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Make decode() produce Vec<[u8; 4]> instead of Vec<u8>. #25

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Merged
bschwind merged 2 commits into from
Sep 23, 2025
Merged

Make decode() produce Vec<[u8; 4]> instead of Vec<u8>. #25

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283715f
Make `decode()` produce `Vec<[u8; 4]>` instead of `Vec<u8>`.
kpreid Sep 13, 2025
e3a8baf
Document option of using `Vec::into_flattened()`.
kpreid Sep 22, 2025
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33 changes: 18 additions & 15 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -677,7 +677,7 @@ fn defilter(
fn process_scanlines(
header: &PngHeader,
scanline_data: &mut [u8],
output_rgba: &mut [u8],
output_rgba: &mut [[u8; 4]],
ancillary_chunks: &AncillaryChunks,
pixel_type: PixelType,
) -> Result<(), DecodeError> {
Expand Down Expand Up @@ -720,15 +720,11 @@ fn process_scanlines(
for (idx, (r, g, b, a)) in scanline_iter.enumerate() {
let (output_x, output_y) = (idx, y);

let output_idx =
(output_y as u64 * header.width as u64 * 4) + (output_x as u64 * 4);
let output_idx = (output_y as u64 * header.width as u64) + (output_x as u64);
let output_idx: usize =
output_idx.try_into().map_err(|_| DecodeError::IntegerOverflow)?;

output_rgba[output_idx] = r;
output_rgba[output_idx + 1] = g;
output_rgba[output_idx + 2] = b;
output_rgba[output_idx + 3] = a;
output_rgba[output_idx] = [r, g, b, a];
}

last_scanline.copy_from_slice(current_scanline);
Expand Down Expand Up @@ -846,14 +842,11 @@ fn process_scanlines(
};

let output_idx =
(output_y as u64 * header.width as u64 * 4) + (output_x as u64 * 4);
(output_y as u64 * header.width as u64) + (output_x as u64);
let output_idx: usize =
output_idx.try_into().map_err(|_| DecodeError::IntegerOverflow)?;

output_rgba[output_idx] = r;
output_rgba[output_idx + 1] = g;
output_rgba[output_idx + 2] = b;
output_rgba[output_idx + 3] = a;
output_rgba[output_idx] = [r, g, b, a];
}

last_scanline.copy_from_slice(current_scanline);
Expand Down Expand Up @@ -886,7 +879,16 @@ fn paeth_predictor(a: i16, b: i16, c: i16) -> u8 {
}
}

pub fn decode(bytes: &[u8]) -> Result<(PngHeader, Vec<u8>), DecodeError> {
/// Decodes the provided PNG into RGBA pixels.
///
/// The returned [`PngHeader`] contains the image’s size, and other PNG metadata which is not
/// necessary to make use of the pixels (the returned format is always 8-bit-per-component RGBA).
///
/// The returned [`Vec`] contains the pixels, represented as `[r, g, b, a]` arrays.
/// Its length will be equal to `header.width * header.height`.
Comment thread
bschwind marked this conversation as resolved.
/// (If you need a `Vec<u8>` of length `header.width * header.height * 4` instead, you can use
/// [`Vec::into_flattened()`] to convert it.)
pub fn decode(bytes: &[u8]) -> Result<(PngHeader, Vec<[u8; 4]>), DecodeError> {
if bytes.len() < PNG_MAGIC_BYTES.len() {
return Err(DecodeError::MissingBytes);
}
Expand Down Expand Up @@ -931,7 +933,7 @@ pub fn decode(bytes: &[u8]) -> Result<(PngHeader, Vec<u8>), DecodeError> {
})?;

// For now, output data is always RGBA, 1 byte per channel.
let mut output_rgba = vec![0u8; header.width as usize * header.height as usize * 4];
let mut output_rgba = vec![[0u8; 4]; header.width as usize * header.height as usize];

process_scanlines(
&header,
Expand Down Expand Up @@ -962,7 +964,7 @@ mod tests {
if extension.to_ascii_lowercase().as_str() == "png" {
let png_bytes = std::fs::read(&path).unwrap();

let (_header, decoded) = if path
let (_header, decoded): (PngHeader, Vec<[u8; 4]>) = if path
.file_stem()
.expect("expected png path to be a file")
.to_string_lossy()
Expand All @@ -973,6 +975,7 @@ mod tests {
} else {
decode(&png_bytes).unwrap()
};
let decoded: Vec<u8> = decoded.into_flattened();

// Uncomment to inspect output.png for debugging.
// let image_buf: image::ImageBuffer<image::Rgba<u8>, _> =
Expand Down
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