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The API is extremely inaccurate for sRGB conversions because it uses an 8-bit intermediate linear value and it does not use the sRGB transform, so it suffers from the known instability in gamma transforms for values close to 0 (see Poynton). The net result is that the calculation has a maximum error of 14.99/255; 0.5/255^(1/2.2). pngstest now uses 15 for the permitted 8-bit error. This may still not be enough because of arithmetic error.