Handle paletted images with implicit transparency values

[Drvi]

When loading paletted images, the color and transparency components for the pallet are queried separately. Apparently, there can be fewer transparency values than color values, in which case the color values without a transparency values should be opaque. Previously we assumed there is equal count of color and transparency values -- this PR fixes that.

[t-bltg]

Can't we use png_get_palette_max(png_ptr, info_ptr) documented in http://www.libpng.org/pub/png/libpng-manual.txt, and thus avoid using PNG_MAX_PALETTE_LENGTH ?

[Drvi]

I tried that and I can't get any other output than 0 (I did call it after png_read_image). I think I tried it before and gave up:). It is a good point, though, just don't know how to make it work. I'll keep the comment for now.

[Drvi]

I'll create an issue for this. If the approach you discovered (using transformations) is faster, it would be nice to get rid of this hack:)

[johnnychen94]

Looks good to me; can't say too much without a test sample to diagnose this.

[Drvi]

Hey, @t-bltg, would you mind if I add https://aws1.discourse-cdn.com/business5/uploads/julialang/original/3X/5/c/5c5cdaa477dcae516c7183c3e710ef65a0bb6ab0.png as a test case? I don't have any other image where palette color count != palette transparency count

[t-bltg]

Hey, @t-bltg, would you mind if I add https://aws1.discourse-cdn.com/business5/uploads/julialang/original/3X/5/c/5c5cdaa477dcae516c7183c3e710ef65a0bb6ab0.png as a test case?

Of course you can ! Its size makes it ideal for testing ;)

[t-bltg]

I tried that and I can't get any other output than 0 (I did call it after png_read_image).

So num_palette_max can only be set in png_do_check_palette_indexes (pngtrans.c) private function.
This method is only called in png_do_read_transformations (pngrtran.c)
In turn, png_do_read_transformations is called in png_read_row (pngread.c) only if any png_ptr->transformations is set, which is not the case in our implementation, when read_as_paletted=true.

I can make it work by applying a transformation and if the whole palette computation block is moved after png_read_image in _load!.