[PD] color keying using pd and Gem for OSX

chris clepper cclepper at artic.edu
Sun Jun 22 21:20:17 CEST 2003

>	i am having limited success achieving color keying using 2 
>stream of video. Have tried using the [pix_alpha] method and can 
>achieve color keying using to picture sources
>or using video as the background and an image as the keyed 
>foreground, but replacing the foreground image with a movie does not 
>	tried converting the foreground movie using [pix_rgba] but no 
>use, it just screws up the color. using  [pix_hsv2rgb] results in 
>some sort of keying but not through the pix_alpha method just as a 
>result of shifting the color fields around.
>	i was wondering if
>		a) what i am trying to do is just wrong
>		b) some crucial step is missing
>		c) GEM implementation on OS X is lacking

which version of GEM are you using?  there's not a final release for 
OSX yet so you might be using a broken or outdated version.  try the 
one here: http://taproot.dyndns.org/~cgc/downloads/index.html  (make 
sure you grab the appropriate one for either G3 or G4 cpu).

it has an object called pix_chroma_key that i wrote and it does 
keying on both RGB and YUV sources.  it's usage is quite simple it 
takes a message for the key 'value' with three arguments for each 
color channel, and a 'range' message which is the range of values +/- 
the center value.

example for RGB:
value 150 100 50
range 20 10 10

would key using the red channel for a range of 130 to 170 the green 
from 90 to 110 and blue 40 to 50.  if this was for YUV it would be 
the same ranges for Y (luma) Cb (chroma-blue) and  Cr (chroma-red) in 
that order.

there's also a 'direction' message that flips the keyed areas.

cheers, tim
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