oxipng/src/reduction/bit_depth.rs

use crate::colors::{BitDepth, ColorType};
use crate::headers::IhdrData;
use crate::png::PngImage;

/// Attempt to reduce a 16-bit image to 8-bit, returning the reduced image if successful
#[must_use]
pub fn reduced_bit_depth_16_to_8(png: &PngImage) -> Option<PngImage> {
    if png.ihdr.bit_depth != BitDepth::Sixteen {
        return None;
    }

    // Reduce from 16 to 8 bits per channel per pixel
    if png.data.chunks(2).any(|pair| pair[0] != pair[1]) {
        // Can't reduce
        return None;
    }

    Some(PngImage {
        data: png.data.iter().step_by(2).cloned().collect(),
        ihdr: IhdrData {
            color_type: png.ihdr.color_type.clone(),
            bit_depth: BitDepth::Eight,
            ..png.ihdr
        },
    })
}

/// Attempt to reduce an 8/4/2-bit image to a lower bit depth, returning the reduced image if successful
#[must_use]
pub fn reduced_bit_depth_8_or_less(png: &PngImage, mut minimum_bits: usize) -> Option<PngImage> {
    assert!((1..8).contains(&minimum_bits));
    let bit_depth = png.ihdr.bit_depth as usize;
    if minimum_bits >= bit_depth || bit_depth > 8 || png.channels_per_pixel() != 1 {
        return None;
    }
    // Calculate the current number of pixels per byte
    let ppb = 8 / bit_depth;

    if let ColorType::Indexed { palette } = &png.ihdr.color_type {
        // We can easily determine minimum depth by the palette size
        let required_bits = match palette.len() {
            0..=2 => 1,
            3..=4 => 2,
            5..=16 => 4,
            _ => 8,
        };
        if required_bits >= bit_depth {
            // Not reducable
            return None;
        } else if required_bits > minimum_bits {
            minimum_bits = required_bits;
        }
    } else {
        // Finding minimum depth for grayscale is much more complicated
        let mut mask = (1 << minimum_bits) - 1;
        let mut divisions = 1..(bit_depth / minimum_bits);
        for &b in &png.data {
            if b == 0 || b == 255 {
                continue;
            }
            'try_depth: loop {
                let mut byte = b;
                // Loop over each pixel in the byte
                for _ in 0..ppb {
                    // Align the first pixel division with the mask
                    byte = byte.rotate_left(minimum_bits as u32);
                    // Each potential division of this pixel must be identical to successfully reduce
                    let compare = byte & mask;
                    for _ in divisions.clone() {
                        // Align the next division with the mask
                        byte = byte.rotate_left(minimum_bits as u32);
                        if byte & mask != compare {
                            // This depth is not possible, try the next one up
                            minimum_bits <<= 1;
                            if minimum_bits == bit_depth {
                                return None;
                            }
                            mask = (1 << minimum_bits) - 1;
                            divisions = 1..(bit_depth / minimum_bits);
                            continue 'try_depth;
                        }
                    }
                }
                break;
            }
        }
    }

    let mut reduced = Vec::with_capacity(png.data.len());
    let mask = (1 << minimum_bits) - 1;
    for line in png.scan_lines(false) {
        // Loop over the data in chunks that will produce 1 byte of output
        for chunk in line.data.chunks(bit_depth / minimum_bits) {
            let mut new_byte = 0;
            let mut shift = 8;
            for &(mut byte) in chunk {
                // Loop over each pixel in the byte
                for _ in 0..ppb {
                    // Align the current pixel with the mask
                    byte = byte.rotate_left(bit_depth as u32);
                    shift -= minimum_bits;
                    // Take the low bits of the pixel and shift them into the output byte
                    new_byte |= (byte & mask) << shift;
                }
            }
            reduced.push(new_byte);
        }
    }

    // If the image is grayscale we also need to reduce the transparency pixel
    let color_type = if let ColorType::Grayscale {
        transparent_shade: Some(trans),
    } = png.ihdr.color_type
    {
        let reduced_trans = (trans & 0xFF) >> (bit_depth - minimum_bits);
        // Verify the reduction is valid by restoring back to original bit depth
        let mut check = reduced_trans;
        let mut bits = minimum_bits;
        while bits < bit_depth {
            check = check << bits | check;
            bits <<= 1;
        }
        // If the transparency doesn't fit the new bit depth it is therefore unused - set it to None
        ColorType::Grayscale {
            transparent_shade: if trans == check {
                Some(reduced_trans)
            } else {
                None
            },
        }
    } else {
        png.ihdr.color_type.clone()
    };

    Some(PngImage {
        data: reduced,
        ihdr: IhdrData {
            color_type,
            bit_depth: (minimum_bits as u8).try_into().unwrap(),
            ..png.ihdr
        },
    })
}