().

xy

The value returned from FlutterFragCoord is distinct from gl_FragCoord.
gl_FragCoord provides the screen space coordinates and should generally be
avoided to ensure that shaders are consistent across backends. When targeting a
Skia backend, the calls to gl_FragCoord are rewritten to access local
coordinates but this rewriting isn’t possible with Impeller.

Colors

There isn’t a built-in data type for colors. Instead they are commonly
represented as a vec4 with each component corresponding to one of the RGBA
color channels.

The single output fragColor expects that the color value is normalized to be
in the range of 0.0 to 1.0 and that it has premultiplied alpha. This is
different than typical Flutter colors which use a 0-255 value encoding and
have unpremultipled alpha.

Samplers