Custom Fragment Shaders with SkiaSharp - glsl

Question

I'm evaluating SkiaSharp for use in a project which allows for rendering some vector elements on top of a photo. SkiaSharp looks well suited to rendering the vector elements, however I also want to apply transformations to the image -- tone-curve adjustments like contrast or exposure, for example.
The ideal scenario would be a way to execute custom GLSL fragment shader code when rendering the rectangle containing the image (eg an SKColorFilter subclass that just wraps GLSL code, to be applied when calling DrawBitmap, or similar).
It would probably also work to have my existing GLSL code render to a texture, and then have SkiaSharp draw a rectangle using that texture as its contents. However, I can't see a way to do this without a GPU read-back, which feels like it would be prohibitively slow.
What's the best way forward here? More precisely: either, how can I apply transformations to the pixel data of an image, on the GPU, in SkiaSharp, or, what else could I use to provide SkiaSharp like vector rendering primitives and also allow GLSL fragment-shader-like pixel transformations?

Answer 1

You could have a look at some really early previews of the v2.84 series. They have a new SKRuntimeEffect that you can make use of.
Check out the preview feed: https://aka.ms/skiasharp-eap/index.json
You must have a look at the builds for the https://github.com/mono/SkiaSharp/pull/1321 PR:
v2.84.0-pr.1321.*
See this example: https://github.com/mono/SkiaSharp/issues/1319#issuecomment-640529824
// input values
SKCanvas canvas = ...;
float threshold = 1.05f;
float exponent = 1.5f;
// shader
var src = #"
in fragmentProcessor color_map;
uniform float scale;
uniform half exp;
uniform float3 in_colors0;
void main(float2 p, inout half4 color) {
half4 texColor = sample(color_map, p);
if (length(abs(in_colors0 - pow(texColor.rgb, half3(exp)))) < scale)
discard;
color = texColor;
}";
using var effect = SKRuntimeEffect.Create(src, out var errorText);
// input values
var inputs = new SKRuntimeEffectInputs(effect);
inputs.Set("scale", threshold);
inputs.Set("exp", exponent);
inputs.Set("in_colors0", new[] { 1f, 1f, 1f });
// shader values
using var blueShirt = SKImage.FromEncodedData(Path.Combine(PathToImages, "blue-shirt.jpg"));
using var textureShader = blueShirt.ToShader();
var children = new SKRuntimeEffectChildren(effect);
children.Set("color_map", textureShader);
// create actual shader
using var shader = effect.ToShader(inputs, children, true);
// draw as normal
canvas.Clear(SKColors.Black);
using var paint = new SKPaint { Shader = shader };
canvas.DrawRect(SKRect.Create(400, 400), paint);