File:IBM16 Palette Map.png

Portable Network Graphics image, 515 pixels wide, 3089 pixels high, 8 bits per sample, RGB with alpha channel, 3780 pixels per metre (96 pixels per inch) resolution, 1/2.2 (0.45455) gamma, sRGB with relative colorimetric rendering intent, background colour #000000. Created with The GIMP 2.2.8, Netpbm 10.27.0 and pngcrush 1.5.10 on Tuesday the 23rd of May 2006.

With a range of 0–3, the values for Red, Green and Blue (on the left side) can be off, one third, two thirds or full (intensity of 0, 1/3, 2/3, 1). Obviously, having recurring decimals, it would not be practical to draw those numbers in the image itself; and vulgar fractions would be too similar to the binary values to be worth it (the binary number converted to decimal and divided by 3 gives the intensity). The stepping should be upscaled to the target bit-depth, which in this image is 8 bits per sample, giving 0 (hexadecimal 00), 85 (55), 170 (AA) and 255 (FF). The values 84 (54), 168 (A8) and 252 (FC) you see ever so often are the result of doing, from a 6-bit value, left-shift + zero-fill instead of linear scaling or left bit replication:

101010 (2A) → 10101010 (AA) : correct value
101010 (2A) → 10101000 (A8) : rounding error

See also the section Sample depth scaling of ISO/IEC 15948:2003 (section 12.5 on the W3C edition).

For any PNG screenshot from MS-DOS that uses the IBM16 palette or a subset of it, ie any CGA or EGA image (but not VGA), it is recommended to add an sBIT (significant bits) chunk with a value of 2 for each of Red, Green and Blue. In addition, the colour values should be checked for accuracy and fixed if erroneous (see above for the most common inaccuracy).

• Image:IBM16_Palette_Map-reduced.png is a reduced-colour version (no antialising, palette with only one transparency entry) for decoders that ignore the alpha channel or generally for where resources are short.

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