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Lode

PNG filter

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I'm still busy trying to make a png decoder, and while reading the png spec, a question arised. It isn't really clear, but as far as I understand, when using RGB color, the scanlines are stored in the order RGBRGBRGBRGB and not RRRRGGGGBBBB. There's a filter applied to the bytes of scanlines, and it really works on bytes, not on piwels. And that's what made me have this question: because the filter works on bytes, and only uses neighbouring bytes, it seems to me as if you're predicting the G value out of the R and B value next to it, instead of using the G value of the neighbouring pixels. Wouldn't the latter be better for compression, since the G value of two neighbouring pixels is likely to be close to each other?

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The png spec in sectio 9.2 refers to "the byte corresponding to x in the pixel immediately before the pixel containing x". So when filtering the G byte x, a refers to the G value of the previous pixel, 3 bytes back (in 24 bit color mode).

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