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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.sampleVariablesSupport. 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
* 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.
* Use texture() instead of textureProj(), e.g. texture(sampler2D, float3) is
equivalent to GLSL's textureProj(sampler2D, float3)
* some built-in functions and one or two rarely-used language features are not
yet supported (sorry!)
SkSL is still under development, and is expected to diverge further from GLSL
over time.
SkSL Fragment Processors
========================
********************************************************************************
*** IMPORTANT: You must set gn arg "skia_compile_processors = true" to cause ***
*** .fp files to be recompiled! In order for compilation to succeed, you ***
*** must run bin/fetch-clang-format (once) to install our blessed version. ***
********************************************************************************
An extension of SkSL allows for the creation of fragment processors in pure
SkSL. The program defines its inputs similarly to a normal SkSL program (with
'in' and 'uniform' variables), but the 'main()' function represents only this
fragment processor's portion of the overall fragment shader.
Within an '.fp' fragment processor file:
* C++ code can be embedded in sections of the form:
@section_name { <arbitrary C++ code> }
Supported section are:
@header (in the .h file, outside the class declaration)
@headerEnd (at the end of the .h file)
@class (in the .h file, inside the class declaration)
@cpp (in the .cpp file)
@cppEnd (at the end of the .cpp file)
@constructorParams (extra parameters to the constructor, comma-separated)
@constructor (replaces the default constructor)
@initializers (constructor initializer list, comma-separated)
@emitCode (extra code for the emitCode function)
@fields (extra private fields, each terminated with a semicolon)
@make (replaces the default Make function)
@clone (replaces the default clone() function)
@setData(<pdman>) (extra code for the setData function, where <pdman> is
the name of the GrGLSLProgramDataManager)
@test(<testData>) (the body of the TestCreate function, where <testData> is
the name of the GrProcessorTestData* parameter)
@coordTransform(<sampler>)
(the matrix to attach to the named sampler2D's
GrCoordTransform)
@samplerParams(<sampler>)
(the sampler params to attach to the named sampler2D)
* global 'in' variables represent data passed to the fragment processor at
construction time. These variables become constructor parameters and are
stored in fragment processor fields. By default float2/half2 maps to SkPoints,
and float4/half4 maps to SkRects (in x, y, width, height) order. Use ctype
(below) to override this default mapping.
* global variables support an additional 'ctype' layout key, providing the type
they should be represented as from within the C++ code. For instance, you can
use 'layout(ctype=GrColor4f) in half4 color;' to create a variable that looks
like a half4 on the SkSL side of things, and a GrColor4f on the C++ side of
things.
* 'uniform' variables become, as one would expect, top-level uniforms. By
default they do not have any data provided to them; you will need to provide
them with data via the @setData section.
* 'in uniform' variables are uniforms that are automatically wired up to
fragment processor constructor parameters. The fragment processor will accept
a parameter representing the uniform's value, and automatically plumb it
through to the uniform's value in its generated setData() function.
* the 'sk_TransformedCoords2D' array provides access to 2D transformed
coordinates. sk_TransformedCoords2D[0] is equivalent to calling
fragBuilder->ensureCoords2D(args.fTransformedCoords[0]) (and the result is
cached, so you need not worry about using the value repeatedly).
* Uniform variables support an additional 'when' layout key.
'layout(when=foo) uniform int x;' means that this uniform will only be
emitted when the 'foo' expression is true.
* 'in' variables support an additional 'key' layout key.
'layout(key) uniform int x;' means that this uniform should be included in
the program's key. Matrix variables additionally support 'key=identity',
which causes the key to consider only whether or not the matrix is an
identity matrix.
Creating a new .fp file
=======================
1. Ensure that you have set gn arg "skia_compile_processors = true"
2. Create your new .fp file, generally under src/gpu/effects.
3. Add the .fp file to sksl.gni.
4. Build Skia. This will cause the .fp file to be compiled, resulting in a new
.cpp and .h file for the fragment processor.
5. Add the .cpp and .h files to gpu.gni.
6. Add the new processor's ClassID (k<ProcessorName>_ClassID) to
GrProcessor::ClassID.
7. At this point you can reference the new fragment processor from within Skia.
Once you have done this initial setup, simply re-build Skia to pick up any
changes to the .fp file.
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