Yuv420p to Rgb Conversion with SKSL

[Can Günaydın]

Hello,
I am quite new to Skia, 
I have yuv image 3 byte arrays. I want to delegate the conversion( to rgb) of the image to gpu if it is possible. I have tried this shader code.

 var src = @"

uniform shader yImage;
uniform shader uImage;
uniform shader vImage;
uniform float3 canvasResolution;
uniform float3 frameResolution;
half4 main(float2 coord) {
float2 scale = frameResolution.xy / canvasResolution.xy;

half y = sample(yImage,coord * scale).r - 0.0625;
half u = sample(uImage,coord * scale).r - 0.5;
half v = sample(vImage,coord * scale).r - 0.5;

half r = y + 1.596 * v;
half g = y - 0.813 * v - 0.391 * u;
half b = y + 2.018 * u;
return half4(r,g,b,1.0);
}";

this is giving an output as you see below.

I use single channel. I have tried Alpha8 and GrayScale, but same result. what can be wrong any idea about it? Cause same math formula works when i try it on opengl. 

[Brian Salomon]

Skia's image shaders generally take the sample position in pixel units. It looks like you need to scale up the U/V plane sampling by a factor of 2 to account for the subsampling.

Not that Skia has APIs that can ingest YUV planes and make a single SkImage from them that is converted to RGB on each draw (SkImages::TextureFromYUVAPixmaps and SkImages::TextureFromYUVATextures).

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[Brian Salomon]

Sorry, scale the UV coords by 1/2.

[Can Günaydın]

You make my day, I was struggling with this for 2 days. thank you super much.  code below makes it work.

half y = sample(yImage,coord * scale).r - 0.0625;

half u = sample(uImage,(coord / 2) * scale).r - 0.5;
half v = sample(vImage,(coord / 2) * scale).r - 0.5;

[Jim Van Verth]

I'll reiterate what Brian said, that SkImages::TextureFromYUVAPixmaps and SkImages::TextureFromYUVATextures can take care of this for you. The Ganesh GPU backend also handles odd sizes and any adjustment of sampling to handle siting (Graphite fixes are coming soon).

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/c97839a9-90e7-4ea0-ac43-64e686773e4dn%40googlegroups.com.

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Jim Van Verth | Software Engineer | Google.com

[Can Günaydın]

Thank you for the information. For the project that i am working on, i was using skiasharp, but as i understand SkImages::TextureFromYUVAPixmaps and SkImages::TextureFromYUVATextures is not adopted by SkiaSharp. (or maybe it is but i couldn't find it)
Now to check what can i do more with skia, i have built it and i am doing some experiments on it. I will definitely try with SkImages::TextureFromYUVAPixmaps and SkImages::TextureFromYUVATextures. If i can manage to work, my plan is to adopt it with a custom build. Thanks again for all your help.

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/CAM5o3G3xE_hWdp4r8W_9HQJ1p5oOkDED97MzwzcS9L08gqHvxw%40mail.gmail.com.