Function composite_solid 

pub fn composite_solid(
    dest: [u8; 4],
    src_rgb: [u8; 3],
    src_a: u8,
    coverage: u8,
    mode: BlendMode,
) -> [u8; 4]

Composite a straight RGBA8 source pixel over a premultiplied destination pixel, blending with mode and scaling the source by coverage.

The same composite as composite_premultiplied, entered one step earlier — and that step is not free: premultiplying a straight colour and un-premultiplying it back quantises it, because a premultiplied byte at alpha a can only express a + 1 of the 256 straight values. Every caller holding a straight colour — every Brush::Solid — must use this; a source already premultiplied (an image sample, a composited layer) has no straight colour to preserve and uses composite_premultiplied.

use pdfrum_page::BlendMode;
use pdfrum_render::blend::composite_solid;

let dest = [0, 0, 255, 255];

// A straight colour, entered one step earlier so the round trip
// through premultiplication cannot quantise it.
assert_eq!(
    composite_solid(dest, [221, 0, 0], 255, 255, BlendMode::default()),
    [221, 0, 0, 255],
);