been transformed into having 12 significant bits. The transformed pixel is then input into a summer 304 along with five other inputs designated 306A, 306B, 306C, 306D and 340, to form a 16-bit adjusted pixel value 308.
Input error 306A is the partial error received by the current pixel from the transformed pixel in the preceding row and to the left (“previous back”). Input error 306A is the product of an 8 to 10 bit representation of the truncated running error from transformed “previous back” pixel and a 4-bit representation of the “receive backward coefficient” 350A, both of which are input to a first integer multiplier 352A.
Input error 306B is the partial error received by the current pixel from the transformed pixel in the preceding row and directly above (“previous above”). Input error 306B is the product of an 8 to 10 bit representation of the truncated running error from transformed “previous above” pixel and a 4-bit representation of the “receive above coefficient” 350B, both of which are input to a second integer multiplier 352B.
Input error 306C is the partial error received by the current pixel from the transformed pixel in the preceding row and to the right (“previous forward”). Input error 306C is the product of an 8 to 10 bit representation of the truncated running error from transformed “previous forward” pixel and a 4-bit representation of the “receive forward coefficient” 350C, both of which are input to a third integer multiplier 352C.
Input error 306D is the partial error received by the current pixel from the just-transformed pixel immediately to the left in the current row (“current left”). Input error 306D is delayed by delay 334 and is the delayed product of an 8 to 10 bit representation of the truncated running error from the immediately preceding (i.e., just-transformed pixel) and a 4-bit representation of the “receive left coefficient” 350D, both of which are input to a fourth integer multiplier 352D.
In the foregoing discussion of the input errors 306A, 306B, 306C, 306D, the term ‘truncated running error’ refers to the fact that the least significant bits of the various running errors have been set to zero by retaining only the most significant bits and/or shifting to the right, as discussed further below.
Finally, input error 340 is delayed by delay 336 and is the delayed version of a 4 to 6 bit current remainder 352 (152B in
Once the 16-bit adjusted current pixel value 308 has been formed by the summer 304, a decision 312 is made to determine whether it exceeds a threshold. If the 16-bit adjusted current pixel value 308 exceeds the threshold 142, then, in block 314, the current pixel is set to “1” and the current running error 322 is calculated. If, on the other hand, the 16-bit adjusted current pixel value 308 does not exceed the threshold 142, then, in block 318, the current pixel is set to “0” and the 16-bit adjusted pixel value is used as the current running error 322. Thus, the current running error 322 is the outcome of the halftoning decision represented by decision block 312 and blocks 314, 318.
In either case, the current running error 322 is tapped, as shown by line 315, and input to a shifter 326 where it undergoes a right shift (i.e., a divide by 16). The output of the shifter 326 is then input to the aforementioned fourth multiplier 352D to help form the ‘current left’ error 306D which is delayed by delay 334 and which is to be provided to the pixel immediately to the right on the same line for use in the next iteration.
One consequence of the design shown in
The current running error 322 is also input to the error compression/decompression block 320, representing the error compression/decompression subsystem 190 of
Returning to
During operation, the first error compressor 150 checks to see the position of the most significant bit in the current running error 122.
If it is determined that the position of the most significant bit in the current running error 122 is between bit positions J=0 and J=11, then the current running error 122 is shifted to the right by k=4. The thus-shifted version of the current running error 122 is considered to be the compressed current running error 152A (since the most significant bit, after shifting, is now between bit positions 0-7), and the lowest four bits of current running error 122 (originally in bit positions 0-3) are simply returned 152B (352 in
If, however, it is determined that the position of the most significant bit in the current running error 122 is between bit positions J=12 and J=14 then the current running error 122 is shifted to the right by an amount necessary to cause the m=8 most significant bits to occupy bit positions 0-7 to thereby create the compressed current running error 152A.
For example, given that the most significant bit is in bit position J=12, the first error compressor 150 shifts the entire current running error value to the right an appropriate amount (in this example, k=5 shifts to the right) until the most significant bit falls into the bit position 7. This way, only m=8 bits need to be stored as the compressed current running error 152A, along with the shift data value 154.
Continuing with this example, the k=5 least significant bits (LSBs) are packed into an 8-bit word and returned 152B (352 in
In a sense, one can consider the m=8 MSBs of the compressed running error to be a mantissa and the error shift value k=5 to be an exponent. The m=8 bit mantissa can then be shifted to the left by the k=5 error shift value to form a truncated previous line running error 124 which has a magnitude on the order of the its original running error 122, and differs from its original running error 122 by just the k=5 least significant bits which, in any event, have been recycled as remainder 152B (352 in
As stated above, N is a 16 bit value, and so the current running error has bit positions in the range [15:0], with 0 denoting the least significant bit. It is understood in the foregoing example that if the most significant bit were in bit position J=11, instead of bit position J=12 in the N=16 bit current running error 122 (322 in
From the foregoing, it can be seen that shifting effectively compresses the original 16-bit current running error value 122 from the original N=16 bits down to m=8 bits. The resulting shifted value is output as an m=8 bit “compressed current running error” 152A and sent to the compressed running error buffer 180 where it is stored.
The error shift value 154 (which in this example is k=5) is passed on to shift data compressor 160. The error shift values 154 (the “exponents”) corresponding to the compressed current running errors 152A for a number of successive 16-bit current running error values 122 are often the same, or vary by 1, at most. Therefore, run length encoding (RLE) of the error shift values can be performed by the shift data compressor 160. The shift data compressor 160 outputs RLE compressed shift data 162, in a form such as a packets, for storage in the compressed shift data buffer 182. These RLE shift data packets 162 can be of variable length, or alternatively, of fixed length, depending on the RLE implementation chosen.
The compressed running error buffer 180 is a FIFO buffer. Current error compressor 150 compresses a current line running error 122 for a pixel in a current line of an image to thereby form the compressed current running error 152A which is stored in buffer 180.
The calculation of the current line running error 122 itself depends, in part, on the partial errors 306A, 306B, 306C contributed by pixels in the previous line, as discussed above with reference to
To perform this decompression, the shift data decompressor 168 first retrieves the appropriate compressed shift data 166 from the compressed shift data buffer 182, then decompresses this to reconstitute the error shift value for each needed pixel's compressed running error in the previous line, and lastly supplies the corresponding reconstituted error shift value 170 to the previous error decompressor 158. The previous error decompressor 158 then uses this reconstituted error shift value 170 to shift the compressed previous running error 156 by the appropriate amount to form the truncated previous line running error 124 that is supplied to error diffusion processor 120.
In the present context, “truncation” refers to the fact that while the order of magnitude of the truncated running error is comparable to that of the original current running error, its least significant bits are not contained in that value.
In summary, then, it can be seen that the compression/decompression subsystem 190 includes a first compressor 150 connected to the error diffusion processor 120 and configured to retain, at most, only the m most significant bits of each of a plurality of current running errors to thereby form a corresponding plurality of compressed current running errors 152A. The compression/decompression subsystem 190 also includes a second compressor 160 configured to compress information sufficient to create a truncated previous line running error 124 corresponding to its original current running error 122, from the compressed previous running error 156.
As mentioned above, error diffusion is performed in line order, and so all the pixels belonging to a single line are processed one after the other. Then, in the general case, one may consider the ith pixel in a line of image data to have an N-bit current running error with the most significant bit in bit position Ji, N>Ji. If Ji<12, the current running error is shifted to the right by ki=4 bits, the m=8 bits in bit positions [7:0) are stored in the compressed error buffer, and the shift value ki itself is sent to the second compressor 160. If Ji≧12, the current running error is shifted to the right by a number of bits ki such that its most significant bit ends up in bit position 7, the m=8 bits in bit positions [7:0] are again stored in the compressed error buffer, and the shift value ki itself is again sent to the second compressor 160. Finally, the various corresponding ki shift values are compressed.
Since the compressed running error buffer 180 is a FIFO buffer, as the first compressor 150 accepts a new running error 122, the previous error decompressor 158 outputs a truncated previous line running error 124, the appropriate location in the compressed running error buffer 180 being overwritten in the process. It is further understood from
Considerable savings in buffer memory from using the two compressors 150, 160 with the dynamic shifting can be realized. Assuming that m=8 MSBs are stored in the compressed running error buffer 180, and further assuming that RLE compression of the error shift values 154 requires 1 bit for every 10 pixels, a line of 5400 pixels requires that roughly 5400×8+540=43,740 bits of data be stored by compressed running error buffer 180 and compressed shift data buffer 182. This is a savings of about 42,660 bits, or roughly 49% fewer bits than would be required if all 5400×16=86,400 bits of the current running error 122 were stored. If, instead, only m=4 MSBs were stored (in which case the remainder 122 (352 in
In
The error diffusion processor 120 uses each of these 4-bit values as a relative weighting, and so multiplies each of the truncated running errors by an appropriate corresponding 4 bit value to create four partial errors 306A, 306B, 306C and 306D used in the summer 304 within the error diffusion processor 120. When the 16-bit error spread vector 210 is provided to the error diffusion processor 120, the latter understands the meaning of the four groupings of bits and uses them accordingly.
In
In the case of error spread coefficient subsystem 130B, each of the 256 error spread vectors comprises four 4-bit weights dictating how to spread the running error from the current pixel to adjacent untransformed pixels. This contrasts with embodiment 130A where the fixed error spread vector 210 dictates how to weight the total error from each of the previously transformed pixels in preparation for summer 304.
In response to a particular 8-bit pixel value input thereto, the lookup table 224 supplies the appropriate 16-bit error spread vector and splits it in two parts. The first part comprises a 4-bit weight that is sent via output 222A to the error diffusion processor 120 for use as the “receive left” coefficient 350D for the next pixel (and also happens to be the “send right” coefficient for the current pixel). The second part is a 12 bit value 228 which comprises the “send back”, “send below” and “send forward”, coefficients for the current pixel. These are stored in a 5400×12-bit spread buffer 226 (12 bits for each pixel in a row) again assuming 600 ppi by 9 inch line length. The error diffusion processor 120 retrieves the appropriate set of previous line “receive” coefficients via output 222B of the error spread coefficient subsystem 130B.
The error spread coefficient subsystem 130C of
Block compressor accepts a plurality of consecutive sets of 12-bit error spread coefficients, and outputs compressed error spread data 234A corresponding to these consecutive 12-bit error spread coefficients. These compressed error spread data can be in the form of multi-bit, such as 64-bit, compressed data blocks 234A. These blocks 234A of compressed 12-bit error spread coefficients are then stored in compressed spread buffer 236. Thus, in this third embodiment 130C, the compressor 234 is configured to compress a plurality of sets of at least three of said four error spread coefficients to thereby form compressed error spread data 234A.
When the error diffusion processor 120 needs to retrieve a set of previous line “receive” coefficients, the decompressor 238 selectively retrieves the appropriate compressed error spread coefficient block 236A from the compressed spread buffer 236, decompresses it, and provides the required information to the error diffusion processor 120 via output 242B.
Compression of consecutive 12-bit sets is possible because of their redundancy. This redundancy is due to a combination of two factors: (1) although the lookup table 232 stores one error spread vector for each gray level value, these vectors are not unique—as few as only 16 or so different vectors may need to be stored—thus, two gray level values that are close to each other typically will index entries comprising identical error spread vectors; and (2) in a line of an image, due to the relatively low spatial frequencies, it is not uncommon for runs of adjacent pixels' gray level values to be identical or very close to one another, and so these map onto the same error spread vector. The degree of compression depends on such factors as the spatial frequencies present in image, the number of different error spread vectors in the lookup table 232, the correlation between neighboring gray level values and the error spread vectors onto which they map, and the like.
In this embodiment, incoming 8-bit pixel data 106 first indexes 256×4 bit pointer lookup table 252. In response to a pixel value, the pointer lookup table 252 outputs a 4-bit error spread pointer 252A to left pixel coefficient array 264 and to pointer compressor 254.
The 4-bit pointer 252A selects one from among 16 possible (24) entries in the 16×4 bit left pixel coefficient array 264. In response to the 4-bit error spread pointer 252A, left pixel coefficient array 264 provides the error diffusion processor 120, via output 262A, the “receive left” coefficient 350D for use by the next pixel that is processed.
The 4-bit error spread pointer 252A is also supplied to a pointer compressor 254 which compresses a plurality of consecutive pointers to thereby from compressed error spread pointer information 254A. In one embodiment this compressed error spread pointer information 254A is formed as 64-bit compressed pointer blocks 254A which are then stored in a compressed pointer buffer 256.
When the error diffusion processor 130 needs the “receive back”, “receive above” and “receive forward” coefficients 350A, 350B, 350C, respectively, it sends appropriate control signals 121D to the pointer decompressor 258 within the error spread coefficient subsystem 130D.
Decompressor 258 then obtains the correct compressed pointer block(s) 256A from the compressed pointer buffer 256, decompresses the compressed error spread pointer information and thereby forms at least one decompressed 4-bit error spread pointer. The appropriate decompressed 4-bit error spread pointer(s) are then used to retrieve the needed coefficients. In this regard, it should be noted that in embodiments where the coefficient array 266 comprises the triplet of “send backward”, “send below” and “send forward” 4-bit error spread coefficients, more than one such 12-bit triplet may need to be retrieved, since the “receive backward”, “received above” and “receive forward” coefficients 350A, 350B, 350C, respectively, may belong to as many as 3 different entries within the 16 entry coefficient array 266.
The present invention has been described with respect to specific embodiments. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.
