void draw(SkCanvas* canvas) {

  // Make a very wide-gamut offscreen surface:

  auto rec2020 = SkColorSpace::MakeRGB(SkNamedTransferFn::kLinear,

                                       SkNamedGamut::kRec2020);

  auto info = SkImageInfo::Make(128, 64, kRGBA_F16_SkColorType,

                                kPremul_SkAlphaType, rec2020);

  auto surface = SkSurfaces::Raster(info);

​

  // Effect draws horizontal gradients. Top half uses the simple uniform,

  // bottom half uses a uniform tagged as a color:

  const char* sksl = R"(

                  uniform vec4 not_a_color;

    layout(color) uniform vec4 color;

​

    vec4 main(vec2 xy) {

      vec4 c = xy.y < 32 ? not_a_color : color;

      return (c * (xy.x / 128)).rgb1;

    })";

​

  auto [effect, err] = SkRuntimeEffect::MakeForShader(SkString(sksl));

​

  // Set both uniforms to be "red":

  SkRuntimeShaderBuilder builder(effect);

  builder.uniform("not_a_color") = SkV4{ 1, 0, 0, 1 };  // Red?

  builder.uniform("color")       = SkV4{ 1, 0, 0, 1 };  // sRGB Red

​

  // Fill the offscreen surface:

  SkPaint paint;

  paint.setShader(builder.makeShader());

  surface->getCanvas()->drawPaint(paint);

​

  // Draw our offscreen image back to the original canvas:

  canvas->drawImage(surface->makeImageSnapshot(), 0, 0);

}