1 <html><head><title>SGI Image File Format Specification</title></head><body bgcolor="#ffffff">
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2 <font face="Helvetica, Arial, sans-serif" size="-1">
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4 <h3><font face="Helvetica, Arial, sans-serif" size="-1">The SGI Image File Format</font></h3>
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6 <font face="Helvetica, Arial, sans-serif" size="-1">Version 1.00
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8 <font face="Helvetica, Arial, sans-serif" size="-1">Paul Haeberli<br>
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9 Silicon Graphics Computer Systems<br>
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10 paulhaeberli@yahoo.com<br>
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11 </font></p></center>
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12 <h3><font face="Helvetica, Arial, sans-serif" size="-1">Introduction</font></h3>
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14 <font face="Helvetica, Arial, sans-serif" size="-1">This is the definitive document describing the SGI image file format. This
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15 is a low level spec that describes the actual byte level format of SGI image
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16 files. On SGI machines the preferred way of reading and writing SGI image
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17 files is to use the image library -limage. This library provides a set
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18 of functions that make it easy to read and write SGI images. If you are
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19 on an SGI workstation you can get info on -limage by doing:
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20 </font></p><blockquote><pre><font face="Helvetica, Arial, sans-serif" size="-1">% man 4 rgb
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21 </font></pre></blockquote>
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22 <h3><font face="Helvetica, Arial, sans-serif" size="-1">A note on byte order of values in the SGI image files</font></h3>
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24 <font face="Helvetica, Arial, sans-serif" size="-1">In the following description a notation like bits[7..0] is used to denote
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25 a range of bits in a binary value. Bit 0 is the lowest order bit in
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28 <font face="Helvetica, Arial, sans-serif" size="-1">All short values are represented by 2 bytes. The first byte stores the
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29 high order 8 bits of the value: bits[15..8]. The second byte stores
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30 the low order 8 bits of the value: bits[7..0].
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32 <font face="Helvetica, Arial, sans-serif" size="-1">So this function will read a short value from the file:
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33 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> unsigned short getshort(inf)
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36 unsigned char buf[2];
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39 return (buf[0]<<8)+(buf[1]<<0);
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43 <font face="Helvetica, Arial, sans-serif" size="-1">All long values are represented by 4 bytes. The first byte stores the
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44 high order 8 bits of the value: bits[31..24]. The second byte stores
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45 bits[23..16]. The third byte stores bits[15..8]. The forth byte stores
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46 the low order 8 bits of the value: bits[7..0].
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48 <font face="Helvetica, Arial, sans-serif" size="-1">And this function will read a long value from the file:
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49 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> static long getlong(inf)
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52 unsigned char buf[4];
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55 return (buf[0]<<24)+(buf[1]<<16)+(buf[2]<<8)+(buf[3]<<0);
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58 <font face="Helvetica, Arial, sans-serif" size="-1">
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60 </p><h3><font face="Helvetica, Arial, sans-serif" size="-1">The general structure of an SGI image file</font></h3>
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61 <font face="Helvetica, Arial, sans-serif" size="-1"> The header indicates whether the image is run length encoded (RLE).
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63 <font face="Helvetica, Arial, sans-serif" size="-1"> If the image is not run length encoded, this is the structure:
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64 </font></p><blockquote>
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65 <font face="Helvetica, Arial, sans-serif" size="-1"> The Header<br>
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67 </font></blockquote>
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68 <font face="Helvetica, Arial, sans-serif" size="-1">
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69 If the image is run length encoded, this is the structure:
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71 <font face="Helvetica, Arial, sans-serif" size="-1"> The Header<br>
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72 The Offset Tables<br>
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74 </font></blockquote>
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75 <font face="Helvetica, Arial, sans-serif" size="-1">
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76 </font><h3><font face="Helvetica, Arial, sans-serif" size="-1">The Header</font></h3>
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78 <font face="Helvetica, Arial, sans-serif" size="-1">The header consists of the following:
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79 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> Size | Type | Name | Description
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81 2 bytes | short | MAGIC | IRIS image file magic number
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82 1 byte | char | STORAGE | Storage format
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83 1 byte | char | BPC | Number of bytes per pixel channel
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84 2 bytes | ushort | DIMENSION | Number of dimensions
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85 2 bytes | ushort | XSIZE | X size in pixels
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86 2 bytes | ushort | YSIZE | Y size in pixels
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87 2 bytes | ushort | ZSIZE | Number of channels
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88 4 bytes | long | PIXMIN | Minimum pixel value
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89 4 bytes | long | PIXMAX | Maximum pixel value
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90 4 bytes | char | DUMMY | Ignored
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91 80 bytes | char | IMAGENAME | Image name
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92 4 bytes | long | COLORMAP | Colormap ID
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93 404 bytes | char | DUMMY | Ignored
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96 <p><font face="Helvetica, Arial, sans-serif" size="-1">
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97 Here is a description of each field in the image file header:
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98 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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99 MAGIC - This is the decimal value 474 saved as a short. This
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100 identifies the file as an SGI image file.
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101 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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102 STORAGE - specifies whether the image is stored using run
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103 length encoding (RLE) or not (VERBATIM). If RLE is used, the value
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104 of this byte will be 1. Otherwise the value of this byte will be 0.
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105 The only allowed values for this field are 0 or 1.
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106 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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107 BPC - describes the precision that is used to store each
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108 channel of an image. This is the number of bytes per pixel
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109 component. The majority of SGI image files use 1 byte per
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110 pixel component, giving 256 levels. Some SGI image files use
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111 2 bytes per component. The only allowed values for this field
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114 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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115 DIMENSION - described the number of dimensions in the data stored
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116 in the image file. The only allowed values are 1, 2, or 3. If
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117 this value is 1, the image file consists of only 1 channel and
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118 only 1 scanline (row). The length of this scanline is given by the
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119 value of XSIZE below. If this value is 2, the file consists of a
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120 single channel with a number of scanlines. The width and height
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121 of the image are given by the values of XSIZE and YSIZE below.
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122 If this value is 3, the file consists of a number of channels.
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123 The width and height of the image are given by the values of
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124 XSIZE and YSIZE below. The number of channels is given by the
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125 value of ZSIZE below.
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126 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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127 XSIZE - The width of the image in pixels
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128 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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130 YSIZE - The height of the image in pixels
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131 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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133 ZSIZE - The number of channels in the image. B/W (greyscale) images
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134 are stored as 2 dimensional images with a ZSIZE or 1. RGB color
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135 images are stored as 3 dimensional images with a ZSIZE of 3. An RGB
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136 image with an ALPHA channel is stored as a 3 dimensional image with
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137 a ZSIZE of 4. There are no inherent limitations in the SGI image
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138 file format that would preclude the creation of image files with more
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141 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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142 PINMIN - The minimum pixel value in the image. The value of
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143 0 may be used if no pixel has a value that is smaller than 0.
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144 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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146 PINMAX - The maximum pixel value in the image. The value of
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147 255 may be used if no pixel has a value that is greater than 255.
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148 This is the value that is considered to be full brightness in
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151 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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152 DUMMY - This 4 bytes of data should be set to 0.
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153 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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155 IMAGENAME - An null terminated ascii string of up to 79 characters
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156 terminated by a null may be included here. This is not commonly
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159 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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160 COLORMAP - This controls how the pixel values in the file should be
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161 interpreted. It can have one of these four values:
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163 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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164 0: NORMAL - The data in the channels represent B/W values
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165 for images with 1 channel, RGB values for images with 3
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166 channels, and RGBA values for images with 4 channels.
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167 Almost all the SGI image files are of this type.
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169 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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170 1: DITHERED - The image will have only 1 channel of data.
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171 For each pixel, RGB data is packed into one 8 bit value.
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172 3 bits are used for red and green, while blue uses 2 bits.
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173 Red data is found in bits[2..0], green data in bits[5..3],
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174 and blue data in bits[7..6]. This format is obsolete.
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176 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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177 2: SCREEN - The image will have only 1 channel of data.
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178 This format was used to store color-indexed pixels.
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179 To convert the pixel values into RGB values a colormap
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180 must be used. The appropriate color map varies from
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181 image to image. This format is obsolete.
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183 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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184 3: COLORMAP - The image is used to store a color map from
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185 an SGI machine. In this case the image is not displayable
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186 in the conventional sense.
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188 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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189 DUMMY - This 404 bytes of data should be set to 0. This makes the
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190 header exactly 512 bytes.
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191 </font></p></blockquote>
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192 <font face="Helvetica, Arial, sans-serif" size="-1">
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193 </font><p><font face="Helvetica, Arial, sans-serif" size="-1">
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194 </font></p><h3><font face="Helvetica, Arial, sans-serif" size="-1">The Image Data (if not RLE)</font></h3>
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195 <p><font face="Helvetica, Arial, sans-serif" size="-1">
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196 If the image is stored verbatim (without RLE), then image data directly
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197 follows the 512 byte header. The data for each scanline of the first
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198 channel is written first. If the image has more than 1 channel, all
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199 the data for the first channel is written, followed by the remaining
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200 channels. If the BPC value is 1, then each scanline is written as XSIZE
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201 bytes. If the BPC value is 2, then each scanline is written as XSIZE
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202 shorts. These shorts are stored in the byte order described above.
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203 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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204 </font></p><h3><font face="Helvetica, Arial, sans-serif" size="-1">The Offset Tables (if RLE)</font></h3>
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205 <p><font face="Helvetica, Arial, sans-serif" size="-1">
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206 If the image is stored using run length encoding, offset tables
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207 follow the header that describe what the file offsets are to the
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208 RLE for each scanline. This information only applies if the value
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209 for STORAGE above is 1.
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210 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> Size | Type | Name | Description
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212 tablen longs | long | STARTTAB | Start table
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213 tablen longs | long | LENGTHTAB | Length table
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214 </font></pre><font face="Helvetica, Arial, sans-serif" size="-1">
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215 </font><p><font face="Helvetica, Arial, sans-serif" size="-1">
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216 One entry in each table is needed for each scanline of RLE data. The
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217 total number of scanlines in the image (tablen) is determined by the
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218 product of the YSIZE and ZSIZE. There are two tables of longs that
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219 are written. Each consists of tablen longs of data. The first
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220 table has the file offsets to the RLE data for each scanline in the
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221 image. In a file with more than 1 channel (ZSIZE > 1) this table first
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222 has all the offsets for the scanlines in the first channel, followed
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223 be offsets for the scanlines in the second channel, etc. The second
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224 table has the RLE data length for each scanline in the image. In a
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225 file with more than 1 channel (ZSIZE > 1) this table first has all the
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226 RLE data lengths for the scanlines in the first channel, followed
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227 be RLE data lengths for the scanlines in the second channel, etc.
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229 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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230 To find the the file offset, and the number of bytes in the RLE data
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231 for a particular scanline, these two arrays may be read in and indexed as
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234 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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235 To read in the tables:
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236 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> unsigned long *starttab, *lengthtab;
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238 tablen = YSIZE*ZSIZE*sizeof(long);
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239 starttab = (unsigned long *)mymalloc(tablen);
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240 lengthtab = (unsigned long *)mymalloc(tablen);
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241 fseek(inf,512,SEEK_SET);
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242 readlongtab(inf,starttab);
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243 readlongtab(ing,lengthtab);
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244 </font></pre><font face="Helvetica, Arial, sans-serif" size="-1">
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246 </font><p><font face="Helvetica, Arial, sans-serif" size="-1">
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247 To find the file offset and RLE data length for a scanline:
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248 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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249 rowno is an integer in the range 0 to YSIZE-1
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250 channo is an integer in the range 0 to ZSIZE-1
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251 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> rleoffset = starttab[rowno+channo*YSIZE]
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252 rlelength = lengthtab[rowno+channo*YSIZE]
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254 <p><font face="Helvetica, Arial, sans-serif" size="-1">
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255 It is possible for two identical rows (scanlines) to share compressed
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256 data. A completely white image could be written as a single compressed
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257 row and having all table entries point to that row. Another little hack
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258 that should work is if you are writing out a RGB RLE file, and a
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259 particular scanline is achromatic (greyscale), you could just make the
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260 r, g and b rows point to the same data!!
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261 </font></p><h3><font face="Helvetica, Arial, sans-serif" size="-1">The Image Data (if RLE)</font></h3>
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262 <p><font face="Helvetica, Arial, sans-serif" size="-1">
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263 This information only applies if the value for STORAGE above is 1. If
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264 the image is stored using run length encoding, the image data follows
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265 the offset tables above. The RLE data is not in any particular order.
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266 The offset tables above are used to locate the rle data for any scanline.
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267 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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268 The RLE data must be read in from the file and expanded into pixel
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269 data in the following manner:
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270 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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271 If BPC is 1, then there is one byte per pixel. In this case the
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272 RLE data should be read into an array of chars. To expand
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273 data, the low order seven bits of the first byte: bits[6..0]
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274 are used to form a count. If the high order bit of the first
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275 byte is 1: bit[7], then the count is used to specify how many
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276 bytes to copy from the RLE data buffer to the destination.
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277 Otherwise, if the high order bit of the first byte is 0: bit[7],
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278 then the count is used to specify how many times to repeat the
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279 value of the following byte, in the destination. This process
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280 continues until a count of 0 is found. This should decompress
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281 exactly XSIZE pixels.
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282 </font></p><p><font face="Helvetica, Arial, sans-serif" size="-1">
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283 Here is example code to decompress a scanline:
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284 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> expandrow(optr,iptr,z)
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285 unsigned char *optr, *iptr;
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288 unsigned char pixel, count;
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293 if ( !(count = (pixel & 0x7f)) )
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295 if(pixel & 0x80) {
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305 </font></pre><font face="Helvetica, Arial, sans-serif" size="-1">
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307 <font face="Helvetica, Arial, sans-serif" size="-1"> If BPC is 2, there is one short (2 bytes) per pixel. In this
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308 case the RLE data should be read into an array of shorts. To
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309 expand data, the low order seven bits of the first short: bits[6..0]
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310 are used to form a count. If bit[7] of the first short is 1, then
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311 the count is used to specify how many shorts to copy from the RLE
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312 data buffer to the destination. Otherwise, if bit[7] of the first
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313 short is 0, then the count is used to specify how many times to
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314 repeat the value of the following short, in the destination. This
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315 process proceeds until a count of 0 is found. This should decompress
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316 exactly XSIZE pixels. Note that the byte order of short data in
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317 the input file should be used, as described above.
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318 </font></p><h3><font face="Helvetica, Arial, sans-serif" size="-1">Implementation notes</font></h3>
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320 <font face="Helvetica, Arial, sans-serif" size="-1"> Implementation of both RLE and VERBATIM format for images with
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321 BPC of 1 is required since the great majority of SGI images are in
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322 this format. Support for images with a 2 BPC is encouraged.
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324 <font face="Helvetica, Arial, sans-serif" size="-1"> If the ZSIZE of an image is 1, it is assumed to represent B/W
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325 values. If the ZSIZE is 3, it is assumed to represent RGB data,
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326 and if ZSIZE is 4, it is assumed to contain RGB data with alpha.
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328 <font face="Helvetica, Arial, sans-serif" size="-1"> The origin for all SGI images is the lower left hand corner. The
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329 first scanline (row 0) is always the bottom row of the image.
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330 </font></p><h3><font face="Helvetica, Arial, sans-serif" size="-1">Naming Conventions</font></h3>
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332 <font face="Helvetica, Arial, sans-serif" size="-1"> On SGI systems, SGI image files end with the extension .bw if
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333 they are B/W images, they end in .rgb if they contain RGB image
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334 data, and end in .rgba if they are RGB images with alpha channel.
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336 <font face="Helvetica, Arial, sans-serif" size="-1"> Sometimes the .sgi extension is used as well.
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337 </font></p><h3><font face="Helvetica, Arial, sans-serif" size="-1">An example</font></h3>
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339 <font face="Helvetica, Arial, sans-serif" size="-1">This program will write out a valid B/W SGI image file:
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340 </font></p><pre><font face="Helvetica, Arial, sans-serif" size="-1"> #include "stdio.h"
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342 #define IXSIZE (23)
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343 #define IYSIZE (15)
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349 unsigned char buf[1];
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352 fwrite(buf,1,1,outf);
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357 unsigned short val;
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359 unsigned char buf[2];
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361 buf[0] = (val>>8);
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362 buf[1] = (val>>0);
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363 fwrite(buf,2,1,outf);
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366 static int putlong(outf,val)
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370 unsigned char buf[4];
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372 buf[0] = (val>>24);
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373 buf[1] = (val>>16);
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374 buf[2] = (val>>8);
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375 buf[3] = (val>>0);
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376 return fwrite(buf,4,1,outf);
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383 unsigned char outbuf[IXSIZE];
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386 of = fopen("example.rgb","w");
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388 fprintf(stderr,"sgiimage: can't open output file\n");
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391 putshort(of,474); /* MAGIC */
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392 putbyte(of,0); /* STORAGE is VERBATIM */
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393 putbyte(of,1); /* BPC is 1 */
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394 putshort(of,2); /* DIMENSION is 2 */
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395 putshort(of,IXSIZE); /* XSIZE */
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396 putshort(of,IYSIZE); /* YSIZE */
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397 putshort(of,1); /* ZSIZE */
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398 putlong(of,0); /* PIXMIN is 0 */
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399 putlong(of,255); /* PIXMAX is 255 */
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400 for(i=0; i<4; i++) /* DUMMY 4 bytes */
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402 strcpy(iname,"No Name");
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403 fwrite(iname,80,1,of); /* IMAGENAME */
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404 putlong(of,0); /* COLORMAP is 0 */
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405 for(i=0; i<404; i++) /* DUMMY 404 bytes */
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408 for(y=0; y<IYSIZE; y++) {
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409 for(x=0; x<IXSIZE; x++)
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410 outbuf[x] = (255*x)/(IXSIZE-1);
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411 fwrite(outbuf,IXSIZE,1,of);
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projects / imager.git / blob