Need Help with my Chroma Key Pixel Shader Algorithm

Need Help with my Chroma Key Pixel Shader Algorithm | Microsoft Share Point

Need Help with my Chroma Key Pixel Shader Algorithm | Microsoft Share Point — Sat, 16 May 2009 02:01:40 GMT

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re: Need Help with my Chroma Key Pixel Shader Algorithm

Ted Howard — Sat, 16 May 2009 13:31:33 GMT

I would try converting to HSV color space in the pixel shader and having a separate tolerance for each component (use a float3 constant and there's no perf diff). You can either calculate it or use a texture as a lookup table (not sure about Silverlight 3's texture/shader compat). HSV is more intuitive and should give a better distance function. There are other color spaces you could try. YUV might work well but it's designed to deemphasize chrominance.

http://en.wikipedia.org/wiki/HSV_color_space

You should probably use a continuous function for alpha. Anything but a step should give a nice improvement.

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re: Need Help with my Chroma Key Pixel Shader Algorithm

Chris Cavanagh — Tue, 19 May 2009 09:21:16 GMT

Very very nice! :) I attempted a WPF version a while ago, but HLSL / shader model 2.0 didn't allow enough statements to accomplish RGB -> HSV -> RGB conversion. Wonder if it could be done with multiple shaders + inputs somehow? (or maybe some close approximation of HSV?). I'll be watching this with great interest :)

re: Need Help with my Chroma Key Pixel Shader Algorithm

Mark — Wed, 20 May 2009 12:21:13 GMT

Interesting boko here on Chroma:

http://books.google.co.nz/books?id=IpSRykrRamgC&pg=PA22&lpg=PA22&dq=HSV+chroma+tolerance&source=bl&ots=ptiPVPtJyC&sig=ryfxFXqnx4Epu45HBYb1fGk3mPs&hl=en&ei=RMcTSpveKovW6gOR-s28Dg&sa=X&oi=book_result&ct=result&resnum=1#PPA23,M1

Definitely worth trying HSV

There is a great HSV -> RGB library here:

http://www.markbetz.net/2009/01/21/color-tools-for-silverlight-2/

re: Need Help with my Chroma Key Pixel Shader Algorithm

Ted Howard — Mon, 25 May 2009 22:18:46 GMT

Relative luminance (http://en.wikipedia.org/wiki/Luminance_%28relative%29) is a linear conversion from RGB, but it just makes the threshold more intuitive.

Preprocessing is the way to go ideally, but I'm sure that's not an option. In general, for big calculations in shaders, you try using a texture as a look-up-table for parts of the calculations that won't fit in shader limits, but I don't think that helps here.

re: Need Help with my Chroma Key Pixel Shader Algorithm

slyi — Thu, 23 Jul 2009 03:30:09 GMT

Is this any better? http://daron.yondem.com/tr/PermaLink.aspx?guid=84d6a965-6c16-453f-be33-d89fd49fad1d