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In Blender's VSE (Video Sequence Editor), on trying to overlay a background video or image sequence with multiple foreground images (each containing transparency, or alpha channel, information), transparency is intermittently lost. Some images are shown with a black background, others retain their transparency.

Possible reasons?

NOTE: This question has been posted so that I can provide an answer derived from my own tests. As the question relates specifically to VSE, it has nothing to do with answers addressing 'Import Image as Plane', those referencing use of the RGBA output setting (which no longer appears to exist), nor those relating to alpha over / alpha under settings - which under these conditions have no effect.

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Blender won't handle an alpha channel in a codec. Alpha in a codec is largely a nonstandard practice.

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This answer applies to Blender's Video Sequence Editor (VSE) on Mac (in my case Yosemite).

I have found that batch pre-compressing images using ImageOptim on Mac appears to intermittently compromise any contained alpha channel (transparency) information.

This effect is not so great that it is revealed in Mac's Preview or many other graphics utilities (confusing!), but certainly has effect in Blender.

So, if removing an image background, simply avoid any use of ImageOptim prior to adding the image into Blender's VSE.

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  • $\begingroup$ When you pause on a frame that seems broken, what happens when you press the Refresh Sequencer button in the VSE timeline? Does it restore the alpha? $\endgroup$ – 3pointedit Dec 2 '17 at 12:31

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