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.sampleVariablesSupport. 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* you do not need to include ".0" to make a number a float (meaning that
49  "float2(x, y) * 4" is perfectly legal in SkSL, unlike GLSL where it would
50  often have to be expressed "float2(x, y) * 4.0". There is no performance
51  penalty for this, as the number is converted to a float at compile time)
52* type suffixes on numbers (1.0f, 0xFFu) are both unnecessary and unsupported
53* creating a smaller vector from a larger vector (e.g. float2(float3(1))) is
54  intentionally disallowed, as it is just a wordier way of performing a swizzle.
55  Use swizzles instead.
56* Use texture() instead of textureProj(), e.g. texture(sampler2D, float3) is
57  equivalent to GLSL's textureProj(sampler2D, float3)
58* some built-in functions and one or two rarely-used language features are not
59  yet supported (sorry!)
60
61SkSL is still under development, and is expected to diverge further from GLSL
62over time.
63
64
65SkSL Fragment Processors
66========================
67
68********************************************************************************
69*** IMPORTANT: You must set gn arg "skia_compile_processors = true" to cause ***
70*** .fp files to be recompiled! In order for compilation to succeed, you     ***
71*** must run bin/fetch-clang-format (once) to install our blessed version.   ***
72********************************************************************************
73
74An extension of SkSL allows for the creation of fragment processors in pure
75SkSL. The program defines its inputs similarly to a normal SkSL program (with
76'in' and 'uniform' variables), but the 'main()' function represents only this
77fragment processor's portion of the overall fragment shader.
78
79Within an '.fp' fragment processor file:
80
81* C++ code can be embedded in sections of the form:
82
83  @section_name { <arbitrary C++ code> }
84
85  Supported section are:
86    @header            (in the .h file, outside the class declaration)
87    @headerEnd         (at the end of the .h file)
88    @class             (in the .h file, inside the class declaration)
89    @cpp               (in the .cpp file)
90    @cppEnd            (at the end of the .cpp file)
91    @constructorParams (extra parameters to the constructor, comma-separated)
92    @constructor       (replaces the default constructor)
93    @initializers      (constructor initializer list, comma-separated)
94    @emitCode          (extra code for the emitCode function)
95    @fields            (extra private fields, each terminated with a semicolon)
96    @make              (replaces the default Make function)
97    @clone             (replaces the default clone() function)
98    @setData(<pdman>)  (extra code for the setData function, where <pdman> is
99                        the name of the GrGLSLProgramDataManager)
100    @test(<testData>)  (the body of the TestCreate function, where <testData> is
101                        the name of the GrProcessorTestData* parameter)
102    @coordTransform(<sampler>)
103                       (the matrix to attach to the named sampler2D's
104                        GrCoordTransform)
105    @samplerParams(<sampler>)
106                       (the sampler params to attach to the named sampler2D)
107* global 'in' variables represent data passed to the fragment processor at
108  construction time. These variables become constructor parameters and are
109  stored in fragment processor fields. By default float2/half2 maps to SkPoints,
110  and float4/half4 maps to SkRects (in x, y, width, height) order. Use ctype
111  (below) to override this default mapping.
112* global variables support an additional 'ctype' layout key, providing the type
113  they should be represented as from within the C++ code. For instance, you can
114  use 'layout(ctype=GrColor4f) in half4 color;' to create a variable that looks
115  like a half4 on the SkSL side of things, and a GrColor4f on the C++ side of
116  things.
117* 'uniform' variables become, as one would expect, top-level uniforms. By
118  default they do not have any data provided to them; you will need to provide
119  them with data via the @setData section.
120* 'in uniform' variables are uniforms that are automatically wired up to
121  fragment processor constructor parameters. The fragment processor will accept
122  a parameter representing the uniform's value, and automatically plumb it
123  through to the uniform's value in its generated setData() function.
124* the 'sk_TransformedCoords2D' array provides access to 2D transformed
125  coordinates. sk_TransformedCoords2D[0] is equivalent to calling
126  fragBuilder->ensureCoords2D(args.fTransformedCoords[0]) (and the result is
127  cached, so you need not worry about using the value repeatedly).
128* Uniform variables support an additional 'when' layout key.
129  'layout(when=foo) uniform int x;' means that this uniform will only be
130  emitted when the 'foo' expression is true.
131* 'in' variables support an additional 'key' layout key.
132  'layout(key) uniform int x;' means that this uniform should be included in
133  the program's key. Matrix variables additionally support 'key=identity',
134  which causes the key to consider only whether or not the matrix is an
135  identity matrix.
136
137
138Creating a new .fp file
139=======================
140
1411. Ensure that you have set gn arg "skia_compile_processors = true"
1422. Create your new .fp file, generally under src/gpu/effects.
1433. Add the .fp file to sksl.gni.
1444. Build Skia. This will cause the .fp file to be compiled, resulting in a new
145   .cpp and .h file for the fragment processor.
1465. Add the .cpp and .h files to gpu.gni.
1476. Add the new processor's ClassID (k<ProcessorName>_ClassID) to
148   GrProcessor::ClassID.
1497. At this point you can reference the new fragment processor from within Skia.
150
151Once you have done this initial setup, simply re-build Skia to pick up any
152changes to the .fp file.