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lvandeve / lodepng

PNG encoder and decoder in C and C++.
zlib License
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Would it be possible to support ARGB? #15

Closed LinusU closed 9 years ago

LinusU commented 9 years ago

Hi, thank for a great library!

I was wondering if it would be possible to add LCT_ARGB to support storing the raw data with the order ARGB instead of RGBA. This is the format that cairo uses natively.

Having this would save me from doing the conversion in memory before hand, which I would guess is much slower.

Thanks!

LinusU commented 9 years ago

Or maybe solve it by setting some other options/flags. libpng has

           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
lvandeve commented 9 years ago

Hello,

The decision is for LodePNG to not do color conversions other than to exactly those models that exist in the PNG specification.

The reason for this is that there exist billions of possible color models, so converting to them would require a library of its own.

In your case, would it work for you to load to RGBA, and then use a for loop to swap the bytes to the order you want? This should not cost speed, since, if LodePNG did it internally it had to do the same thing anyway.

Just to give an idea about the amount of possible color models: -any permutation of order of R, G, B and A, almost anything is getting used somewhere, ARGB, ABGR, RGBA, BGRA, ... -whether or not A is present -bits per pixel or per individual component (e.g. there exists 565 RGB and so on) -whether alpha is premultiplied -whether scanlines end at bytes or can end at bits in between, or even at a multiple of 4 bytes like BMP does -whether it's mirrored (like BMP also does) -and this didn't even get into YUV, HSL, and so on :)

So supporting anything outside of the PNG spec would mean having to support them all, and really that's outside of the scope of an image loading library imho :)

Regards,

-Lode

LinusU commented 9 years ago

Cool, yeah I actually realised after posting that those corresponded. Thats why I switched to options flags.

But I think that you are right, it shouldn't be the responsibility of lodepng to do that. Thank you for your time.