On 10.10.2017 10:10, clipka wrote:
> Am 10.10.2017 um 09:27 schrieb Stephen:
> 
>>> I seem to remember to have had problems generating transparent png
>>> images while I never had problems with tga.
>>>
>>
>> I recognised the form of the spell. I had the same problem donkey's
>> years ago.
>> There was a time when PNGs behaved badly. I think it was not writing the

>> Alpha values in the output.
> 
> If I may clarify:
> 
> PNG output in POV-Ray v3.6 had the alpha mode wrong - i.e. how the alpha
> channel is supposed to interact with the colour channels.
> 
> The PNG specification mandates so-called "straight alpha", i.e. the
> colour channels should hold the colour as if the image was fully opaque,
> and blending should be done entirely by the display software, using
> Alpha:(1-Alpha) as the ratio between stored colour and background. This
> is akin to the way in which semi-transparent pigments are specified in
> POV-Ray's SDL.
> 
> However, POV-Ray's internal computations yield colour channel values
> that already take opacity into account, which is akin to so-called "gay
> alph..." uh, I mean, "associated alpha" aka "premultiplied alpha", in
> which the display software is only responsible for the background
> portion of the blending, using 1:(1-Alpha) as the ratio between stored
> colour and background.
> 
> POV-Ray v3.6 neglected the issue of alpha mode entirely: In file output
> it directly wrote the computed colour values, thus always writing in
> associated alpha mode (which happens to be wrong for PNG files). In file
> input, on the other hand, it interpreted the colour values in much the
> same way as in SDL, thus always reading in straight alpha mode (which
> happens to be correct for PNG files but wrong for certain other files).
> 
> So not only was POV-Ray v3.6 in violation of the PNG file format specs,
> it also was inconsistent in itself.
> 
> In POV-Ray v3.7, a proper alpha handling framework was implemented, and
> the problem fixed for PNG files.
> 
> 
> (Interestingly, even though the modes are referred to as "associated
> alpha" and "straight alpha", it's not actually the alpha channel that's
> different between the two modes -- the value stored is actually
> identical for both modes -- but the colour channel.)
> 
> 
> The advantage of "straight alpha" is that you can display an opaque
> version of the image simply by ignoring the alpha channel.
> 
> The advantages of "associated alpha" are that (a) it takes less
> computational effort to display, and (b) it can account for situations
> where bright light is to be shown against a highly transparent
> background (e.g. a transparent sphere with highlights).
> 
Interesting insight, thanks. :-)

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