Function rgb_frame 

fn rgb_frame(rgba: &[u8], width: u32, height: u32) -> Rgb

Available on crate feature snapshot only.

Turn an RGBA8 frame into the RGB the metric reads.

The metric works in XYB, which it derives from linear RGB, so the values handed over are the sRGB ones the frame holds: the transfer characteristic named here is what linearizes them, and naming it is also what says the frame is sRGB rather than something else.