[PD] Alpha+pix_film

IOhannes m zmoelnig zmoelnig at iem.at
Mon Jan 26 11:13:18 CET 2009

info at thesaddj.com wrote:
> HI everybody,
> thanks to all, i solved the issue. But is still not clear to
> me what's happening.
> I mixed the different suggestions you gave me (cause
> independently no one was working) and i got this:
> [gemhead]
> |
> [pix_film]/[colorspace RGB(
> |
> [pix_alpha]
> |
> [colorRGB]
> |
> [alpha]
> |
> [pix_texture]
> |
> [rectangle]
> it looks a bit weird but it's working and my cpu is drinking
> a coffee while i mix four videos of 160/200mb each.
> but I'm really keen to understand where was my problem...
> as Philippe said i have to send the [colorspace RGB( message
> first and then load videos. 
> [pix_alpha] seems to be the obj making the work done, but

well, [pix_alpha] will overwrite whatever alpha is in the channel.
so if the original alpha is 0 (full transparency), [pix_alpha] will 
eventually raise this to 1 (full opacity), thus making your object not 

what bothers me a bit is the use of RGB rather than RGBA in the 
colorspace message....

> anyway i had to add [colorRGB] to be able to change
> transparency. 

well, that was the original idea, wasn't it?

> @IOhannes: i'm on windows xp pro. do you think it could be
> because of this?

don't know, it just makes it easier to debug...(e.g. on osx the default 
colorspace is YUV, having no alpha-channel at all, which would make it 
unlikely that each pixel would have an alpha of 0)
which movie are you opening? an AVI, a QuickTime movie?

could you share it for testing purposes? or does it happen with the 
example movies that come with Gem as well?

> what do you mean with video-importer?

i mean the actual backend that reads the file and passes the frame to 
e.g. the Gem code that accesses QuickTime/W32 or DirectShow or gmerlin.
if the movie does not contain an alpha-channel, "somebody" has to 
provide values for alpha when doing RGBA; there might be a bug there...

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