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[devel] Revised commentary about png_set_alpha_mode() to make it clearer

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that this only affects how composited pixels are returned to the calling
application; they don't affect the pixels that are written to a PNG file.
This commit is contained in:
Glenn Randers-Pehrson 2011-05-08 22:56:42 -05:00
parent 96cec0e1a3
commit 44e26d4149
1 changed files with 11 additions and 7 deletions
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18
png.h
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@ -1118,15 +1118,19 @@ PNG_EXPORT(35, void, png_build_grayscale_palette, (int bit_depth,
#ifdef PNG_READ_ALPHA_MODE_SUPPORTED #ifdef PNG_READ_ALPHA_MODE_SUPPORTED
/* How the alpha channel is interpreted - this affects how the color channels of /* How the alpha channel is interpreted - this affects how the color channels of
* a PNG file are output when an alpha channel, or tRNS chunk is a palette file, * a PNG file are returned when an alpha channel, or tRNS chunk in a palette
* is present. * file, is present.
* *
* The default is to output data according to the PNG specification: the alpha * This has no effect on the way pixels are written into a PNG output
* datastream. The color samples in a PNG datastream are never premultiplied
* with the alpha samples.
*
* The default is to return data according to the PNG specification: the alpha
* channel is a linear measure of the contribution of the pixel to the * channel is a linear measure of the contribution of the pixel to the
* corresponding output pixel. The gamma encoded color channels must be scaled * corresponding composited pixel. The gamma encoded color channels must be
* according to the contribution and to do this it is necessary to undo the * scaled according to the contribution and to do this it is necessary to undo
* encoding, scale the color values, perform the composition and reencode the * the encoding, scale the color values, perform the composition and reencode
* values. This is the 'PNG' format. * the values. This is the 'PNG' mode.
* *
* The alternative is to 'associate' the alpha with the color information by * The alternative is to 'associate' the alpha with the color information by
* storing color channel values that have been scaled by the alpha. The * storing color channel values that have been scaled by the alpha. The