1Overview 2======== 3 4SkSL ("Skia Shading Language") is a variant of GLSL which is used as Skia's 5internal shading language. SkSL is, at its heart, a single standardized version 6of GLSL which avoids all of the various version and dialect differences found 7in GLSL "in the wild", but it does bring a few of its own changes to the table. 8 9Skia uses the SkSL compiler to convert SkSL code to GLSL, GLSL ES, or SPIR-V 10before handing it over to the graphics driver. 11 12 13Differences from GLSL 14===================== 15 16* Precision modifiers are not used. 'float', 'int', and 'uint' are always high 17 precision. New types 'half', 'short', and 'ushort' are medium precision (we 18 do not use low precision). 19* Vector types are named <base type><columns>, so float2 instead of vec2 and 20 bool4 instead of bvec4 21* Matrix types are named <base type><columns>x<rows>, so float2x3 instead of 22 mat2x3 and double4x4 instead of dmat4 23* "@if" and "@switch" are static versions of if and switch. They behave exactly 24 the same as if and switch in all respects other than it being a compile-time 25 error to use a non-constant expression as a test. 26* GLSL caps can be referenced via the syntax 'sk_Caps.<name>', e.g. 27 sk_Caps.canUseAnyFunctionInShader. The value will be a constant boolean or int, 28 as appropriate. As SkSL supports constant folding and branch elimination, this 29 means that an 'if' statement which statically queries a cap will collapse down 30 to the chosen branch, meaning that: 31 32 if (sk_Caps.externalTextureSupport) 33 do_something(); 34 else 35 do_something_else(); 36 37 will compile as if you had written either 'do_something();' or 38 'do_something_else();', depending on whether that cap is enabled or not. 39* no #version statement is required, and it will be ignored if present 40* the output color is sk_FragColor (do not declare it) 41* use sk_Position instead of gl_Position. sk_Position is in device coordinates 42 rather than normalized coordinates. 43* use sk_PointSize instead of gl_PointSize 44* use sk_VertexID instead of gl_VertexID 45* use sk_InstanceID instead of gl_InstanceID 46* the fragment coordinate is sk_FragCoord, and is always relative to the upper 47 left. 48* use sk_Clockwise instead of gl_FrontFacing. This is always relative to an 49 upper left origin. 50* you do not need to include ".0" to make a number a float (meaning that 51 "float2(x, y) * 4" is perfectly legal in SkSL, unlike GLSL where it would 52 often have to be expressed "float2(x, y) * 4.0". There is no performance 53 penalty for this, as the number is converted to a float at compile time) 54* type suffixes on numbers (1.0f, 0xFFu) are both unnecessary and unsupported 55* creating a smaller vector from a larger vector (e.g. float2(float3(1))) is 56 intentionally disallowed, as it is just a wordier way of performing a swizzle. 57 Use swizzles instead. 58* Swizzle components, in addition to the normal rgba / xyzw components, can also 59 be LTRB (meaning "left/top/right/bottom", for when we store rectangles in 60 vectors), and may also be the constants '0' or '1' to produce a constant 0 or 61 1 in that channel instead of selecting anything from the source vector. 62 foo.rgb1 is equivalent to float4(foo.rgb, 1). 63* All texture functions are named "sample", e.g. sample(sampler2D, float3) is 64 equivalent to GLSL's textureProj(sampler2D, float3). 65* Functions support the 'inline' modifier, which causes the compiler to ignore 66 its normal inlining heuristics and inline the function if at all possible 67* some built-in functions and one or two rarely-used language features are not 68 yet supported (sorry!) 69