Function save_opaque 

pub fn save_opaque(
    path: impl AsRef<Path>,
    w: u32,
    h: u32,
    rgba: &[u8],
) -> Result<()>

Save an RGBA8 buffer as an opaque image: the alpha channel is ignored.

This is what frame capture uses. The alpha left in a window’s back buffer is whatever the last blend wrote, and is not meant to be seen.