1 Overview 2 ======== 3 4 SkSL ("Skia Shading Language") is a variant of GLSL which is used as Skia's 5 internal shading language. SkSL is, at its heart, a single standardized version 6 of GLSL which avoids all of the various version and dialect differences found 7 in GLSL "in the wild", but it does bring a few of its own changes to the table. 8 9 Skia uses the SkSL compiler to convert SkSL code to GLSL, GLSL ES, or SPIR-V 10 before handing it over to the graphics driver. 11 12 Differences from GLSL 13 ===================== 14 15 SkSL is based on GLSL 4.5. For the most part, write SkSL exactly as you would 16 desktop GLSL, and the SkSL compiler will take care of version and dialect 17 differences (for instance, you always use "in" and "out", and skslc will handle 18 translating them to "varying" and "attribute" as appropriate). Be aware of the 19 following differences between SkSL and GLSL: 20 21 * GLSL caps can be referenced via the syntax 'sk_Caps.<name>', e.g. 22 sk_Caps.sampleVariablesSupport. The value will be a constant boolean or int, 23 as appropriate. As SkSL supports constant folding and branch elimination, this 24 means that an 'if' statement which statically queries a cap will collapse down 25 to the chosen branch, meaning that: 26 27 if (sk_Caps.externalTextureSupport) 28 do_something(); 29 else 30 do_something_else(); 31 32 will compile as if you had written either 'do_something();' or 33 'do_something_else();', depending on whether that cap is enabled or not. 34 * no #version statement is required, and will be ignored if present 35 * the output color is sk_FragColor (do not declare it) 36 * use sk_VertexID instead of gl_VertexID 37 * the fragment coordinate is sk_FragCoord, and is always relative to the upper 38 left. 39 * lowp, mediump, and highp are always permitted (but will only be respected if 40 you run on a device which supports them) 41 * you do not need to include ".0" to make a number a float (meaning that 42 "vec2(x, y) * 4" is perfectly legal in SkSL, unlike GLSL where it would often 43 have to be expressed "vec2(x, y) * 4.0". There is no performance penalty for 44 this, as the number is converted to a float at compile time) 45 * type suffixes on numbers (1.0f, 0xFFu) are both unnecessary and unsupported 46 * creating a smaller vector from a larger vector (e.g. vec2(vec3(1))) is 47 intentionally disallowed, as it is just a wordier way of performing a swizzle. 48 Use swizzles instead. 49 * Use texture() instead of textureProj(), e.g. texture(sampler2D, vec3) is 50 equivalent to GLSL's textureProj(sampler2D, vec3) 51 * some built-in functions and one or two rarely-used language features are not 52 yet supported (sorry!) 53 54 SkSL is still under development, and is expected to diverge further from GLSL 55 over time. 56
