j2k-jpeg 0.6.1

JPEG inspect/decode and fallback encode support for j2k
Documentation 
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// SPDX-License-Identifier: Apache-2.0

//! `Gray8Writer` — 1-byte-per-pixel grayscale output. Multi-component YCbCr
//! inputs project to luminance via `Y` (ignoring chroma).

use crate::error::JpegError;
use crate::output::OutputWriter;

pub(crate) struct Gray8Writer<'o> {
    out: &'o mut [u8],
    stride: usize,
    width: u32,
}

impl<'o> Gray8Writer<'o> {
    pub(crate) fn new(out: &'o mut [u8], stride: usize, width: u32) -> Self {
        Self { out, stride, width }
    }
}

impl OutputWriter for Gray8Writer<'_> {
    fn write_rgb_row(
        &mut self,
        y: u32,
        r_row: &[u8],
        g_row: &[u8],
        b_row: &[u8],
    ) -> Result<(), JpegError> {
        let dst_start = (y as usize) * self.stride;
        let width = self.width as usize;
        let dst = &mut self.out[dst_start..dst_start + width];
        for i in 0..width {
            let r = u32::from(r_row[i]);
            let g = u32::from(g_row[i]);
            let b = u32::from(b_row[i]);
            dst[i] = ((77 * r + 150 * g + 29 * b + 128) >> 8) as u8;
        }
        Ok(())
    }

    fn write_ycbcr_row(
        &mut self,
        y: u32,
        y_row: &[u8],
        _cb_row: &[u8],
        _cr_row: &[u8],
    ) -> Result<(), JpegError> {
        let dst_start = (y as usize) * self.stride;
        let width = self.width as usize;
        self.out[dst_start..dst_start + width].copy_from_slice(&y_row[..width]);
        Ok(())
    }

    fn write_gray_row(&mut self, y: u32, gray_row: &[u8]) -> Result<(), JpegError> {
        let dst_start = (y as usize) * self.stride;
        let width = self.width as usize;
        self.out[dst_start..dst_start + width].copy_from_slice(&gray_row[..width]);
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use alloc::vec;

    #[test]
    fn copies_gray_row_verbatim() {
        let mut buf = vec![0u8; 4];
        let mut w = Gray8Writer::new(&mut buf, 4, 4);
        w.write_gray_row(0, &[10, 20, 30, 40]).unwrap();
        assert_eq!(buf, vec![10, 20, 30, 40]);
    }

    #[test]
    fn ycbcr_row_projects_to_y_channel_only() {
        let mut buf = vec![0u8; 4];
        let mut w = Gray8Writer::new(&mut buf, 4, 4);
        w.write_ycbcr_row(0, &[10, 20, 30, 40], &[250; 4], &[5; 4])
            .unwrap();
        assert_eq!(buf, vec![10, 20, 30, 40]);
    }

    #[test]
    fn respects_stride_across_rows() {
        let mut buf = vec![0u8; 16];
        let mut w = Gray8Writer::new(&mut buf, 8, 2);
        w.write_gray_row(0, &[1, 2]).unwrap();
        w.write_gray_row(1, &[3, 4]).unwrap();
        assert_eq!(buf[0..2], [1, 2]);
        assert_eq!(buf[8..10], [3, 4]);
    }
}