SkSL-Overview.md

Overview

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SkSL ("Skia Shading Language") is a variant of GLSL which is used as Skia's internal shading language. SkSL is, at its heart, a single standardized version of GLSL which avoids all of the various version and dialect differences found in GLSL "in the wild", but it does bring a few of its own changes to the table.

Skia uses the SkSL compiler to convert SkSL code to GLSL, GLSL ES, or SPIR-V before handing it over to the graphics driver.

Differences from GLSL

• The entry point is vec4 main( vec2 fragcoord ) instead of void mainImage( out vec4 fragColor, in vec2 fragCoord ), when translating you will need to change the fragColor assignment to a return() statement.
• Macros (#define) are not supported. Either convert them to constats or functions.
• Precision modifiers are not used. float, int, and uint are always high precision. New types half, short, and ushort are medium precision (we do not use low precision).
• Vector types are named <base type><columns>, so float2 instead of vec2 and bool4 instead of bvec4
• Matrix types are named <base type><columns>x<rows>, so float2x3 instead of mat2x3 and double4x4 instead of dmat4.
• "@if" and "@switch" are static versions of if and switch. They behave exactly the same as if and switch in all respects other than it being a compile-time error to use a non-constant expression as a test.
• GLSL caps can be referenced via the syntax sk_Caps.\<name\>, e.g. sk_Caps.canUseAnyFunctionInShader. The value will be a constant boolean or int, as appropriate. As SkSL supports constant folding and branch elimination, this means that an if statement which statically queries a cap will collapse down to the chosen branch, meaning that:

    if (sk_Caps.externalTextureSupport)
        do_something();
    else
        do_something_else();

will compile as if you had written either do_something(); or do_something_else();, depending on whether that cap is enabled or not.

• No #version statement is required, and it will be ignored if present
• Use sk_Position instead of gl_Position. sk_Position is in device coordinates rather than normalized coordinates.
• Use sk_PointSize instead of gl_PointSize
• Use sk_VertexID instead of gl_VertexID
• Use sk_InstanceID instead of gl_InstanceID
• The fragment coordinate is sk_FragCoord, and is always relative to the upper left.
• Use sk_Clockwise instead of gl_FrontFacing. This is always relative to an upper left origin.
• You do not need to include .0 to make a number a float (meaning that float2(x, y) * 4 is perfectly legal in SkSL, unlike GLSL where it would often have to be expressed float2(x, y) * 4.0. There is no performance penalty for this, as the number is converted to a float at compile time)
• Type suffixes on numbers (1.0f, 0xFFu) are both unnecessary and unsupported
• Creating a smaller vector from a larger vector (e.g. float2(float3(1))) is intentionally disallowed, as it is just a wordier way of performing a swizzle. Use swizzles instead.
• Swizzle components, in addition to the normal rgba / xyzw components, can also be LTRB (meaning "left/top/right/bottom", for when we store rectangles in vectors), and may also be the constants 0 or 1 to produce a constant 0 or 1 in that channel instead of selecting anything from the source vector. foo.rgb1 is equivalent to float4(foo.rgb, 1).
• All texture functions are named "sample", e.g. sample(sampler2D, float3) is equivalent to GLSL's textureProj(sampler2D, float3).
• Functions support the inline modifier, which causes the compiler to ignore its normal inlining heuristics and inline the function if at all possible
• Some built-in functions and one or two rarely-used language features are not yet supported (sorry!)

Refer to the full documetation for more details.