LodePNG unnecessarily remaps colours when reading a paletted PNG (requesting LCT_PALETTE paletted ouput)

[rversteegen]

I have a png file with a palette, and the palette contains duplicate colors. I read the file, passing

	state.info_raw.colortype = LCT_PALETTE
	state.info_raw.bitdepth = 8

to lodepng_decode. (I want both the value of each pixel, as a palette index, and the palette.) Unfortunately, lodepng_decode performs an unnecessary call to lodepng_convert, which causes the original palette indices to be lost.

The problem is this check in lodepng_color_mode_equal:

  /*if one of the palette sizes is 0, then we consider it to be the same as the
  other: it means that e.g. the palette was not given by the user and should be
  considered the same as the palette inside the PNG.*/
  if(1/*a->palettesize != 0 && b->palettesize != 0*/) {

In this case a->palettesize is 0, because I didn't pass a palette to lodepng_decode.
I don't know why this is commented out; the git log is uninformative. Uncommenting it fixes the bug for me but maybe it breaks something else.

Luckily there is a very easy workaround:

	state.decoder.color_convert = 0

[rversteegen]

Oh, and here's a testcase. Some examples of pixels:
at (0,0): lodepng_decode returns 194, should be 144
at (2,1): lodepng_decode returns 200, should be 152
at (3,1): lodepng_decode returns 208, should be 0

(BTW, very nice library, and extremely well documented. Thank you!)

[lvandeve]

Thanks for reporting!

I removed the "if(1)" and found a better location to fix it. d03d7df should fix it