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
=====================

SkSL is based on GLSL 4.5. For the most part, write SkSL exactly as you would
desktop GLSL, and the SkSL compiler will take care of version and dialect
differences (for instance, you always use "in" and "out", and skslc will handle
translating them to "varying" and "attribute" as appropriate). Be aware of the
following differences between SkSL and GLSL:

* "@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 will be ignored if present
* the output color is sk_FragColor (do not declare it)
* use sk_VertexID instead of gl_VertexID
* the fragment coordinate is sk_FragCoord, and is always relative to the upper
  left.
* lowp, mediump, and highp are always permitted (but will only be respected if
  you run on a device which supports them)
* you do not need to include ".0" to make a number a float (meaning that
  "vec2(x, y) * 4" is perfectly legal in SkSL, unlike GLSL where it would often
  have to be expressed "vec2(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. vec2(vec3(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, vec3) is
  equivalent to GLSL's textureProj(sampler2D, vec3)
* 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.