SkSL & Runtime Effects
SkSL is Skia’s
SkRuntimeEffect is a Skia C++ object that can be used to create
SkBlender objects with behavior controlled by
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
SkShadercan actually be a tree of objects (due to
SkShaders::Blendand other features described below).
- Similar logic for the
SkColorFilter(which can also be a tree, due to
SkColorFilters::Blend, and features described below).
- Blending code (for certain
SkBlendModes, or for custom blending specified with
Even if the
SkPaint has a complex tree of objects in the
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
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
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
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:
When dealing with transparent colors, there are two (common)
Skia calls these unpremultiplied (what Wikipedia calls straight), and
premultiplied. In the Skia pipeline, every
SkShader returns premultiplied
If you’re familiar with OpenGL blending, you can think of it in terms of the
blend equation. For common alpha blending (called
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
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
- 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.
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
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,
(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: