Created attachment 290061 [details]
this is how the ass images look like (BGRA) before blending

i'm trying to render ass subtitles on top of "empty" frames so that i can blit them afterwards, expecting that the pixels that actually have subtitles rendered become visible, and the rest ("the background") stays transparent.

currently, the output frames keep the input alpha everywhere in the image, so i can either blit nothing, or mask my video with the background color.

the problem may be that the assrender blitting function tries to create a premultiplied bitmap. i did some funny ascii art output though ^_^ 
it looks like in case of a fully transparent input pixel (alpha = 0x00), it doesn't do premultiplication but instead keeps the alpha.
that's visible in the log
(i'm simply doing a 
printf("%02x%02x%02x%02x ",dst[0],dst[1],dst[2],dst[3]);
in the blit function)

however, the transparency byte of the ass images seems to get overwritten somewhere later on during the composition process, maybe the gst_video_overlay_composition_blend operation