SkSL & Runtime Effects

Overview

SkSL is Skia’s shading language. SkRuntimeEffect is a Skia C++ object that can be used to create SkShader, SkColorFilter, and SkBlender objects with behavior controlled by SkSL code.

You can experiment with SkSL at https://shaders.skia.org/. The syntax is very similar to GLSL. When using SkSL effects in your Skia application, there are important differences (from GLSL) to remember. Most of these differences are because of one basic fact: With GPU shading languages, you are programming a stage of the GPU pipeline. With SkSL, you are programming a stage of the Skia pipeline.

In particular, a GLSL fragment shader controls the entire behavior of the GPU between the rasterizer and the blending hardware. That shader does all of the work to compute a color, and the color it generates is exactly what is fed to the fixed-function blending stage of the pipeline.

SkSL effects exist as part of the larger Skia pipeline. When you issue a canvas drawing operation, Skia (generally) assembles a single GPU fragment shader to do all of the required work. This shader typically includes several pieces. For example, it might include:

  • Evaluating whether a pixel falls inside or outside of the shape being drawn (or on the border, where it might apply antialiasing).
  • Evaluating whether a pixel falls inside or outside of the clipping region (again, with possible antialiasing logic for border pixels).
  • Logic for the SkShader on the SkPaint. The SkShader can actually be a tree of objects (due to SkShaders::Blend and other features described below).
  • Similar logic for the SkColorFilter (which can also be a tree, due to SkColorFilters::Compose, SkColorFilters::Blend, and features described below).
  • Blending code (for certain SkBlendModes, or for custom blending specified with SkPaint::setBlender).

Even if the SkPaint has a complex tree of objects in the SkShader, SkColorFilter, or SkBlender fields, there is still only a single GPU fragment shader. Each node in that tree creates a single function. The clipping code and geometry code each create a function. The blending code might create a function. The overall fragment shader then calls all of these functions (which may call other functions, e.g. in the case of an SkShader tree).

Your SkSL effect contributes a function to the GPU’s fragment shader.


Evaluating (sampling) other SkShaders

In GLSL, a fragment shader can sample a texture. With runtime effects, the object that you bind (in C++) is an SkShader, represented by a shader in SkSL. To make it clear that you are operating on an object that will emit its own shader code, you don’t use sample. Instead, the shader object has a .eval() method. Regardless, Skia has simple methods for creating an SkShader from an SkImage, so it’s easy to use images in your runtime effects:

Because the object you bind and evaluate is an SkShader, you can directly use any Skia shader, without necessarily turning it into an image (texture) first. For example, you can evaluate a linear gradient. In this example, there is no texture created to hold the gradient. Skia generates a single fragment shader that computes the gradient color, samples from the image’s texture, and then multiplies the two together:

Of course, you can even invoke another runtime effect, allowing you to combine shader snippets dynamically:


Premultiplied Alpha

When dealing with transparent colors, there are two (common) possible representations. Skia calls these unpremultiplied (what Wikipedia calls straight), and premultiplied. In the Skia pipeline, every SkShader returns premultiplied colors.

If you’re familiar with OpenGL blending, you can think of it in terms of the blend equation. For common alpha blending (called source-over), you would normally configure your blend function as (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA). Skia defines source-over blending as if the blend function were (GL_ONE, GL_ONE_MINUS_SRC_ALPHA).

Skia’s use of premultiplied alpha implies:

  • If you start with an unpremultiplied SkImage (like a PNG), turn that into an SkImageShader, and evaluate that shader… the resulting colors will be [R*A, G*A, B*A, A], not [R, G, B, A].
  • If your SkSL will return transparent colors, it must be sure to multiply the RGB by A.
  • For more complex shaders, you must understand which of your colors are premultiplied vs. unpremultiplied. Many operations don’t make sense if you mix both kinds of color together.

The image below demonstrates this: properly premultiplied colors produce a smooth gradient as alpha decreases. Unpremultipled colors cause the gradient to display incorrectly, becoming too bright and shifting hue as the alpha changes.


Coordinate Spaces

To understand how coordinates work in SkSL, you first need to understand how they work in Skia. If you’re comfortable with Skia’s coordinate spaces, then just remember that the coordinates supplied to your main() are local coordinates. They will be relative to the coordinate space of the SkShader. This will match the local space of the canvas and any localMatrix transformations. Additionally, if the shader is invoked by another, that parent shader may modify them arbitrarily.

In addition, the SkShader produced from an SkImage does not use normalized coordinates (like a texture in GLSL). It uses (0, 0) in the upper-left corner, and (w, h) in the bottom-right corner. Normally, this is exactly what you want. If you’re evaluating an SkImageShader with coordinates based on the ones passed to you, the scale is correct. However, if you want to adjust those coordinates (to do some kind of re-mapping of the image), remember that the coordinates are scaled up to the dimensions of the image: