[PD] snapshot misteries, bang to gemhead

Matteo Sisti Sette matteosistisette at gmail.com
Mon Dec 7 13:16:12 CET 2009

Yes the issue is indeed much simpler and [pix_snap] is not involved.

Attached is a very simple patch with a square and a sphere. The bang 
forces a render. If you move the sphere far behind the square and hit 
the bang, you can see a "flash" of the red sphere just close to the eye, 
as if it was in the opposite position than usual.

Is it normal, and if so, what is the explanation???

Matteo Sisti Sette escribió:
> Hi,
> I'm working on a patch (not the attached one which is an example that 
> isolates the problem), where from time to time I need to take a snapshot 
> of the whole scene and "draw" it as a texture on a rectangle which is on 
> the background.
> (when the patch is finished I think I will be allowed to share it, for 
> what it's worth)
> I have it all working fine, but than I came into a problem: sometimes, I 
> have to send a message to take the snapshot, and immediately after, send 
> some other message that changes something on the scene, which has to 
> happen AFTER the snapshot.
> Since I am taking snapshots in the "classic" way, that is, opening a 
> spigot on the gemlist that comes to the [pix_snap], the snapshot is 
> actually deferred until the next render; this is a problem because I 
> need to "defer" any other message that has to take effet just after the 
> sbapshot; this can be done but i'd like to avoid all the complications.
> I thought that a solution would be to force a render just after any 
> snapshot, by sending a bang to the gemhead that is rendering everithing.
> However, the result is not what I expected and I really can't understand 
> why.
> That's why I am asking for your help.
> So, the attached patch is a simplification, in which I draw an initially 
> white square and a sphere. If you hit the big bang (sorry for the pun), 
> a snapshot of the scene is taken via [pix_snap] and it becomes the 
> texture of the square. You can use the number box to move the sphere 
> around.
> Now, if I make the connection where it says "connect this", what I am 
> doing is just provoking a bang to the gemhead just after opening the 
> spigot to the pix_snap. What I would expect is that it should work 
> exactly the same as before (with the difference that an extra frame is 
> rendered between two "normal" frames, and that's the moment the snapshot 
> is taken; but this difference should not be "visible").
> However, the observed behaviour is not this. The snapshot that is taken 
> and "printed" on the texture is not the scene, but some aberration of 
> it. If you leave the sphere wher it is, it will usually render and snap 
> an all-black scene; but if you move the sphere to the back, behind the 
> square, the snapshot will take the sphere very near to the "eye"......
> I really don't understand. Doesn't the bang to the gemhead provoke a 
> full render of everything just the same as it is rendered when a new 
> frame occurs? Then why is the scene rendered differently when I do the 
> bang????
> I guess this could be further isolated to something regarding the bang 
> to gemhead, i.e. without involving pix_snap; of course with a "fast" 
> enough to see what happens during a single extra frame....
> Thanks in advance
> By the way, thanks to all who gave me example patches some days ago 
> which have helped me building this patch.
> m.

Matteo Sisti Sette
matteosistisette at gmail.com
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