clipka <ano### [at] anonymousorg> wrote:
> That means POV-Ray 3.7 /is/ actually doin' it plain wrong, as this 
> behavior effectively constitutes writing premultiplied alpha. I don't 
> know about TGA, but the PNG file format is explicitly specified to use 
> /non/-premultiplied alpha. Which, when viewed with some piece of 
> software that totally ignores the alpha channel, may look like crap for 
> some scenes (e.g. a glass sphere on an opaque plane), but that's how the 
> file format is specified.

  It's specified like that for a good reason. When you take a PNG with an
alpha channel and overlay it on top of something else (eg. on an image
manipulation program or on a web page), you get the proper behavior with
respect to the alpha blending.

  POV-Ray 3.6 did this right. If you rendered something with +UA and the
background (ie. nothingness) was "visible", you could then take the resulting
png and use it on top of something else (eg. on a web page) and it would just
work. The image would perfectly blend with the background (at least assuming
you have used antialiasing; also semi-transparent surfaces would work fine).

  With +UA the actual color of the background was completely ignored, as it
should be.

-- 
                                                          - Warp

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