Function find_chessboard_corners 

pub fn find_chessboard_corners(
    gray: &[u8],
    w: usize,
    h: usize,
    pattern_w: usize,
    pattern_h: usize,
) -> Option<Vec<(f32, f32)>>

Detect a pattern_w x pattern_h (inner corners) chessboard in a grayscale image. Returns the inner corners row-major, or None if no board is found.

pattern_w/pattern_h are the inner-corner counts (e.g. 9x6).