I'm working on a video in the sequencer, and as part of this I want to overlay a scene containing some text. The text is rendered via node-based shading, and in particular uses a "clouds" texture to define the alpha-value of the final composite.
If I render the scene by itself, or set the relevant scene-strip to render using a mode like "replace", all looks as expected. See here:
However, if I set the scene-strip to render using the "alpha-over" mode, then I get a strange artefact: in places where the scene is completely transparent, I see the colour of whatever was present there (if anything), and using what looks to be additive blending!
Here you can see the same text rendered over a simple blank "colour" strip:
And here over a video-strip; note how the areas that one might expect to be fully-transparent seem to "glow" against the background colour, as though blended additively:
I'm really stuck as to what might be causing this.
It looks almost as though, when presented with an alpha-value of zero, the "alpha-over" blending switches to blending the RGB colour additively with full alpha--but that makes little sense.
It also occurred to me that perhaps I was getting negative alpha-values due to the application of the "clouds" texture, and in particular my handling of it.
But if that were so, I would expect simple high-opacity rendering, not this odd additive-like rendering. And furthermore, the alpha-values go through a "math" node that has the "clamp" check-box ticked, so it should, presumably, only be getting values between 0 and 1.
So I'm stuck. Does anyone here know what might be the problem, and how I might fix it? :/
 Oh, I forgot to mention: this is using Blender 2.78c, I believe.
[edit 2] Amusingly, the actual additive-blending strip-mode works as expected, with fully-transparent areas being invisible.
[edit 3] I've also discovered that I can get the expected result if I place the background on top of the text-scene and set the former to use the "alpha under" mode. If called for, this might at least provide a workaround.
[edit 4] It seems that if I add a very small amount to the final alpha-value given to the "composite" node in my "compositing" node-system, the problem disappears. It really does look like there's an issue with alpha values of zero, specifically, for some reason. o_0
[edit 5] A stripped-down version of the blend-file: Test Blend-file
In this version, there's just a simple plane being rendered over a colour-strip. Note that, when the scene alone is rendered in the "Default" layout, it has transparency as expected. However, when rendered as part of the video in the "Video Editing" layout, the transparency shows the above-described issue.
Video Editing is set up in the "Video Editing" layout, and compositing in the "Default" layout.