How can i write yuv420p to rgb Skia Shader with SKSL Language?

[cangunaydin]

Hello,
I have 3 byte arrays which presents y, u and v plane. Since yuv to rgb conversion takes long time, i want to do the conversion on gpu side.
There are many yuv to rgb conversion algorithms around the internet. So my first try is below, I am inspired from this thread. #1319 (comment)

 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);
                            }";
                using var effect = SKRuntimeEffect.Create(src, out var errorText);

                if (!string.IsNullOrEmpty(errorText))
                {
                    Console.WriteLine(errorText);
                    return;
                }


                using var yBufferData = SKData.CreateCopy(_frame.YBuffer);
                using var yImage =
                    SKImage.FromPixels(
                        new SKImageInfo(_frame.FrameWidth, _frame.FrameHeight, SKColorType.Gray8),
                        yBufferData);
                
                using var uBufferData = SKData.CreateCopy(_frame.UBuffer);
                using var uImage =
                    SKImage.FromPixels(
                        new SKImageInfo(_frame.FrameWidth / 2, _frame.FrameHeight / 2, SKColorType.Gray8),
                        uBufferData);

                var vBufferData = SKData.CreateCopy(_frame.VBuffer);
                using var vImage =
                    SKImage.FromPixels(
                        new SKImageInfo(_frame.FrameWidth / 2, _frame.FrameHeight / 2, SKColorType.Gray8),
                        vBufferData);
                
                
                using var yImageShader = yImage.ToShader();
                using var uImageShader = uImage.ToShader();
                using var vImageShader = vImage.ToShader();


                var children = new SKRuntimeEffectChildren(effect)
                {
                    { "yImage", yImageShader },
                    { "uImage", uImageShader },
                    { "vImage", vImageShader }
                };
                
                var uniforms = new SKRuntimeEffectUniforms(effect);
                uniforms["canvasResolution"] = new[] { _frame.CanvasWidth,_frame.CanvasHeight,1.0f  };
                uniforms["frameResolution"] = new[] { _frame.FrameWidth, _frame.FrameHeight,1.0f };
                
                using var shader = effect.ToShader(true, uniforms, children);
                using var paint = new SKPaint();
                paint.Shader = shader;
                canvas.DrawPaint(paint);

the outputs you can see here.

I am pretty sure that there is nothing wrong with math formula. Cause i use the same formula in an opengl shader. And it works fine over there. However the problem is more likely about how i pass the data to the sampler. What i do in opengl was sth like this.

so i use red channel only. But here i don't have that option. Instead i use one channel grayscale. So my assumption is to get the data from r channel. But it turns out that after sampling g and b values also exists. So sth wrong with channel or how i pass the byte arrays to gpu or how i approach to the problem :)

I appreciate any feedback and help. Thank you very much.

[cangunaydin]

thanks to help from skia conversation groups i figured it out.
you can see the discussion over here.
https://groups.google.com/g/skia-discuss/c/OSySAGVVwKw/m/OzWTBrKiAAAJ?utm_medium=email&utm_source=footer
Basically coordinates comes as pixel units to shader so you need to scale uv coordinates with 1/2

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;

hope it can help someone