Function write_pgm 

pub fn write_pgm<P: Pixel, W: Write>(
    bitmap: &Bitmap<P>,
    out: W,
) -> Result<(), EncodeError>

Write bitmap to out as a binary PGM (P5) image.

The sink out is consumed; wrap in std::io::BufWriter if buffering is needed.

Errors

Returns EncodeError::UnsupportedMode for non-grayscale modes — use write_ppm for colour bitmaps. Returns EncodeError::Io on any I/O failure.