| Overview |
| ======== |
| |
| 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 |
| ===================== |
| |
| * 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.integerSupport. 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.integerSupport) |
| 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 |
| * the output color is sk_FragColor (do not declare it) |
| * 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!) |