POV-Ray versions beginning with 3.70  have changed the way 'Output_Alpha' works
when rendering an image to have a transparent background (for compositing later
for example.) As the included documentation says, "In version 3.7 alpha handling
for image file output has changed. Effectively, the background *now requires* a
filter or transmit value in order for alpha transparency to work properly." And
the resulting PNG image file has a nice alpha channel mask.

I have a suggestion to improve this feature-- although not for a 'typical'
Output_Alpha render of a scene.

Instead, let's say I've rendered a 'straight' scene-- i.e., NO Output_Alpha or
transparency, just the usual kind of render. Or better yet, make it a 1000-frame
animation. A further detail is that the animation has taken *days* to complete,
with AA,  radiosity, etc. etc. But after the entire animation is finished, I
notice that one small section of the scene has a visual problem, for whatever
reason; maybe I want to replace a small object or its color that I didn't notice
before, simply because I don't like it.

Instead of re-rendering the entire full-scene animation to include this change,
I decide to pick a small subsection of the frame to re-render--just that area
for 1000 frames, to save lots of rendering time-- the idea being to 'composite'
that small area into the animation later (either within POV-Ray itself as simple
layered image_maps, or in some other app.) For this to work, the 1000 small
subsection frames all need to have an alpha-channel mask (and in the proper
location, of course)-- with the entire area around the rectangular section being
completely transparent.

But there's no *direct* way of doing this in v3.7xx, as far as I know (although
there are possible workarounds later, when compositing). When turning
Output_Alpha on and choosing a small area of a frame to render, the resulting
alpha channel is completely white (i.e., completely opague, no transparency) and
can't be used as-is. This is different behavior from pre-3.7xx versions--
although v3.6xx had its own kind of problem, IIRC.

BTW, I do understand why this happens in 3.7xx: There is no 'filter or transmit'
value for the scene's 'background'-- as there's NO background in this case, in
the usual meaning. But it would be nice to have an additional 'different'
behavior for Output_Alpha, when rendering a subsection of an image.

**I do realize that changing an object or its color for subsequent re-animation
would probably create a subtle difference in that small area's radiosity
lighting-- but that's a different problem ;-) **

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