YCbCr continued
YCbCr continued
- Subject: YCbCr continued
- From: Brad Wright <email@hidden>
- Date: Mon, 04 Aug 2008 15:23:25 -0700
Hi Pro Apps Team:
Here's what I did. I created a core image filter that generates a
color that fills the entire screen. All this filter does is display
what ever values you feed into it for the vec4 color that is returned
(pre-multiplied of course). I then used the built in Final Cut
Scopes to read the color values back. It appears that the color
neutral point is when both U and V are at 0.42 - 0.44 depending on
the value for Y. At least that's what the Final Cut Scopes are
saying. I looked at both the RGB parade and the Vectorscope to
verify that these values line up. I'm apply this filter directly
over a video frame from an FxTexture mapped into a CIImage. I've
been through the documentation and I can't figure out how Final Cut
is mapping these values. The U and V values should stay color
neutral while the Y value varies up or down. I'm not seeing that
happening. It appears the Hue is being shifted around as the Y
varies. I'm not sure how the U and V values arrive at between 0.42
and 0.44.
The best I can figure is that Cb and Cr are mapped at between 0 - 224
without the offset that the documentation says there is.
If I assume that the color neutral point is .42-0.44 in float point,
then I can get this value by mapping:
Zero Saturation point 112
Maximum Cb and Cr value of 255
112/255 = .43921569
approximately .439
Does anyone know why this would happen?
Brad Wright
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