Error diffusion filter for use in a DMD display

Abstract: An improved error diffusion filter for a DMD display includes both an inverse gamma LUT (Look-up Table) (51) and an Error LUT (Look-up Table) (82), both responsive to the raster scan red, green, or blue video data for when the video data calls for an intensity level different from the achievable level of a DMD device. The Error LUT (82) provides an error value for that difference. The error is distributed to neighboring DMD devices.

Description: Technical Field of the Invention This invention relates to digital imaging and more particularly to an error diffusion filter for a digital micromirror device (DMD) display of the type defined in the precharacterizing portion of claim 1. Background of the Invention A new projection display that utilizes reflections from hundreds of thousands of micromirrors, each mounted above its own semiconductor memory cell is described in IEEE Spectrum, November 1993, vol. 30, no. 11, written by Jack M. Younse of Texas Instruments Incorporated. The digital micromirror device (DMD) comprises a special light modulator that was invented in 1987 by Larry J. Hornbeck, a Texas Instruments Incorporated scientist. The DMD, or digital micromirror device, covers each memory cell of a CMOS static RAM with a movable micromirror. Electrostatic forces based on the data in this cell tilt the mirror either plus or minus 10 degrees, modulating the light incident on the surface. The light reflected from any of the mirrors passes through a projection lens and creates an image on a large screen. Light from the remaining off mirrors is reflected away from the projection lens and trapped. The portion of the time during each video frame that the mirror remains in the on state determines the shades of grey- from black for zero on time to white for 100 percent on time. Color may be added in two ways, by a color wheel or a 3-DMD set up. Some DMD devices may have the capability to display only a low number of bits representing the on and off times and, therefore, the shades of grey or shades of color, leading to degradation of the video quality. Also, the use of digital degamma in the DMD display systems entails some loss of resolution (blockiness) in the low intensity regions. Finally, even the best of DMDs can have some defects (pixels stuck on, off, or flat). It is desirable to find some method to provide a correction for these display errors and to provide a more pleasing picture without significantly increasing the time for processing by increasing the number of bits for each on or off time. An error diffusion filter of the type defined in the precharacterizing portion of claim 1 is known from WO-A-9209064. Using that error diffusion filter, bits having a duration of less than the shortest displayable duration can be displayed by adjusting the turn-on time of a proportion of the adjacent mirrors. From US-A-5 289 172 a method of mitigating the effects of a bright defective electromechanical pixel which transmits light into an observer's field of view is known. That method comprises the step of applying a voltage sufficient to damage the electromechanical pixel, which has a beam and a hinge, in such a manner that the pixel no longer transmits light into the observer's field of view. Summary of the Invention According to one aspect of the present invention there is provided an error diffusion filter for a DMD display of the type defined in the precharacterizing portion of claim 1 and comprising the features of the characterizing portion of claim 1. According to a further aspect of the present invention there is provided a method of filtering a DMD display comprising the steps defined in claim 7. According to a still further aspect of the present invention there is provided method of correcting for defective pixel elements in a DMD display comprising the steps defined in claim 8. In accordance with the present invention the problem of low number of bits, blockiness due to degamma and DMD defects is alleviated by an improved diffusion filter. The improved filter includes both an inverse gamma look-up table and an error look-up table that is responsive to the video data for providing an error value that is distributed to neighboring pixel elements. Description of the Drawings Reference will now be made, by way of example, in which: Fig. 1 is an overall block diagram of a Digital Micromirror Display System; Fig. 2 is a sketch of a micromirror element in Fig. 1; Fig. 3 is a timing diagram illustrating the on-time of MSB and LSB; Fig. 4 is a plot of CRT response, gamma transmission sent, and inverse gamma; Fig. 4a is a plot in more detail of the inverse degamma of Fig. 4; Fig. 5 illustrates the stepped DMD response with LUT; Fig. 6 is a sketch of an error diffusion filter according to one embodiment of the present invention; and Fig. 7 illustrates the operation of the filter of Fig. 6. Description of Preferred Embodiments of the Present Invention Referring to Fig. 1, there is illustrated an overall block diagram of a digital micromirror device (DMD) display system. Referring to Fig. 1, an example of a DMD system 10 is illustrated, wherein the light from a light source 11 is applied through a first condenser lens 13 and through a color wheel 15, which is rotating at about sixty cycles, or hertz, or 60 frames per second. The light passing through the color wheel 15 passes through a second condenser lens 17 onto a DMD chip 19. The DMD chip includes an array of tiny mirror elements, or micromirrors, where each mirror element is hinged by a torsion hinge and support post above a memory cell of a CMOS static RAM as shown in Fig. 2. The movable micromirror tilts into the on or off states by electrostatic forces based on data in the cell. The tilt of the mirror is either plus 10 degrees (on) or minus 10 degrees (off) to modulate the light that is incident on the surface. For additional details, see U.S. Patent No. 5,061,049 entitled, "Spatial Light Modulator" and U.S. Patent No. 5,280,277 entitled, "Field Updated Deferrable Mirror Device," both by Larry J. Hornbeck. As shown, the light reflected from any of the mirrors may pass through a projection lens 20 and create images on a large screen 21. As stated previously, the portion of time during which the video frame that a mirror remains in the on state determines the shade of grey. The time duration in which the cell is in the positive direction, or on, is represented by 8 bits of data sent to that cell. The color wheel 15 is divided into red, green, and blue sectors. In the color wheel example, the maximum red would be when the red, for example, would be reflecting the maximum period of time as when the light is on the longest period of time in the red sector. Applicants' invention can also be used in DMD display systems that use three DMD displays in place of a color wheel. The same would go for the other two colors. The minimum would be where the micromirror would not be reflecting through the color wheel and the lens, etc. at all during the color cycle. The intensity resolution in this pulse width modulation (PWM) is limited by the response time of the DMD mirrors. The total time available to display a color frame and the least time required to turn a mirror to the "on" state and back to "off" state defines the resolution of present systems. In the arrangement for the 8 bits, the most significant bit, as illustrated in Fig. 3, is the 7th bit with that bit representing the widest "on" time, the 6th bit being then the next widest "on" time, and the 5th bit representing the third longest "on" time, etc., all the way down to the least significant 0 bit, which is represented by the shortest time period. For example, a sequential color DMD system might have 5 (five) msec (milliseconds) available for a color frame. For 8-bit binary PWM, the least significant bit (0 bit on only), the shortest period would be "on" for about 19.6 microseconds. The mirror on/off time would have to be less than 19.6 msec to implement this scheme with the current method. In a system where the DMD device has the capability of only 6 bits, or even those that have the full eight bits, the system would have too few number of grades of grey or shades of color and, therefore, tend to show blockiness between portions of the picture. This would represent one of the errors that the present invention is to overcome. Another error is due to the degamma effect in the display. On a typical CRT television display system, the intensity of the picture is a function of the voltage, which is represented by the CRT response in Curve A of the sketch in Fig. 4. Note that the intensity for the lower voltage is nearly flat in the low voltage region, but increases rapidly at middle to highest voltage inputs. In order to correct for this, the transmission that is sent to the display has a gamma characteristic of curve B so that the overall response is linear as represented by the linear solid line C. In order to duplicate the CRT response for the digital micromirror device, a digital degamma characteristic is made to follow curve A of Fig. 4. Fig. 4a is a plot in more detail of Curve A of Fig. 4 for conventional inverse degamma, SMPTE inverse degamma, and Texas Instruments' degamma. A plot of conventional degamma input and output values follows Table 1 in Appendix A. There may be different gamma for red, green and blue, and, therefore, different degamma output tables for red, green and blue. This degamma correction is done, for example, for each color by providing the raster scanned gamma corrected red, green, or blue video data as shown in Fig. 1 to a separate degamma look-up table (LUT) 50 where for given input threshold levels the mirrors are turned on for given durations. However, due to the micromirror being digital in nature, the output is stepped as shown in Fig. 5 rather than smooth between the thresholds of the bits and, therefore, the grade levels again take on a blockiness particularly in the low intensity regions. In accordance with the present invention, Applicants solve the problems of the low number of bits and blockiness due to degamma, and defective DMDs by an error diffusion filter, as shown in Fig. 6 (FIL in Fig. 1), for each color path (red, green, and blue) on the raster scanned video output that would normally be written into the frame RAM buffer 53. For each input intensity, an output intensity N will be displayed on the DMD device. If the degamma was perfect and there was no lack of bits, the value displayed on the DMD would be some other value N1. We compute the difference between N and N1 and distribute this difference (error) among the neighboring pixels. The error can be distributed among the neighbors in various ways. One implementation is shown in Fig. 6. Temporal error diffusion can be a post processing step in this algorithm to improve frame to frame visual response and is being investigated. In accordance with one embodiment of the present invention, there is illustrated error diffusion filter according to one embodiment of the present invention. Referring to Fig. 6, the difference between N and N1 is distributed among its neighboring pixels as illustrated in Fig. 7. The implementation is illustrated in Fig. 6 wherein the raster scan gamma corrected red, green, or blue video 8-bit pixel intensity level data is applied, for example, via summer device 81 to both an error look-up table 82 and inverse gamma look-up (LUT) 51. For a gamma corrected video sent that follows Curve B, a perfect inverse gamma is known and follows Curve A. Table 2 in Appendix A for a 8-bit display shows for a given input value (IN) the value N provided by LUT 51 is in column marked "OUT" and is applied to corresponding DMD device. The error or difference value from the ideal N1 is N - N1 and is taken from the error look-up table (LUT) 82 and for an 8-bit display the value is marked as "ERROR" in Table 2. For example, if the input data value is 38, the output value from LUT 51 is 8 and the error value from Error LUT Table 82 is 1. This error is distributed for example as indicated in Fig. 7. The value of pixel 90 has one-quarter of the error from the previous horizontal line r-1 and previous column c-1 via H-1 delay 83 and V-1 delay 85, respectively, divider 88 and summer 92. The value at Pixel 90 has one-quarter of the error from the previous picture element in the column C-1 and this value is applied to summer 81 via H-1 delay 83, divider 88 and summer 92. Another one-quarter of the error from the previous line and same column is provided over V-1 delay 87 and divider 88. Another quarter of the error from the previous horizontal line R-1 and the next or C+1 column is applied via V-1 delay 87, H+1 delay 89 and divider 88. The H-1 delay is for one pixel delay, the V-1 delay is one line delay and the H+1 delay would reduce the V-1 delay by one pixel delay. This could be done by a separate FIFO delay by parsing V-1 delay 87. In this manner, the error added at summer 81 enhances the apparent intensity resolution of the video display system at the adjacent pixels. If the DMD is a 7-bit display, the Tables 51 and 82 provide the values of Table 3 of Appendix A. A further advantage of the present invention is that the defect compensation can be performed as part of this algorithm. In accordance with the teaching herein, a defective pixel is corrected for by treating this as an error. For this, the DMD coordinates of defective pixels at source 95 is provided to look-up table 82 and the error diffusion needs to be modified to account for the fact that at those locations the pixel displays either bright (stuck ON), dark (stuck OFF), or neutral (flat pixel). The surrounding pixels to the defective pixels are modified in intensity to correct for the failed position of the micromirrors. The look-up table LUT 82 stores error values for these defective locations based on the type of error and the neighboring pixels are modified at summer 81. The error is distributed as discussed previously, such as the distribution as in Fig. 7 by the delays and summer 92 and divider of Fig. 6. Other Embodiments Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope of the invention as defined by the appended claims. APPENDIX A Inverse Degamma Values (Conventional) IN OUT IN OUT IN OUT 0 0 43 5 85 23 1 0 44 5 86 23 2 0 45 6 87 24 3 0 46 6 88 25 4 0 47 6 89 25 5 0 48 6 90 26 6 0 49 7 91 26 7 0 50 7 92 27 8 0 51 7 93 28 9 0 52 8 94 28 10 0 53 8 95 29 11 0 54 8 96 30 12 0 55 9 97 30 13 0 56 9 98 31 14 0 57 9 99 32 15 1 58 10 100 33 16 1 59 10 101 33 17 1 60 11 102 34 18 1 61 11 103 35 19 1 62 11 104 35 20 1 63 12 105 36 21 1 64 12 106 37 22 1 65 13 107 38 23 1 66 13 108 39 24 1 67 13 109 39 25 2 68 14 110 40 26 2 69 14 111 41 27 2 70 15 112 42 28 2 71 15 113 43 30 2 72 16 114 43 31 2 73 16 115 44 32 3 74 17 116 45 33 3 75 17 117 46 34 3 76 18 118 47 35 3 77 18 119 48 36 3 78 19 120 49 37 4 79 19 121 49 38 4 80 20 122 50 39 4 81 20 123 51 40 4 82 21 124 52 41 5 83 22 125 53 42 5 84 22 126 54 127 55 169 103 211 168 128 56 170 105 212 170 129 57 171 106 213 172 130 58 172 107 214 173 131 59 173 109 215 175 132 60 174 110 216 177 133 61 175 111 217 179 134 62 176 113 218 181 135 63 177 114 219 182 136 64 178 116 220 184 137 65 179 117 221 186 138 66 180 119 222 188 139 67 181 120 223 190 140 68 182 121 224 192 141 69 183 123 225 194 142 70 184 124 226 196 143 71 185 126 227 197 144 73 186 127 228 199 145 74 187 129 229 201 146 75 188 130 230 203 147 76 189 132 231 205 148 77 190 133 232 207 149 78 191 135 233 209 150 79 192 137 234 211 151 81 193 138 235 213 152 82 194 140 236 215 153 83 195 141 237 217 154 84 196 143 238 219 155 85 197 145 239 221 156 87 198 146 240 223 157 88 199 148 241 225 158 89 200 149 242 227 159 90 201 151 243 229 160 91 202 153 244 231 161 93 203 154 245 234 162 94 204 156 246 236 163 95 205 158 247 238 164 97 206 159 248 240 165 98 207 161 249 242 166 99 208 163 250 244 167 100 209 165 251 246 168 102 210 166 252 248 253 251 254 253 255 255 Conventional APPENDIX A Look-up Tables for 8-bit Display IN LUT 51 OUT LUT 82 ERROR IN LUT 51 OUT LUT 82 ERROR IN LUT 51 OUT LUT 82 ERROR 0 0 0 44 11 0 88 34 0 1 0 1 45 11 1 89 35 0 2 0 2 46 12 0 90 35 1 3 0 3 47 12 1 91 36 0 4 0 4 48 13 0 92 37 0 5 0 5 49 13 1 93 37 1 6 0 6 50 14 0 94 38 0 7 0 7 51 14 1 95 39 0 8 0 8 52 15 0 96 40 0 9 0 9 53 15 1 97 40 1 10 1 0 54 15 2 98 41 0 11 1 1 55 16 0 99 42 0 12 1 2 56 16 1 100 43 0 13 1 3 57 17 0 101 43 1 14 1 4 58 17 1 102 44 0 15 2 0 59 18 0 103 45 0 16 2 1 60 18 1 104 46 0 17 2 2 61 19 0 105 46 1 18 2 3 62 19 1 106 47 0 19 2 4 63 20 0 107 48 0 20 3 0 64 20 1 108 49 0 21 3 1 65 21 0 109 50 0 22 3 2 66 21 1 110 51 0 23 3 3 - 67 22 0 111 51 1 24 4 0 68 22 1 112 52 0 25 4 1 69 23 0 113 53 0 26 4 2 70 23 1 114 54 0 27 4 3 71 24 0 115 55 0 28 5 0 72 24 1 116 56 0 29 5 1 73 25 0 117 57 1 30 5 2 74 25 1 118 57 0 31 6 0 75 26 0 119 58 0 32 6 1 76 27 0 120 59 0 33 6 2 77 27 1 121 60 0 34 7 0 78 28 0 122 61 0 35 7 1 79 28 1 123 62 0 36 7 2 80 29 0 124 63 0 37 8 0 81 30 0 125 64 0 38 8 1 82 30 1 126 65 0 39 8 2 83 31 0 127 66 0 40 9 0 84 31 1 128 67 0 41 9 1 85 32 0 129 68 0 42 10 0 86 33 0 43 10 1 87 33 1 130 69 0 173 118 0 216 183 0 131 70 0 174 119 0 217 184 0 132 71 0 175 121 0 218 186 0 133 72 0 176 122 0 219 188 0 134 73 0 177 124 0 220 190 0 135 74 0 178 125 0 221 191 0 136 75 0 179 126 0 222 193 0 137 76 0 180 128 0 223 195 0 138 77 0 181 129 0 224 197 0 139 78 0 182 130 0 225 198 0 140 79 0 183 132 0 226 200 0 141 80 0 184 133 0 227 202 0 142 81 0 185 135 0 228 204 0 143 82 0 186 136 0 229 205 0 144 83 0 187 138 0 230 207 0 145 84 0 188 139 0 231 209 0 146 85 0 189 140 0 232 211 0 147 86 0 190 142 0 233 213 0 148 88 0 191 143 0 234 214 0 149 89 0 192 145 0 235 216 0 150 90 0 193 146 0 236 218 0 151 91 0 194 148 0 237 220 0 152 92 0 195 149 0 238 222 0 153 93 0 196 151 0 239 224 0 154 94 0 197 152 0 240 226 0 155 96 0 198 154 0 241 228 0 156 97 0 199 155 0 242 229 0 157 98 0 200 157 0 243 231 0 158 99 0 201 158 0 244 233 0 159 100 0 202 160 0 245 235 0 160 102 0 203 162 0 246 237 0 161 103 0 204 163 0 247 239 0 162 104 0 205 165 0 248 241 0 163 105 0 206 166 0 249 243 0 164 107 0 207 168 0 250 245 0 165 108 0 208 170 0 251 247 0 166 109 0 209 171 0 252 249 0 167 110 0 210 173 0 253 251 0 168 112 0 211 174 0 254 253 0 169 113 0 212 176 0 255 255 0 170 114 0 213 178 0 "TI_Gamma 171 116 0 214 179 0 172 117 0 215 181 0 APPENDIX A Look-up Tables for 7-bit Display IN OUT ERROR IN OUT ERROR IN OUT ERROR 0 0 0 44 10 2 88 34 0 1 0 1 45 10 3 89 34 1 2 0 2 46 12 0 90 34 2 3 0 3 47 12 1 91 36 0 4 0 4 48 12 2 92 36 1 5 0 5 49 12 3 93 36 2 6 0 6 50 14 0 94 38 0 7 0 7 51 14 1 95 38 1 8 0 8 52 14 2 96 40 0 9 0 9 53 14 3 97 40 1 10 0 10 54 14 4 98 40 2 11 0 11 55 16 0 99 42 0 12 0 12 56 16 1 100 42 1 13 0 13 57 16 2 101 42 2 14 0 14 58 16 3 102 44 0 15 2 0 59 18 0 103 44 1 16 2 1 60 18 1 104 46 0 17 2 2 61 18 2 105 46 1 18 2 3 62 18 3 106 46 2 19 2 4 63 20 0 107 48 0 20 2 5 64 20 1 108 48 1 21 2 6 65 20 2 109 50 0 22 2 7 66 20 3 110 50 1 23 2 8 67 22 0 111 50 2 24 4 0 68 22 1 112 52 0 25 4 1 69 22 2 113 52 1 26 4 2 70 22 3 114 54 0 27 4 3 71 24 0 115 54 1 28 4 4 72 24 1 116 56 0 29 4 5 73 24 2 117 56 1 30 4 6 74 24 3 118 56 2 31 6 0 75 26 0 119 58 0 32 6 1 76 26 1 120 58 1 33 6 2 77 26 2 121 60 0 34 6 3 78 28 0 122 60 1 35 6 4 79 28 1 123 62 0 36 6 5 80 28 2 124 62 1 37 8 0 81 30 0 125 64 0 38 8 1 82 30 1 126 64 1 39 8 2 83 30 2 127 66 0 40 8 3 84 30 3 128 66 1 41 8 4 85 32 0 129 68 0 42 10 0 86 32 1 43 10 1 87 32 2 130 68 1 173 118 0 216 182 0 131 70 0 174 118 1 217 184 0 132 70 1 175 120 0 218 186 0 133 72 0 176 122 0 219 188 0 134 72 1 177 124 0 220 190 0 135 74 0 178 124 1 221 190 1 136 74 1 179 126 0 222 192 0 137 76 0 180 128 0 223 194 0 138 76 1 181 128 1 224 196 0 139 78 0 182 130 0 225 198 0 140 78 1 183 132 0 226 200 0 141 80 0 184 132 1 227 202 0 142 80 1 185 134 0 228 204 0 143 82 0 186 136 0 229 204 1 144 82 1 187 138 0 230 206 0 145 84 0 188 138 1 231 208 0 146 84 1 189 140 0 232 210 0 147 86 0 190 142 0 233 212 0 148 88 0 191 142 1 234 214 0 149 88 1 192 144 0 235 216 0 150 90 0 193 146 0 236 218 0 151 90 1 194 148 0 237 220 0 152 92 0 195 148 1 238 222 0 153 92 1 196 150 0 239 224 0 154 94 0 197 152 0 240 226 0 155 96 0 198 154 0 241 228 0 156 96 1 199 154 1 242 228 1 157 98 0 200 156 0 243 230 0 158 98 1 201 158 0 244 232 0 159 100 0 202 160 0 245 234 0 160 102 0 203 162 0 246 236 0 161 102 1 204 162 1 247 238 0 162 104 0 205 164 0 248 240 0 163 104 1 206 166 0 249 242 0 164 106 0 207 168 0 250 244 0 165 108 0 208 170 0 251 246 0 166 108 1 209 170 1 252 248 0 167 110 0 210 172 0 253 250 0 168 112 0 211 174 0 254 252 0 169 112 1 212 176 0 255 254 0 170 114 0 213 178 0 "TI_Gamma 171 116 0 214 178 1 172 116 1 215 180 0

Claims: An error diffusion filter for a DMD display comprising: an inverse gamma look-up table (51); and means responsive to an error output value for distributively changing pixel data intensity levels to pixel elements neighbouring a given pixel element of a DMD (19), said changing being in proportion to said error output value; characterized in that said inverse gamma look-up table (51) is responsive to raster scanned, gamma corrected video data for providing a digitised pixel data intensity level to said given pixel element, said digitised intensity level being at the closest achievable pixel data intensity level to an exact inverse gamma intensity level; and that said error diffusion filter further comprises an error look-up table (82) responsive to said video data for providing said error output value, said error output value corresponding to the difference between said exact inverse gamma intensity level and said closest immediately achievable intensity level for said given pixel element. The error diffusion filter of claim 1 wherein said error look-up table (82) further includes a table providing error values based on location of defective pixel elements and wherein said means for changing pixel data intensity levels includes means responsive to said error values based on location of said defective pixel elements for changing pixel data intensity level to pixel elements surrounding each said defective pixel element to visually adjust for said defective pixel elements. The error diffusion filter of claim 1 or claim 2 wherein said means responsive to error output value, in order to change a neighbouring pixel element, includes means (92) for summing 1/4 of the error from a previous given horizontal pixel data element, 1/4 of the error from a previous given vertical pixel data element, 1/4 of the error from the previous given horizontal pixel data element to said previous vertical pixel data element, and 1/4 of the error from the next given horizontal data element to said previous vertical pixel data element. The error diffusion filter of claim 3 wherein said means (92) for summing includes a line delay (85, 87). The error diffusion filter of any preceding claim wherein there is a separate inverse gamma look-up table and diffusion operation for red, green and blue video data. The error diffusion filter of any preceding claim, wherein the means responsive to error output comprises: a summer (81) coupled to pixel elements neighbouring said given pixel element and responsive to said error output value for summing said error output value with the pixel data intensity levels for said neighbouring pixel elements to visually achieve said exact inverse gamma intensity level. A method of filtering a DMD display comprising the steps of: providing a digitised pixel intensity level to each pixel element that is the closest intensity level to an exact inverse gamma intensity level using an inverse gamma look-up table (51); providing error output value corresponding to the difference between said exact intensity level and the closest achievable intensity level using an error look-up table (82), and changing the pixel data intensity levels to pixel elements surrounding pixel elements having an error output value, said surrounding pixel elements being changed in proportion to said error output value. A method of correcting for defective pixel elements in a DMD display comprising the steps of: determining the location of each of said defective pixel elements; storing error values in a table (82) based on the location of each of said defective pixel elements; and changing pixel intensity levels of pixel elements surrounding each of said defective pixel elements to visually adjust for the defect.

Inventors: MARKANDEY VISHAL, PETTITT GREGORY S, MARKANDEY, VISHAL, PETTITT, GREGORY S.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1995
Main classification: G09G3
IPC codes: G02B26, G09G3, H04N5, H04N9