I have a Bitmap file type Format8bppIndexed. I need to get by JavaScript the value for each pixels (0-255), which corresponds to the index color of the palette. I tried with getImageData, but the returned value is not the color index, but the RGBA value of the painted pixel color.
Get color Index of (indexed) Bitmap using JavaScript
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You need to use the corresponding palette definitions. The position in the Format8bppIndexed bitmap (technically bitplanes) will return the index for the palette.
Canvas is always RGBA format, each component 8-bit. 24-bit (or 32-bit if you include alpha) do not need a palette as the values are directly related to the RGB buffer.
To convert a RGB value into a palette index, you need the palette, then you need a way to find the shortest distance from RGB to the value the index represent (f.ex. using
r*r+g*g+b*bfor each index value, the shortest distance difference between the RGB and the RGB of the palette wins).