1 Advanced usage instructions for the Independent JPEG Group's JPEG software |
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2 ========================================================================== |
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3 |
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4 This file describes cjpeg's "switches for wizards". |
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5 |
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6 The "wizard" switches are intended for experimentation with JPEG by persons |
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7 who are reasonably knowledgeable about the JPEG standard. If you don't know |
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8 what you are doing, DON'T USE THESE SWITCHES. You'll likely produce files |
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9 with worse image quality and/or poorer compression than you'd get from the |
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10 default settings. Furthermore, these switches must be used with caution |
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11 when making files intended for general use, because not all JPEG decoders |
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12 will support unusual JPEG parameter settings. |
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13 |
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14 |
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15 Quantization Table Adjustment |
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16 ----------------------------- |
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17 |
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18 Ordinarily, cjpeg starts with a default set of tables (the same ones given |
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19 as examples in the JPEG standard) and scales them up or down according to |
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20 the -quality setting. The details of the scaling algorithm can be found in |
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21 jcparam.c. At very low quality settings, some quantization table entries |
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22 can get scaled up to values exceeding 255. Although 2-byte quantization |
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23 values are supported by the IJG software, this feature is not in baseline |
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24 JPEG and is not supported by all implementations. If you need to ensure |
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25 wide compatibility of low-quality files, you can constrain the scaled |
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26 quantization values to no more than 255 by giving the -baseline switch. |
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27 Note that use of -baseline will result in poorer quality for the same file |
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28 size, since more bits than necessary are expended on higher AC coefficients. |
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29 |
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30 You can substitute a different set of quantization values by using the |
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31 -qtables switch: |
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32 |
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33 -qtables file Use the quantization tables given in the named file. |
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34 |
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35 The specified file should be a text file containing decimal quantization |
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36 values. The file should contain one to four tables, each of 64 elements. |
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37 The tables are implicitly numbered 0,1,etc. in order of appearance. Table |
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38 entries appear in normal array order (NOT in the zigzag order in which they |
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39 will be stored in the JPEG file). |
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40 |
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41 Quantization table files are free format, in that arbitrary whitespace can |
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42 appear between numbers. Also, comments can be included: a comment starts |
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43 with '#' and extends to the end of the line. Here is an example file that |
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44 duplicates the default quantization tables: |
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45 |
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46 # Quantization tables given in JPEG spec, section K.1 |
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47 |
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48 # This is table 0 (the luminance table): |
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49 16 11 10 16 24 40 51 61 |
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50 12 12 14 19 26 58 60 55 |
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51 14 13 16 24 40 57 69 56 |
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52 14 17 22 29 51 87 80 62 |
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53 18 22 37 56 68 109 103 77 |
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54 24 35 55 64 81 104 113 92 |
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55 49 64 78 87 103 121 120 101 |
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56 72 92 95 98 112 100 103 99 |
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57 |
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58 # This is table 1 (the chrominance table): |
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59 17 18 24 47 99 99 99 99 |
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60 18 21 26 66 99 99 99 99 |
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61 24 26 56 99 99 99 99 99 |
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62 47 66 99 99 99 99 99 99 |
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63 99 99 99 99 99 99 99 99 |
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64 99 99 99 99 99 99 99 99 |
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65 99 99 99 99 99 99 99 99 |
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66 99 99 99 99 99 99 99 99 |
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67 |
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68 If the -qtables switch is used without -quality, then the specified tables |
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69 are used exactly as-is. If both -qtables and -quality are used, then the |
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70 tables taken from the file are scaled in the same fashion that the default |
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71 tables would be scaled for that quality setting. If -baseline appears, then |
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72 the quantization values are constrained to the range 1-255. |
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73 |
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74 By default, cjpeg will use quantization table 0 for luminance components and |
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75 table 1 for chrominance components. To override this choice, use the -qslots |
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76 switch: |
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77 |
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78 -qslots N[,...] Select which quantization table to use for |
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79 each color component. |
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80 |
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81 The -qslots switch specifies a quantization table number for each color |
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82 component, in the order in which the components appear in the JPEG SOF marker. |
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83 For example, to create a separate table for each of Y,Cb,Cr, you could |
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84 provide a -qtables file that defines three quantization tables and say |
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85 "-qslots 0,1,2". If -qslots gives fewer table numbers than there are color |
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86 components, then the last table number is repeated as necessary. |
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87 |
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88 |
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89 Sampling Factor Adjustment |
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90 -------------------------- |
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91 |
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92 By default, cjpeg uses 2:1 horizontal and vertical downsampling when |
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93 compressing YCbCr data, and no downsampling for all other color spaces. |
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94 You can override this default with the -sample switch: |
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95 |
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96 -sample HxV[,...] Set JPEG sampling factors for each color |
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97 component. |
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98 |
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99 The -sample switch specifies the JPEG sampling factors for each color |
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100 component, in the order in which they appear in the JPEG SOF marker. |
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101 If you specify fewer HxV pairs than there are components, the remaining |
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102 components are set to 1x1 sampling. For example, the default YCbCr setting |
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103 is equivalent to "-sample 2x2,1x1,1x1", which can be abbreviated to |
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104 "-sample 2x2". |
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105 |
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106 There are still some JPEG decoders in existence that support only 2x1 |
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107 sampling (also called 4:2:2 sampling). Compatibility with such decoders can |
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108 be achieved by specifying "-sample 2x1". This is not recommended unless |
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109 really necessary, since it increases file size and encoding/decoding time |
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110 with very little quality gain. |
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111 |
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112 |
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113 Multiple Scan / Progression Control |
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114 ----------------------------------- |
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115 |
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116 By default, cjpeg emits a single-scan sequential JPEG file. The |
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117 -progressive switch generates a progressive JPEG file using a default series |
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118 of progression parameters. You can create multiple-scan sequential JPEG |
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119 files or progressive JPEG files with custom progression parameters by using |
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120 the -scans switch: |
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121 |
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122 -scans file Use the scan sequence given in the named file. |
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123 |
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124 The specified file should be a text file containing a "scan script". |
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125 The script specifies the contents and ordering of the scans to be emitted. |
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126 Each entry in the script defines one scan. A scan definition specifies |
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127 the components to be included in the scan, and for progressive JPEG it also |
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128 specifies the progression parameters Ss,Se,Ah,Al for the scan. Scan |
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129 definitions are separated by semicolons (';'). A semicolon after the last |
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130 scan definition is optional. |
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131 |
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132 Each scan definition contains one to four component indexes, optionally |
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133 followed by a colon (':') and the four progressive-JPEG parameters. The |
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134 component indexes denote which color component(s) are to be transmitted in |
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135 the scan. Components are numbered in the order in which they appear in the |
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136 JPEG SOF marker, with the first component being numbered 0. (Note that these |
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137 indexes are not the "component ID" codes assigned to the components, just |
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138 positional indexes.) |
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139 |
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140 The progression parameters for each scan are: |
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141 Ss Zigzag index of first coefficient included in scan |
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142 Se Zigzag index of last coefficient included in scan |
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143 Ah Zero for first scan of a coefficient, else Al of prior scan |
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144 Al Successive approximation low bit position for scan |
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145 If the progression parameters are omitted, the values 0,63,0,0 are used, |
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146 producing a sequential JPEG file. cjpeg automatically determines whether |
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147 the script represents a progressive or sequential file, by observing whether |
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148 Ss and Se values other than 0 and 63 appear. (The -progressive switch is |
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149 not needed to specify this; in fact, it is ignored when -scans appears.) |
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150 The scan script must meet the JPEG restrictions on progression sequences. |
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151 (cjpeg checks that the spec's requirements are obeyed.) |
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152 |
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153 Scan script files are free format, in that arbitrary whitespace can appear |
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154 between numbers and around punctuation. Also, comments can be included: a |
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155 comment starts with '#' and extends to the end of the line. For additional |
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156 legibility, commas or dashes can be placed between values. (Actually, any |
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157 single punctuation character other than ':' or ';' can be inserted.) For |
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158 example, the following two scan definitions are equivalent: |
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159 0 1 2: 0 63 0 0; |
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160 0,1,2 : 0-63, 0,0 ; |
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161 |
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162 Here is an example of a scan script that generates a partially interleaved |
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163 sequential JPEG file: |
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164 |
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165 0; # Y only in first scan |
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166 1 2; # Cb and Cr in second scan |
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167 |
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168 Here is an example of a progressive scan script using only spectral selection |
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169 (no successive approximation): |
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170 |
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171 # Interleaved DC scan for Y,Cb,Cr: |
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172 0,1,2: 0-0, 0, 0 ; |
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173 # AC scans: |
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174 0: 1-2, 0, 0 ; # First two Y AC coefficients |
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175 0: 3-5, 0, 0 ; # Three more |
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176 1: 1-63, 0, 0 ; # All AC coefficients for Cb |
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177 2: 1-63, 0, 0 ; # All AC coefficients for Cr |
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178 0: 6-9, 0, 0 ; # More Y coefficients |
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179 0: 10-63, 0, 0 ; # Remaining Y coefficients |
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180 |
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181 Here is an example of a successive-approximation script. This is equivalent |
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182 to the default script used by "cjpeg -progressive" for YCbCr images: |
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183 |
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184 # Initial DC scan for Y,Cb,Cr (lowest bit not sent) |
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185 0,1,2: 0-0, 0, 1 ; |
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186 # First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits: |
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187 0: 1-5, 0, 2 ; |
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188 # Send all Cr,Cb AC coefficients, minus lowest bit: |
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189 # (chroma data is usually too small to be worth subdividing further; |
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190 # but note we send Cr first since eye is least sensitive to Cb) |
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191 2: 1-63, 0, 1 ; |
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192 1: 1-63, 0, 1 ; |
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193 # Send remaining Y AC coefficients, minus 2 lowest bits: |
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194 0: 6-63, 0, 2 ; |
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195 # Send next-to-lowest bit of all Y AC coefficients: |
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196 0: 1-63, 2, 1 ; |
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197 # At this point we've sent all but the lowest bit of all coefficients. |
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198 # Send lowest bit of DC coefficients |
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199 0,1,2: 0-0, 1, 0 ; |
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200 # Send lowest bit of AC coefficients |
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201 2: 1-63, 1, 0 ; |
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202 1: 1-63, 1, 0 ; |
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203 # Y AC lowest bit scan is last; it's usually the largest scan |
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204 0: 1-63, 1, 0 ; |
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205 |
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206 It may be worth pointing out that this script is tuned for quality settings |
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207 of around 50 to 75. For lower quality settings, you'd probably want to use |
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208 a script with fewer stages of successive approximation (otherwise the |
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209 initial scans will be really bad). For higher quality settings, you might |
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210 want to use more stages of successive approximation (so that the initial |
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211 scans are not too large). |
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