Summary

SkImageFilters::RuntimeShader allows running custom SkSL shader programs as image filters, enabling users to write arbitrary GPU-accelerated image processing effects (custom blurs, distortions, color grading, edge detection, etc.) and compose them with built-in filters in the image filter graph.

This API was introduced in Skia m98 and enhanced with a maxSampleRadius parameter in m116 for proper boundary handling when shaders sample neighboring pixels.

Why this matters

Today, SkiaSharp users who want custom image filter effects must either:

• Build a shader and wrap it manually with SKImageFilter.CreateShader (which does not receive the filtered input image — it is a leaf filter)
• Chain multiple built-in filters together, which is limited to pre-defined operations

CreateRuntimeShader bridges this gap: it lets any SkSL shader receive the filtered image as a child shader input, process it per-pixel on the GPU, and participate in the filter composition graph. This is the most powerful image filter factory — essentially "run any SkSL as a filter."

Use cases

• Custom blur variants — directional blur, tilt-shift, lens blur
• Distortion effects — ripple, swirl, barrel distortion, heat haze
• Color grading — film LUTs, split toning, selective color adjustment
• Edge detection and outline — Sobel, Laplacian, artistic outlines
• Compositing — custom blend modes beyond built-in options
• Post-processing pipelines — chain with built-in blur/shadow/color filters

API surface

// Apply a runtime shader as an image filter
SKImageFilter.CreateRuntimeShader(
    SKRuntimeShaderBuilder builder,
    string childShaderName,
    SKImageFilter? input = null);

// With explicit sample radius for shaders that read neighboring pixels
SKImageFilter.CreateRuntimeShader(
    SKRuntimeShaderBuilder builder,
    float maxSampleRadius,
    string childShaderName,
    SKImageFilter? input = null);

Upstream references

• SkImageFilters::RuntimeShader (C++ header)
• Added in Skia m98, maxSampleRadius added in m116

Part of #3680