Colour filters

A colour filter runs on the pixels a draw (or layer) produces, before mask blur, so the blur spreads the filtered result.

paint.color_filter = Some(ColorFilter::Matrix([
    0.2126, 0.7152, 0.0722, 0.0, 0.0,
    0.2126, 0.7152, 0.0722, 0.0, 0.0,
    0.2126, 0.7152, 0.0722, 0.0, 0.0,
    0.0, 0.0, 0.0, 1.0, 0.0,
]));

paint.setColorFilter(ColorFilter.matrix(new Float32Array([
  0.2126, 0.7152, 0.0722, 0, 0,
  0.2126, 0.7152, 0.0722, 0, 0,
  0.2126, 0.7152, 0.0722, 0, 0,
  0, 0, 0, 1, 0,
])));

Left to right: unfiltered, greyscale matrix, violet Modulate blend.

Matrix

A row-major 4×5 transform over straight colour in 0..=1. Each output channel is row · [r, g, b, a, 1], then clamped (Skia's SkColorMatrix).

Flutter's ColorFilter.matrix hands the translation column in 0..=255. Divide entries 4, 9, 14, and 19 by 255 before they arrive here.

Blend

ColorFilter::Blend(color, mode) composites a constant source colour over each produced pixel. mode is any BlendMode.

paint.color_filter = Some(ColorFilter::Blend(
    Color::rgb(0.49, 0.29, 1.0),
    BlendMode::Modulate,
));

paint.setColorFilter(ColorFilter.blend(0.49, 0.29, 1, 1, BlendMode.Modulate));

On a solid paint the filter can fold into the colour on the CPU. On an image shader it bakes a filtered texture; on a direct image draw it runs in the fragment.