EP0378780B1 - Images en demi-teintes avec propagation d'erreurs avec décalage de phase variant dans le temps - Google Patents

Images en demi-teintes avec propagation d'erreurs avec décalage de phase variant dans le temps Download PDF

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Publication number
EP0378780B1
EP0378780B1 EP19890121390 EP89121390A EP0378780B1 EP 0378780 B1 EP0378780 B1 EP 0378780B1 EP 19890121390 EP19890121390 EP 19890121390 EP 89121390 A EP89121390 A EP 89121390A EP 0378780 B1 EP0378780 B1 EP 0378780B1
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Prior art keywords
image
display
intensity
elements
value
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German (de)
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EP0378780A1 (fr
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Ronald I. Feigenblatt
Carl G. Powell
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International Business Machines Corp
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International Business Machines Corp
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    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/2007—Display of intermediate tones
    • G09G3/2059—Display of intermediate tones using error diffusion

Definitions

  • the present invention relates to multilevel display systems with display elements of relatively few intensity levels and, in particular, the adaptation of such systems for presentation of grey-scale images.
  • a flat panel display system such as a binary liquid crystal display (LCD), having elements capable of displaying relatively few intensity values, does not have many of the disadvantages found in CRT monitor type display systems. Specifically, a binary LCD does not require an electron gun or a vacuum tube, and can therefore be made much thinner than a TV monitor. An LCD has low power and voltage requirements and consequently, gives off relatively little heat during operation, which makes it particularly suited to high density and portable uses.
  • the hardware is relatively durable and can display the same image for a very long time without danger of damage to the elements.
  • a CRT display system can easily display relatively many intensity values between a minimum and maximum intensity value.
  • the input to a CRT display system is typically an analogue intensity signal which is effectively quantized due to the noise associated with the signal, which limits finer differentiation of the input signal.
  • the advantage of this finely quantized intensity capability is two-fold: First, the display gives a relatively accurate portrayal of the tone of the image. For example, the typical signal to noise ratios in video signals "limits" the elements on an analogue T.V. to be displayed at one of 256 values; nonetheless, this is a relatively finely quantized image, and the displayed value of each element is therefore a relatively close approximation of the part of the image being displayed.
  • the display has good "grey-toning", or "halftoning".
  • the display has relatively good spatial resolution. Since the elements follow the intensity of the image well, changes in the intensity between adjacent pixels are also well represented. Therefore, the spatial resolution of the display is limited primarily by the physical separation of the elements.
  • a binary LCD element represents the corresponding portion of the image by being either on or off. This is a poor representation if the image is grey at the part being represented. In other words, an LCD has inherently poor halftoning capability. Also, again related to the poor halftoning, the LCD display has relatively poor spatial resolution. Since the elements are either on or off, the shading of one image into another must be approximated by adjacent elements being either completely on or off. As a result, the shading will either be too abrupt or too spatially diffuse. In the latter case, the displayed image has resolution worse than the spatial separation of adjacent elements.
  • the prior art has considered the task of processing techniques for displaying an input signal adapted for systems with display elements of finely quantized intensity levels on a display with elements of relatively coarsely quantized intensity levels.
  • the fundamental problem addressed by these prior art methods is that the number of intensity values an image element may take is greater than the number a display element may take. Therefore, the processing transforms a relatively finely quantized image element intensity value into one of a fewer number of display element intensity values. By making such a transformation, in most cases, the display element intensity value will differ from the image element intensity value.
  • each image element intensity may take one of 256 values between a minimum and maximum intensity value, while the corresponding display element is either the minimum or maximum value.
  • displaying the display element at the maximum or minimum value is to some degree an error. If the image element has intensity corresponding to the 128th value, i.e., is halfway between the minimum and maximum intensity value, displaying the display element with the minimum or maximum intensity is a relatively large error.
  • displaying the display element with minimum intensity is a very small error.
  • U.S. Patent 3,937,878 to Judice describes one method ("dither") applied to black and white imaging in a binary display system; the image to be reproduced is divided into a matrix of picture elements, each element corresponding to a respective cell of the display panel.
  • a predetermined threshold value is assigned to each display cell.
  • the threshold values repeat in a pattern, typically over 16 square (4x4) elements, and are evenly spaced between a minimum and maximum image intensity value. If the intensity of any given picture element is greater than the threshold value assigned to the corresponding display cell, that cell is turned on, otherwise, it is maintained off.
  • the starting point again is a spatially quantized image.
  • the amount by which the display element exceeds or falls short of the corresponding image intensity value is not simply discarded, as in straight threshold processing, but is added to or subtracted from geometrically nearby image values which are to be quantized into display elements later.
  • halftoning is achieved by adjusting nearby elements to compensate for the excess or deficit in intensity of a given element.
  • the present invention takes error propagation one step further, applying the concepts described above to mosaic color displays.
  • a mosaic color display the element immediately to the right of a given element is not necessarily the same color as the given element; therefore error cannot always be propagated to the next element processed physically.
  • the closest unprocessed element of the same color is, for a mosaic of isochromatic diagonals, for example, the element beneath the element directly to right of the given element.
  • error propagation method applied to such mosaic color error is diffused diagonally among elements.
  • the method is more complex to implement on a hardware level when the elements are processed horizontally corresponding to the standard raster order of video data. For such a case, the error of one element must be stored and retrieved when the diagonally adjacent element is processed.
  • the present invention provides a processing method and system whereby color images with elemental regions of relatively finely quantized intensity values are displayed on an LCD-type mosaic color display with display elements of relatively few intensity values.
  • the inventive method of displaying an image includes the basic error propagation method for mosaic color displays, described above. As described, that method propagates error between elements diagonally for the case where the mosaic color display is patterned such that diagonal rows consist of monochromatic elements. However, the basic method assumes the "error" propagated to the first element in the diagonal to be zero, or, more generally, a temporally stationary, position-independent constant. By an additional feature of the method of the present invention, called “pel interleaving", an "error" is propagated into the first element in the diagonal row that changes with each new image or frame processed.
  • the "error” propagated into the first element in the diagonal row is also called the "preload value”. More specifically, the "error” propagated into the first element of each diagonal increases incrementally with each frame processed until the "error” exceeds the maximum element intensity value. At that point, the error propagation starts cover by subtracting the maximum value.
  • the present invention also achieves color halftoning in the displayed image which is subjectively a high quality representation of the image.
  • the present invention also uses the rapid succession of displayed images to achieve subjective high quality color halftoning in the display.
  • the present invention improves on the prior art by applying error propagation to mosaic color displays and systematically varying the error associated with the first element of each diagonal row, along which error is propagated in a mosaic display with isochromatic diagonals.
  • the systematic variation of the "error" propagated into the first element in the diagonal, or, equivalently, the error preload value leads to the pel interleaving, or spatial drift of "on" elements along the diagonal in the binary case, wherein the time integration of the display images for a succession of images or frames approaches the actual contone of the images inputted.
  • the present invention is not limited to mosaics with a pattern of isochromatic diagonals, although the following description focuses on the diagonal mosaic case.
  • the present invention has two facts: (1) halftoning for mosaic color displays using error propagation, and (2) halftoning using pel interleaving.
  • halftoning for mosaic color displays using error propagation and (2) halftoning using pel interleaving.
  • certain preliminary concepts applying to error propagation must be described in some detail.
  • any method of error propagation can be represented by the flow diagram of Figure 1.
  • a portion of an image 2 is shown, and divided into an image matrix 4, with matrix elements 6, of discrete intensity values.
  • the matrix indices, l and p maintain the spatial relationship of the image matrix to the image.
  • the value of the image matrix elements can be assigned from a spatial and amplitude quantized version of a CRT video signal, for example.
  • the intensity value of each input matrix element 6 is in proportion to the light intensity of the corresponding region of the image 2 represented.
  • the value of the intensity assigned to each image matrix element is discrete and finite, and the number of representative intensity values are relatively numerous with respect to the number of possible display intensity values, further discussed below.
  • the number of discrete intensity values which each image element may take is defined as q.
  • the image matrix intensity values are then transformed by a processor 8 into a display matrix 10 each display matrix element 12 having an intensity.
  • Each display matrix element 12 has a spatially corresponding image matrix element 6; therefore each display element intensity value is mapped one to one from the intensity of the corresponding image matrix element 6, plus an error value, described below.
  • the number of intensity values each display element can handle is discrete and quantized and is referred to as r.
  • the present invention applies to the situation where the number of intensity values the display elements may handle is less than the number of intensity values the image elements may handle; equivalently, the value of r is less than q. Therefore the processing transforms a relatively finely quantized image element intensity value into one of a fewer number of display element intensity values.
  • the image element intensity may be one of 256 intensity values while the corresponding display element is one of two possible values (on or off).
  • FIG 2 a schematic of one of the horizontal rows, l, of the image and display elements of Figure 1 are shown.
  • the arrows show that the mapping of the present invention is one to one between corresponding matrix elements.
  • the processor 8 of Figure 1 processes in real time. Such real time processing by processor 8 is not a necessary requirement for the term "error" propagation to apply to the display method, but such a feature is a preferred embodiment. That is, the processor maps an image element intensity into a display element intensity before the next image element intensity value arrives for input at the processor. If the processor mapped at a rate slower than the input rate of image element intensity values, the processor would have to store the backlogged values prior to processing, known to the art as "frame buffering".
  • the image intensity value of the first element in row l is mapped onto its corresponding display element before the image intensity value of the second element arrives at the processor.
  • the display image corresponds in time to the original image, inputted into the processor as a time sequence of image intensity values.
  • a "frame” represents an “image” and consists of image elements with intensity values.
  • a "new" frame may consist of the identical input signals of the prior frame, as when a display periodically refreshes for a constant image.
  • the pixel pattern of a mosaic color display with isochromatic diagonals is shown.
  • the R corresponds to red pixels, G, green pixels and B, blue pixels.
  • the colors correspond to the primary colors.
  • An analogue input signal or one finely amplitude quantized due to its inherent signal to noise ratio limitation, typically is continuous in time and consists of overlapping signals for each of the three primary color intensities representing the particular region of the analogue image.
  • These three input intensity values are of course discretely but finely quantized by the inherent signal to noise ratio limitation, discussed above.
  • the mapping of color image intensity values onto display intensity values is therefore not as straightforward as in the monochromatic display case.
  • the present invention contemplates two techniques for treatment of an image intensity value consisting of three primary intensity values to be mapped onto a display intensity value of one primary color.
  • the first method effectively disregards the image intensity values of the two primaries which do not correspond to the display element primary. This method is advantageous when spatial resolution is the primary concern.
  • the second method effectively groups temporally adjacent image intensity values in groups of three, averages the image intensity value for each primary color over the three intervals, and maps the averaged image intensity value for each primary color to the display element corresponding spatially to one of the image intensity values with the appropriate primary color. This method is advantageous when accurate contone is more important than spatial resolution.
  • the present invention incorporates both treatments of the input image signal. Since the invention applies in general, it is most convenient for the ensuing description to assume that the input signal has been previously processed according to one of the two methods described above so that a finely quantized image intensity value of color corresponding to the spatially corresponding display element primary color is to be mapped to that display element. With this in mind, the same mapping concepts described above with respect to Figures 1 and 2 can be applied to map a color image represented by a matrix of image elements with color intensities I(m,n) into a matrix of display elements with color intensities D(m,n), where I(m,n) must now be thought of as the resulting image intensity value of a single primary color, determined according to one of the two methods described above.
  • Figure 4 shows an image matrix which has matrix elements referenced to the monochromatic pixel diagonals of the mosaic color display of Figure 3. Since error is diffused diagonally between pixels in the method of error propagation for mosaic color display that the present invention improves upon, it is convenient to reference the matrix of image and display elements diagonally. Because of this, the elements in the "matrix" of Figure 4 do not lie in orthogonal rows and columns as in standard matrices. Furthermore, each diagonal does not necessarily have the same number of elements as other diagonals. As a result, it is most appropriate to refer to Figure 4 and related figures as m diagonals with each diagonal's elements numbered from 1 to n total and referenced as n.
  • each diagonal is numbered and is the first index of the matrix
  • each element from the "top” of the diagonal to the "bottom” is numbered and is the second index of the matrix.
  • a portion of the image elements are shown referenced as i(diagonal number, position in diagonal), or i(m,n).
  • the second element in the 98th diagonal is referenced i(98,2).
  • each diagonal does not have the same number of elements; for example, row 1 has only one element, while row 5 has 5 elements.
  • the maximum number of elements in a row, n total therefore, is a function of m.
  • each image element i(m,n) will have an intensity value I(m,n) associated with it.
  • each display element d(m,n) will have a display intensity value, referenced D(m,n), associated with it.
  • the physical embodiments typically process the elements in horizontal rows. Since error is propagated diagonally in the method of the present invention where the mosaic has isochromatic diagonals, the processor must be able to store the error of one element while intervening elements on the horizontal rows are processed, and retrieve the error when the diagonally adjacent element arrives for processing.
  • the mth diagonal row of a mosaic color image matrix and display matrix are illustrated horizontally.
  • the lines between image and display elements are representative of the one to one correspondence between the image and display matrix elements.
  • An image element, i(m,n), has an intensity value, I(m,n), which is one of q possible values.
  • the display element d(m,n), has an intensity value, D(m,n), which can be one of r values.
  • the possible number of image intensity values I(m,n) may take is greater than the number of display intensity values D(m,n) may take.
  • the r display intensity values are defined as A1, A2,...Ar; thus D(m,n) can equal A1, A2,..., or Ar.
  • Figure 6 illustrates the relative relationship between the q possible intensity values which I(m,n) may take, the r possible display values D(m,n) which range from A1 to Ar, and the r-1 threshold levels T1, T2,..., T(r-1).
  • the intensity values I(m,n) are normalized to range from 0 to 1. It is seen that the q possible image intensity values each image element may take are more numerous than the r possible display intensity values D(m,n) each display element may take. In other words, the image elements have relatively finely quantized intensity values with respect to the display elements.
  • Figure 8 is an enlarged view of the possible intensity values near the xth possible image intensity value.
  • the xth and (x+1)th possible image intensity values lie between threshold Ty and T(y+1); therefore if image element intensity value I(m,n) was at one of these intensity values, display element intensity value D(m,n) would have a mapped intensity value A(y+1), according to the simple algorithm described above.
  • the(x-1)th possible image intensity value lies between Ty and T(y-1); therefore, according to the simple mapping algorithm, an image element at intensity level x-1 would have a displayed intensity value of Ay.
  • Error propagation generally refers to the adjustment of the value of neighboring display element intensities due to the over or under representation of the image element intensity, I(m,n), by the display element intensity, D(m,n).
  • the over or under representation resulting from a mapping is the "error" propagated to the next image element.
  • the nth element of the mth row is generally notated as (m,n) and is used interchangeably with i(m,n).
  • the error propagated into the (m,n)th element of the matrix is E(m,n).
  • the amount of over or under representation resulting from the mapping of the image element intensity I(m,n-1) onto D(m,n-1) is subtracted or added, respectively, from the image element intensity I(m,n) before I(m,n) is mapped, using the mapping described above, onto display element intensity D(m,n).
  • the method maps the sum of the image element intensity I(m,2) and propagated error E(m,2) onto the display element intensity D(m,2).
  • the excess or deficiency of the displayed intensity of the first element is subtracted or added to the second image element intensity value before it is mapped onto the second display element intensity value.
  • the image element intensity value I(m,2) plus the propagated error E(m,2) which is mapped onto the display element intensity value is defined as the "adjusted image element intensity value" of the second image element in the mth row. It is the excess or deficit of the displayed intensity with respect to the adjusted intensity value which is propagated to the next element.
  • E(m,3) [I(m,2) + E(m,2)] - D(m,2) .
  • the adjusted image element intensity value of the 3rd element, I(m,3) + E(m,3) is mapped onto the corresponding display element intensity value, D(m,3).
  • the adjusted image element intensity value I(m,n) + E(m,n) is mapped onto the corresponding display element intensity value D(m,n).
  • the propagated error value E(m,n) [I(m,n-1) + E(m,n-1)] - D(m,n-1) .
  • the present invention further provides a system in which the error value E(m,1) is arbitrarily chosen between 0 and the maximum value the image intensity value may attain.
  • the adjusted intensity value of the first element, 1(m,1) + E(m,1) is mapped onto the corresponding display element D(m,1).
  • the error preload value E(m,1) changes with each frame processed, whether the image is invariant or changes between frames. More specifically the error preload value for each diagonal increases incrementally with each frame processed until it is equal to or exceeds the maximum element intensity value, in which case it is started anew by subtracting the maximum value.
  • the incremental increasing of the error associated with each diagonal's first element, in the binary display case leads to the spatial drift of "on" elements along the diagonals. If all preload values are equally likely, the time integrated ensemble of the displays approaches the exact contone image as the number of displayed images increases. Thus, if the processing is fast, so that the eye integrates a number of displayed images for the same input image, the display perceived using the present image approaches the actual contone of the input image.
  • the present invention can therefore be summarized as a method according to claim 1.
  • m references the groupings of elements for processing based on the particular mosaic pattern
  • n references the order of processing among elements in the mth group, respectively.
  • sequence of input signals need not necessarily correspond to the mapping sequence of elements; equivalently, the mapping need not occur in real time.
  • the input signals for one frame may be stored in a storage matrix and accessed in the particular method's order of processing.
  • the method of error diffusion for the isochromatic diagonal display need not necessarily propagate the error from one display element directly into the adjacent element in the diagonal.
  • a simple extension of the method described above would divide the error from one display element and diffuse it to a number of adjacent elements in the diagonal. Therefore, when processing the nth element in the mth row, the error value E(m,n) would equal the sum of a certain percentage of the error from a number of prior elements in the mth row. For example, the value of E(m,n) may equal 1 ⁇ 2 of the sum of the adjusted intensity of the prior two elements in the row minus their corresponding display elements.
  • E(m,n) would equal 1 ⁇ 2 [[I(m,n-1) + E(m,n-1)] - D(m,n-1)] + 1 ⁇ 2 [[I(m,n-2) + E(m,n-2)] - D(m,n-2)] .
  • the error E(m,n) could be diffused to a number of nearby elements, not necessarily in the mth diagonal. Therefore, when processing the nth element in the mth diagonal, the error value E(m,n) would equal the sum of a certain percentage of error from a number of nearby prior elements in the mth, (m+3)th, (m+6)th, etc., diagonal since those diagonals have the same color as the (m,n)th element for the display with monochromatic diagonals.
  • E(m,n) may equal 1 ⁇ 2 [[I(m,n-1) + E(m,n-1)] - D(m,n-1)] + 1 ⁇ 2 [[I(m+3,n+2) + E(m+3, n+2)] - D(m+3,n+2)] , where the element (m+3, n+2) is a next nearest element previously processed of the same color as the (m,n)th element.
  • Figure 9 relates the diagonally referenced rows of the matrix model to the horizontal processing of a typical physical embodiment.
  • the diagonal row, m, of Figure 9 corresponds to one of the monochromatic diagonals of a mosaic color display along which error is propagated in the present invention.
  • the first image element in the mth diagonal row is the first element physically processed in the horizontal row in which it lies. Since E(m,1) is pre-selected as described above, I(m,1) + E(m,1) is processed onto the corresponding display element D(m,1), not shown in Figure 9, in the physical process.
  • the physical process maps the image element in the horizontally adjacent position to the (m+1,1)th element, since the image and display electronic signals typically correspond to a standard raster display, as noted above.
  • the physical processing continues until the last image element on the horizontal row is processed, and then begins to process the next horizontal row, beginning with image element (m-1,1). Only then is image element (m,2), the second element in the mth diagonal row, physically processed.
  • the line buffer accomplishes this in a manner functionally analogous to a FIFO shift register of size equal to the number of elements in a horizontal row.
  • the resulting value E(m,2) of processed element 21 is loaded into a buffer 20 before beginning the processing of the horizontally adjacent element 22.
  • the value of E(m,2) moves toward the output 30 of the buffer 20, as data corresponding to the error of the intervening elements 23-28 is inputted and withdrawn from the buffer 20.
  • the value E(m,2) is at the output 30 of the buffer 20 and can be accessed for processing.
  • preload buffer 52 is accessed, and E(l,p) is retrieved. If the element under consideration is not at the top of a diagonal, ie., l ⁇ 1 and p ⁇ 1, then the appropriate error value, E(l,p), is retrieved from error storage means 48. However E(l,p) is obtained, it is added to I(l,p) at processing means 50. Threshold determination means 46 is accessed with the sum I(l,p) + E(l,p), which determines which of the r-1 thresholds, described above, the sum I(l,p) + E(l,p) lies between. The appropriate threshold value is used by processing means 50, which outputs a value of display intensity D(l,p), at the output means 44 according to the threshold value.
  • the error storage means can be a line buffer of size equal to the number of vertical columns of image elements.
  • the above device can be applied to a device which propagates error diagonally in general, and is not limited to the specific mapping described above.
  • the device will work for any determination of D(l,p) based on I(l,p) and E(l,p) and need not necessarily use the thresholding method of the present invention. Further, it will work for any value of E(l+1, p+1) determined from I(l,p), E(l,p) and D(l,p).
  • the particular embodiment comprises a device according to claim 13.
  • the output means of that device would be connected to a display with a color mosaic pattern, each element capable of taking on the intensity values A1, A2,... Ar.
  • An alternative embodiment of the device would propagate error to more than one adjacent element on the diagonal or other diagonals.
  • This device would comprise means as claimed in claim 14.
  • the partial error storage means could be a number of line buffers.
  • the device would receive input signals in a sequence, not necessarily a standard raster sequence, and map them according to a different sequence. The device would then require a storage matrix 54 between input means 42 and processing means 50, as shown in Figure 10A. Such a device would therefore comprise:
  • the above device can be applied to mosaic color patterns other than those with isochromatic diagonals, as long as the elements are referenced with matrix notation (l,p), where l represents groupings of elements for processing based on the particular mosaic pattern of the display, and p references the order of processing among elements in the group, respectively.
  • l,p matrix notation
  • references corresponds to the diagonals of the mosaic with isochromatic diagonals as before.
  • m is one of m total monochromatic diagonals of a mosaic color display
  • n is the nth element from the top of the diagonal.
  • the mosaic with isochromatic diagonals is focused on for exemplary purposes, the invention applying to mosaic color patterns in general.
  • E(m,1) for each diagonal row corresponds to an image element which lies on the physical border of the image. There is no error to be propagated from a prior element since element (m,1) begins a diagonal row. Therefore, as described above, E(m,1) is selected for each of the m diagonal rows. The value can be the same or different for all m diagonal rows. It can also be changed between images.
  • the consequence of changing the error value for the first element of the mth row E(m,1) for successive frames processed by the present invention is that changes result in any or all of D(m,n) for all m.
  • the "row" is drawn horizontally, but may correspond to a diagonal row m of a mosaic color display as in prior Figures, or to any mosaic pattern in general.
  • the two particular frames are identical, or, at least, the mth row for the two successive frames are identical.
  • E(m,1) alternates between two values between the minimum and maximum image intensity values for successive frames.
  • the alternating of the nth display element intensity value D(m,n) between two intensity values for successive identical frames is perceived as an average of the two intensities if the alternating is fast enough. If the threshold level is approximately halfway between two possible display intensity values, then an image element with intensity value I(m,n) near the threshold value is not well represented by either adjacent display intensity value.
  • the alternating between the two adjacent display intensity values which is more probable in the present method, gives a perceived intensity approximately equal to the threshold level, or the image element intensity value.
  • a variation in E(m,1) is less likely to result in a change of display intensity between successive frames. This is also a good result, since if I(m,n) is not near a threshold value, it is relatively close to a possible display level of intensity, Ax, and is well represented by the mapping onto Ax.
  • processing speed is not at the state where such a large number of frames may be processed at a rate undetected by the eye.
  • E(m,1) changed between 1000 values for the same image at today's processing speeds, the eye would sense the resulting changes in the intensity of the individual display elements along the row m, rather than perceiving the average of the intensities.
  • the particular embodiments described below are attempts to accommodate these two conflicting requirements: changing E(m,1) for successive frames to achieve a more accurate display of intensity over time, while not changing it so much that changes in the displayed image elements are sensed over time for the same input image.
  • the error value for the first element in the mth row E(m,1) for each m are initially uncorrelated to the value for any other m.
  • Each E(m,1) has an initial value between 0 and the maximum image intensity value.
  • E(m,1) for each m increases incrementally for each frame. When E(m,1) is equal to or exceeds the maximum image intensity value, that maximum is subtracted, and the process continues.
  • A1 and A2 are normalized to be 0 and 1 respectively, and T1 is chosen to be 1 ⁇ 2.
  • the image intensity values correspond to those of a TV input and can take one of 256 values between normalized intensities of 0 and 1.
  • E(m,1) for each m is initially chosen arbitrarily to be either 0 or .5.
  • E(m,1) for each m alternates between 0 and .5. This leads to qualitatively good half toning and is therefore a preferred embodiment.
  • the result of the present invention would be an even temporal and spatial shifting of "on" pixels along the mth row, with the eye perceiving the spatial and temporal average of the "on” elements, rather than sparse, stationary "on” elements on a black background, the result of the time constant preload case.
  • E(m,1) for each m is initially chosen randomly between 0 and 1.
  • E(m,1) for each m alternates between its initial value and either (i) a value 1 ⁇ 2 less than the initial value if the initial value is greater than or equal to 1 ⁇ 2, or (ii) a value 1 ⁇ 2 greater than the initial value if the initial value is less than 1 ⁇ 2.
  • One further feature of the present invention is a method of artifact suppression, unique to the present invention.
  • the present invention eliminates these artifacts by maintaining a record or counter variable, C, of when the last display element in the row was displayed with an intensity greater than A1. If (1) the considered display element is to be mapped with an intensity A2 using the nominal processing algorithm of the present invention and, (2) the record shows that more than a preselect number, N, of display elements of intensity value A1 have been processed since the last element with an intensity value greater than A1, then the display element is mapped with an intensity value A1 instead of A2. If both conditions are not met, then the considered display element is mapped with an intensity A2, the result of the nominal processing algorithm of the present invention
  • the display intensity values are equally spaced and the threshold values are spaced equidistantly between neighboring intensity values.
  • the method of error propagation by pel interleaving could be applied to black and white displays. Then it would not be necessary to propagate diagonally, since horizontally adjacent elements are black and white. Error could be propagated horizontally, and the elements could be referenced so that m corresponds to horizontal rows and n corresponds to vertical columns. Therefore, all i(m,1), ie., the elements for which the error is preselected, would make up the first vertical column of the image.

Claims (19)

  1. Procédé de propagation d'erreur pour le traitement d'éléments d'image et l'affichage d'une image, comprenant les étapes de:
    (a) fournir une image constituée d'une pluralité d'éléments d'image i(m,n), m comprenant les nombres entiers 1 à mtotal, n comprenant les nombres entiers 1 à ntotal, ntotal étant fonction de m, chaque élément d'image i(m,n) ayant une intensité I(m,n) égale à au moins une des q valeurs d'intensité d'image, où q est au moins égal à trois, chaque paire de nombres (m,n) indiquant la position d'un élément d'image sur le plan d'affichage, les nombres adjacents m ou n correspondant aux positions adjacentes.
    (b) fournir un affichage comprenant une pluralité d'éléments d'affichage d(m,n), chacun ayant une position correspondant à la position de l'élément d'image i(m,n), et chaque élément d'affichage d(m,n) étant capable d'émettre de la lumière avec une intensité D(m,n) égale à une des r valeurs d'intensité d'affichage ordonnées en amplitude A1, A2, ...Ar, où r est un nombre entier inférieur à q, et Ax est la xième valeur d'intensité d'affichage;
    (c) définir r-1 valeurs de seuil, (T1, T2, ... T(r-1)), où Tx est la xième valeur de seuil;
    (d) définir une valeur d'erreur E(m,n) pour chaque m et n, où E(m,n) = I(m,n-1) + E(m,n-1) - D(m,n-1)
    Figure imgb0094
    Figure imgb0095
     et où E(m,1), m = 1 à mtotal sont pré-établies pour chaque traitement d'image, ne dépendant pas des positions d'affichage adjacentes, et où E(m,1) m=1 à mtotal changent avec une cadence égale à celle de la trame.
    (e) fournir de nombreuses images temporairement successives à une cadence de trame constante, chaque image étant traitée et affichée une fois pendant une période de trame, ce qui fait que l'image reçue est formée par une série d'images affichées temporairement successives, et
       pour m = 1 à mtotal et, pour chaque m, pour n = 1 à ntotal, afficher l'élément d'affichage d(m,n) avec une intensité.
    (i) D(m,n) = A1
    Figure imgb0096
     , si I(m,n) + E(m,n) < T1
    Figure imgb0097
     ,
    (ii) D(m,n) = Ar
    Figure imgb0098
     , si I(m,n) + E(m,n) ≧ T(r-1)
    Figure imgb0099
     ou
    (iii) pour r> 2 et T1 ≦ I(m,n) + E(m,n) < T(r-1)
    Figure imgb0100
     , D(m,n) = Ax
    Figure imgb0101
     ,
       où x est la valeur entre 2 et r-1 qui satisfait la condition

    Tx-1 ≦ I(m,n) + E(m,n) < Tx.
    Figure imgb0102
  2. Procédé selon la revendication, dans lequel l'étape d) est remplacée par l'étape suivante:
    d) définir une valeur d'erreur E(m,n) pour chaque m et n, où
    Figure imgb0103
     où
    (i) m' se range parmi les références des mtotal groupements d'éléments pour le traitement, tandis que j se range parmi les référence de chaque élément dans le m'-groupe d'éléments.
    (ii) K(m,m',n,j) est un coefficient de propagation pour la propagation d'erreur depuis i(m',j) jusqu'à i(m,n)
    (iii) E(m,1), m = 1 à mtotal, sont pré-établies pour chaque traitement d'image, ne dépendant pas des positions d'affichage adjacentes, et E(m,1), m = 1 à mtotal changent avec une cadence égale à celle de la trame.
  3. Procédé selon las revendications 1 ou 2, pour la suppression d'artefacts provoqués par des éléments d'image successifs qui ont des niveaux d'intensité proches de la plus petite valeur d'intensité d'affichage A1, comprenant les étapes supplémentaires de:
       définir une variable d'enregistrement C, et une constante de suppression N;
       pour chaque m et n considérés,
    (i) si n = 1, établir C = N
    Figure imgb0104
     ,
    (ii) si C<0 et d(m,n) = A2
    Figure imgb0105
     , afficher la valeur d'intensité d'élément d'image D(m,n) avec une valeur d'intensité A1,
    (iii)
    A) si I(m,n) + E(m,n) ≧ T1
    Figure imgb0106
     , établir C = N
    Figure imgb0107
    B) si I(m,n) + E(m,n) < T1
    Figure imgb0108
     , établir C = C-1.
  4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel l'image est fournie par le signal d'entrée sur un écran vidéo CRT, le signal d'entrée comprenant trois signaux analogiques qui se chevauchent correspondant à chaque intensité de couleur primaire de l'image, et l'affichage est fourni par un affichage avec des éléments de trois couleurs agencés en une configuration en couleur en mosaïque, et dans lequel I(m,n) est la valeur d'intensité d'élément d'image discrète de même couleur de l'élément d'affichage correspondant, I(m,n) étant déterminée à partir de signaux analogiques pour la région spatiale de l'image correspondant à l'élément d'image d(m,n), la région spatiale de l'image étant l'élément d'image i(m,n).
  5. Procédé selon la revendication 4, dans lequel l'affichage est un affichage en couleur en mosaïque avec des diagonales isochromatiques, et les éléments d'affichage d(m,n) correspondent aux ntotal éléments dans chacune des mtotal diagonales isochromatiques de l'affichage, ce qui fait que des éléments d'images i(m,n) sont également référencés diagonalement sur l'image pour correspondre spatialement aux éléments d'affichage d(m,n), dans lequel l'image est une parmi de nombreuses images temporairement successives fournies à une cadence de trame constante, l'image est affichée une fois par période de trame, ce qui fait qu'il est affiché une série d'images temporairement successives, éventuellement identiques qui changent à la cadence de la trame.
  6. Procédé selon la revendication 5, dans lequel E(m,1) sont équivalentes pour tous les m dans une image quelconque distincte parmi lesdites images successives.
  7. Procédé selon la revendication 6, dans lequel r = 2, A1 = 0, T1 = A2 2 
    Figure imgb0109
     , et la valeur de E(m,1) pour tous les m alterne à la cadence de la trame entre 0 et T1, ce qui fait que quelques uns des éléments du réseau d'affichage D(m,n), ou tous, ont des valeurs d'intensité d'affichage différentes pour des images successives quoique identiques.
  8. Procédé selon la revendication 5, dans lequel la valeur de E(m,1) pour chaque m n'est pas en corrélation avec E(m,1) pour tous les autres m dans une image quelconque distincte parmi lesdites images successives fournies à la cadence de trame constante E(m,1) pour tous les m se trouvant entre A1 et Ar.
  9. Procédé selon la revendication 8, dans lequel r = 2; A1 = 0, T1 = A2 2 
    Figure imgb0110
     et les valeurs de E(m,1) pour chaque m alternent à la cadence de la trame entre deux valeurs dans la gamme de 0 et A2 séparée par A2 2 
    Figure imgb0111
    .
  10. Procédé selon la revendication 5, dans lequel E(m,1) pour tous les m alterne entre deux valeurs différentes pour chaque image successive.
  11. Procédé selon la revendication 10, dans lequel r= 2 et E(m,1) pour tous les m alternent entre 0 et T1.
  12. Procédé selon l'une quelconque des revendications 1 à 11, dans lequel les valeurs d'intensité d'affichage (A1, A2,...Ar) sont à égale distance et les valeurs de seuil (T1, T2,...T(r-1) sont espacée à équidistance entre des valeurs d'intensité voisines.
  13. Dispositif pour l'affichage d'une image et le traitement d'éléments d'image conformément au procédé de la revendication 1, comprenant:
    (a) des moyens d'entrée (42) pour recevoir dans une séquence de trames classique de la gauche vers la droite de haut en bas une pluralité de signaux codés en intensité I(l,p) correspondant à une pluralité d'éléments d'image i(l,p) qui correspondent, chacun, à une position sur l'image, l comprenant les nombres entiers de 1 à ltotal et correspondant de haut en bas aux ltotal lignes horizontales d'un balayage de trame, p comprenant les nombres entiers 1 à ptotal et correspondant de la gauche vers la droite aux ptotal lignes verticales d'un balayage de trame, où chaque signal codé en intensité I(l,p) correspond à un moins une de q valeurs d'intensité d'image, où q est au moins égal à trois;
    (b) des moyens de sortie (44) pour sortir séquentiellement une pluralité de signaux codés en intensité D(l,p) correspondant à une pluralité d'éléments d'affichage d(l,p), chaque élément d'affichage d(l,p) correspondant à la position d'un élément d'image i(l,p), et chaque signal codé en intensité d'affichage D(l,p) correspondant à une de r valeurs d'intensité d'affichage ordonnées en amplitude A1, A2, ..., Ar, où r est un nombre entier inférieur à q, et Ax est la xième valeur d'intensité d'affichage;
    (c) des moyens d'emmagasinage d'erreur (48) pour emmagasiner une valeur d'erreur E(l,p) correspondant à un signal codé en intensité à l'entrée;
    (d) un tampon de préchargement (52) pour maintenir une valeur d'erreur présélectionnée E(l,p) correspondant à un nombre de signaux codés en intensité à l'entrée;
    (e) des moyens de traitement (50) pour mapper le signal codé en intensité à l'entrée I(l,p) sur le signal codé en intensité à la sortie D(l,p) en
    (1) récupérant la valeur I(l,p) dans les moyens d'entrée;
    (2) si l=1 ou p=1, obtenant la valeur E(l,p) dans le tampon de préchargement;
    (3) si l≠1 et p≠1, obtenant la valeur E(l,p) dans les moyens d'emmagasinage d'erreur;
    (4) déterminant la valeur D(l,p) basée sur les valeurs de I(l,p) et E(l,p);
    (5) envoyant la valeur D(l,p) sur les moyens de sortie;
    (6) calculant la valeur E(l+1, p+1) en prenant comme base I(l,p), D(l,p) et E(l,p);
    (7) emmagasinant E(l+1, p+1) dans lesdits moyens d'emmagasinage d'erreur.
  14. Dispositif selon la revendication 13 pour l'affichage d'une image et le traitement d'éléments d'image conformément au procédé de la revendication 2, dans lequel les caractéristiques (c), (e3), (e6) et (e7) de la revendication 13 sont remplacées par les caractéristiques suivantes:
    (c) des moyens d'emmagasinage d'erreur partielle (48) pour emmagasiner des valeurs d'erreur partielles PE(l,1',p,p') = K(l, l',p,p') x X ([I(l',p') + D(l',p') - E(l',p')]
    Figure imgb0112
     correspondant à un signal codé en intensité I(l,p) à l'entrée provenant des éléments (l',p');
    (e)
    (3) si l≠1 et p≠1, en obtenant les valeurs PE(l,l',p,p') dans les moyens d'emmagasinage d'erreur partielle(48) et en additionnant les valeurs de PE(l,l',p,p') pour obtenir E(l,p);
    (6) calculant les valeurs partielles PE(a,l,b,p), où (a,b) sont les éléments sur lesquels est propagée l'erreur à partir de (l,p);
    (7) emmagasinant lesdites valeurs partielles PE(a,l,b,p) dans lesdits moyens d'emmagasinage d'erreur partielle (48), pour tous les (a,b).
  15. Dispositif selon les revendications 13 ou 14, dans lequel ladite image est fournie par le signal d'entrée sur un écran vidéo CRT, le signal d'entrée comprenant trois signaux analogiques qui se chevauchent correspondant à chaque intensité de couleur primaire de l'image, et l'affichage est fourni par un affichage avec des éléments de trois couleurs agencés en une configuration en mosaïque en couleur, et dans lequel I(l,p) est la valeur d'intensité d'élément d'image discrète de même couleur de l'élément d'affichage correspondant I(l,p) déterminée à partir des signaux analogiques de la région spatiale de l'image correspondant à l'élément d'affichage d(l,p), la région spatiale de l'image étant l'élément d'image i(l,p).
  16. Dispositif selon la revendication 15, dans lequel l'affichage est un affichage en mosaïque en couleur avec des diagonales isochromatiques, et les éléments d'affichage d(l,p) correspondent aux ptotal éléments dans chacune des ltotal diagonales isochromatiques de l'affichage, ptotal étant fonction de l, ce qui fait que les éléments d'image i(l,p) sont également référencés diagonalement sur l'image pour correspondre spatialement aux éléments d'image d(l,p).
  17. Dispositif selon l'une quelconque des revendications 13 à 16, dans lequel le processeur comprend en outre des moyens de détermination de seuil (46) pour déterminer si un signal codé en intensité correspond à une intensité qui est supérieure à l'une quelconque des r-1 valeurs de seuil T1, T2, ...T(r-1), ou à toutes, où Tx est la xième valeur de seuil.
  18. Dispositif selon la revendication 17, dans lequel les moyens de traitement (50) additionnent les valeurs E(l,p) et I(l,p), ont accès aux moyens de détermination de seuil (46), et déterminent les valeurs D(l,p) comme étant:
    (i) D(l,p) = A1
    Figure imgb0113
     , si les moyens de détermination de seuil (46) déterminent I(l,p) + E(l,p) < T1
    Figure imgb0114
     ,
    (ii) D(l,p) = Ar
    Figure imgb0115
     , si les moyens de détermination de seuil (46) déterminent I(l,p) + E(l,p) -T(r-1)
    Figure imgb0116
     , ou,
    (iii) pour r>2 et T1 ≦ I(l,p) + E(l,p) < T(r-1)
    Figure imgb0117
     , D(l,p) = Ax
    Figure imgb0118
     , où x est la valeur entre 2 et r-1, déterminée par les moyens de détermination de seuil (46) qui satisfait la condition T(x-1) ≦ I(l,p) + E(l,p) < Tx
    Figure imgb0119
     .
  19. Dispositif selon l'une quelconque des revendications 13 à 18, pour la suppression d'artefacts soulevés par des éléments d'image consécutifs qui ont des niveaux d'intensité proches de la plus petite valeur d'intensité d'affichage A1, dans lequel sont définies une variable d'enregistrement C et une constante de suppression N et, pour chaque valeur d'intensité d'élément d'image entrée, les moyens de traitement (50) en outre:
    (i) établissent C=N
    Figure imgb0120
     si p = 1,
    (ii) si C<0 et D(l,p) = A2
    Figure imgb0121
     , envoient la valeur d'intensité A1 sur les moyens de sortie pour la valeur d'intensité d'élément d'affichage D(1,p),
    (iii) établissent C= N
    Figure imgb0122
     si I(l,p) + E(l,p) ≧ T1
    Figure imgb0123
     ,
    (iv) établissent C = C-1 si I(l,p) + E(l,p) < R1
    Figure imgb0124
     .
EP19890121390 1989-01-13 1989-11-18 Images en demi-teintes avec propagation d'erreurs avec décalage de phase variant dans le temps Expired - Lifetime EP0378780B1 (fr)

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US5585818A (en) * 1992-05-19 1996-12-17 Canon Kabushiki Kaisha Display control unit and display control method
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US4890167A (en) * 1986-10-17 1989-12-26 Matsushita Electric Industrial Co., Ltd. Apparatus for processing image signal
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DE69007690D1 (de) 1994-05-05
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EP0404933A1 (fr) 1991-01-02
JPH0792650B2 (ja) 1995-10-09

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