The idea is supposed to be that gray_to_rgb is only concerned with the special
case of turning Y->RGB and YA->RGBA (I'm using Y to signify greyscale
luminance). If it's a 2-channel image where the second channel isn't alpha
(let's call it JK), in general you don't want to wreck it by changing it to a
4-channel image JJJK, that would be confusing.
So it seems like the real bug might be that when the original image is read,
it's not setting alpha_channel correctly, so it's seeing a 2-channel image but
not recognizing it as the YA special case.
In png_pvt.h, there is code circa line 7:
if (spec.nchannels == 2) {
// Special case: PNG spec says 2-channel image is Gray & Alpha
spec.channelnames[0] = "Y";
spec.channelnames[1] = "A";
spec.alpha_channel = 1;
}
that sure makes it seem like any 2-channel PNG image that is input will
necessarily know that channel 1 in alpha, so I'm not sure where things go wrong.
Can you maybe send me an image that exhibits this problem? I'd like to trace
through the logic and see how and why in the PNG gets confused about what the
channels mean. (Or, feel free to take a stab at that yourself as well.)
> On Jul 4, 2017, at 12:40 PM, Stefan Werner <[email protected]> wrote:
>
> Hi,
>
> I ran across one texture that I can’t quite figure out how to deal with it
> correctly using TextureSystem. The original texture is a gray scale PNG with
> alpha channel, and I’m converting it with maketx. I’m trying to get that back
> as RGBA via the gray_to_rgb=1 option in the texture cache, but I always get
> the gray channel as R and the alpha channel as G, with B and A empty/default.
>
> I stepped through the code, and I see that in
> TextureSystemImpl::fill_gray_channels(), line 848, the case for two channel
> input files only does the conversion when spec.alpha_channel = 1. When I use
> TIFF for my cache file format, spec.alpha_channel is -1, when I use OpenEXR
> as cache file format, spec.alpha_channel is 0. Maketx apparently only marks
> TIFF as containing an alpha channel when there are 4 or more channels, see
> tiffoutput.cpp line 485. OpenEXR does set the channel name to alpha.
>
> Is there a reason that fill_gray_channels() checks for index of the alpha
> channel rather than just its presence? Is there any way that I can get this
> texture to be read as RGBA through the texture cache?
>
> Thanks,
> Stefan
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--
Larry Gritz
[email protected]
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