[PD] GEM : fx to make persons appear only when they move

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Hi list,

I' trying to make an effect on a live camera feed with GEM for an 
installation in which only persons who move appear on a kind of video 
mirror, as chameleonTv of effecTv 
(http://effectv.sourceforge.net/chameleon.html)
I tryed severals ways :
 > with pix_background and pix_blob, modulating the alpha value of the 
rectangle on which the video is textured in function of the quantity of 
movement but if one person moves, everybody appears.
 > with pix_multiblob and combination of pix_image pix_rectangle and 
pix_mask, the [pix_multiblob 2] seems to eat all the processor, even if 
I [pix_resize 320 240] the image before the multiblob.
 > accumulating many (25) pix_movement and using the result by 
pix_masking it with  the live feed dig a hole in the person that moves a 
little. Maybe I did not the "average" of alpha channel of the 25 
previous frame as I thought, due to the fact that pix_movement works on 
the previous frame ?
is there a better way with opengl instructions ? I tryed to look in that 
direction but I didn't manage yet to find  how to analyze and affect the 
pixels of a live feed....
thanks for any advice
benjamin



for the tech note : the video feed is from a PCI capture card 
(AlchemyTv) connected in s-video to a dv camera with [pix_video 720 576] 
on a G5 10.4 Pd ext7,
for the first trial, I manage to mix this video feed with another DV 
camera connected in firewire, putting into a buffer an image background, 
filling and playing another buffer with PAL images, and playing a non 
compressed video of the same size  >>> output on a second screen 
800x600, with all that, the motion capture was really fast, only 40% of 
the proc used, it was really impressive (and stable), great dev work, thks



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