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Add EncoderParams to make predictor transform optional #111

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Add EncoderParams to make predictor transform optional #111

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4738af3
Add EncoderParams for disabling predictor transform
w-flo Sep 18, 2024
7b74002
Test libwebp roundtrip result and different EncoderParams
w-flo Sep 18, 2024
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102 changes: 78 additions & 24 deletions src/encoder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -347,6 +347,24 @@ fn write_run<W: Write>(
Ok(())
}

/// Allows fine-tuning some encoder parameters.
///
/// Pass to [`WebPEncoder::set_params()`].
#[non_exhaustive]
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I wasn't sure if I should mark this #[non_exhaustive].

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Yes, it's a good idea to have non-exhaustive in public APIs.

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I agree, but it makes using this struct a bit less ergonomic, because

EncoderParams {
    use_predictor_transform: false,
    ..Default::default()
}

(like in the test in my second commit) is forbidden outside of the image-webp crate with non-exhaustive structs. It needs to be

let mut p = EncoderParams::default();
p.use_predictor_transform = false;

instead.

#[derive(Clone, Debug)]
pub struct EncoderParams {
/// Use a predictor transform. Enabled by default.
pub use_predictor_transform: bool,
}

impl Default for EncoderParams {
fn default() -> Self {
Self {
use_predictor_transform: true,
}
}
}

/// Encode image data with the indicated color type.
///
/// # Panics
Expand All @@ -358,6 +376,7 @@ fn encode_frame<W: Write>(
width: u32,
height: u32,
color: ColorType,
params: EncoderParams,
) -> Result<(), EncodingError> {
let w = &mut BitWriter {
writer,
Expand Down Expand Up @@ -392,11 +411,13 @@ fn encode_frame<W: Write>(
w.write_bits(0b101, 3)?;

// predictor transform
w.write_bits(0b111001, 6)?;
w.write_bits(0x0, 1)?; // no color cache
write_single_entry_huffman_tree(w, 2)?;
for _ in 0..4 {
write_single_entry_huffman_tree(w, 0)?;
if params.use_predictor_transform {
w.write_bits(0b111001, 6)?;
w.write_bits(0x0, 1)?; // no color cache
write_single_entry_huffman_tree(w, 2)?;
for _ in 0..4 {
write_single_entry_huffman_tree(w, 0)?;
}
}

// transforms done
Expand Down Expand Up @@ -429,18 +450,20 @@ fn encode_frame<W: Write>(
}

// compute predictor transform
let row_bytes = width as usize * 4;
for y in (1..height as usize).rev() {
let (prev, current) =
pixels[(y - 1) * row_bytes..][..row_bytes * 2].split_at_mut(row_bytes);
for (c, p) in current.iter_mut().zip(prev) {
*c = c.wrapping_sub(*p);
if params.use_predictor_transform {
let row_bytes = width as usize * 4;
for y in (1..height as usize).rev() {
let (prev, current) =
pixels[(y - 1) * row_bytes..][..row_bytes * 2].split_at_mut(row_bytes);
for (c, p) in current.iter_mut().zip(prev) {
*c = c.wrapping_sub(*p);
}
}
for i in (4..row_bytes).rev() {
pixels[i] = pixels[i].wrapping_sub(pixels[i - 4]);
}
pixels[3] = pixels[3].wrapping_sub(255);
}
for i in (4..row_bytes).rev() {
pixels[i] = pixels[i].wrapping_sub(pixels[i - 4]);
}
pixels[3] = pixels[3].wrapping_sub(255);

// compute frequencies
let mut frequencies0 = [0u32; 256];
Expand Down Expand Up @@ -506,8 +529,10 @@ fn encode_frame<W: Write>(
}
if is_alpha {
write_huffman_tree(w, &frequencies3, &mut lengths3, &mut codes3)?;
} else {
} else if params.use_predictor_transform {
write_single_entry_huffman_tree(w, 0)?;
} else {
write_single_entry_huffman_tree(w, 255)?;
}
write_single_entry_huffman_tree(w, 1)?;

Expand Down Expand Up @@ -597,6 +622,7 @@ pub struct WebPEncoder<W> {
icc_profile: Vec<u8>,
exif_metadata: Vec<u8>,
xmp_metadata: Vec<u8>,
params: EncoderParams,
}

impl<W: Write> WebPEncoder<W> {
Expand All @@ -609,6 +635,7 @@ impl<W: Write> WebPEncoder<W> {
icc_profile: Vec::new(),
exif_metadata: Vec::new(),
xmp_metadata: Vec::new(),
params: EncoderParams::default(),
}
}

Expand All @@ -627,6 +654,11 @@ impl<W: Write> WebPEncoder<W> {
self.xmp_metadata = xmp_metadata;
}

/// Set the `EncoderParams` to use.
pub fn set_params(&mut self, params: EncoderParams) {
self.params = params;
}

/// Encode image data with the indicated color type.
///
/// # Panics
Expand All @@ -640,7 +672,7 @@ impl<W: Write> WebPEncoder<W> {
color: ColorType,
) -> Result<(), EncodingError> {
let mut frame = Vec::new();
encode_frame(&mut frame, data, width, height, color)?;
encode_frame(&mut frame, data, width, height, color, self.params)?;

// If the image has no metadata, it can be encoded with the "simple" WebP container format.
if self.icc_profile.is_empty()
Expand Down Expand Up @@ -757,45 +789,67 @@ mod tests {

#[test]
fn roundtrip_libwebp() {
roundtrip_libwebp_params(EncoderParams::default());
roundtrip_libwebp_params(EncoderParams {
use_predictor_transform: false,
..Default::default()
});
}

fn roundtrip_libwebp_params(params: EncoderParams) {
println!("Testing {params:?}");

let mut img = vec![0; 256 * 256 * 4];
rand::thread_rng().fill_bytes(&mut img);

let mut output = Vec::new();
WebPEncoder::new(&mut output)
let mut encoder = WebPEncoder::new(&mut output);
encoder.set_params(params.clone());
encoder
.encode(&img[..256 * 256 * 3], 256, 256, crate::ColorType::Rgb8)
.unwrap();
webp::Decoder::new(&output).decode().unwrap();
let decoded = webp::Decoder::new(&output).decode().unwrap();
assert!(&img[..256 * 256 * 3] == &*decoded);

let mut output = Vec::new();
WebPEncoder::new(&mut output)
let mut encoder = WebPEncoder::new(&mut output);
encoder.set_params(params.clone());
encoder
.encode(&img, 256, 256, crate::ColorType::Rgba8)
.unwrap();
webp::Decoder::new(&output).decode().unwrap();
let decoded = webp::Decoder::new(&output).decode().unwrap();
assert!(&img == &*decoded);

let mut output = Vec::new();
let mut encoder = WebPEncoder::new(&mut output);
encoder.set_params(params.clone());
encoder.set_icc_profile(vec![0; 10]);
encoder
.encode(&img, 256, 256, crate::ColorType::Rgba8)
.unwrap();
webp::Decoder::new(&output).decode().unwrap();
let decoded = webp::Decoder::new(&output).decode().unwrap();
assert!(&img == &*decoded);

let mut output = Vec::new();
let mut encoder = WebPEncoder::new(&mut output);
encoder.set_params(params.clone());
encoder.set_exif_metadata(vec![0; 10]);
encoder
.encode(&img, 256, 256, crate::ColorType::Rgba8)
.unwrap();
webp::Decoder::new(&output).decode().unwrap();
let decoded = webp::Decoder::new(&output).decode().unwrap();
assert!(&img == &*decoded);

let mut output = Vec::new();
let mut encoder = WebPEncoder::new(&mut output);
encoder.set_params(params.clone());
encoder.set_xmp_metadata(vec![0; 7]);
encoder.set_icc_profile(vec![0; 8]);
encoder.set_icc_profile(vec![0; 9]);
encoder
.encode(&img, 256, 256, crate::ColorType::Rgba8)
.unwrap();
webp::Decoder::new(&output).decode().unwrap();
let decoded = webp::Decoder::new(&output).decode().unwrap();
assert!(&img == &*decoded);
}
}
2 changes: 1 addition & 1 deletion src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@
extern crate test;

pub use self::decoder::{DecodingError, LoopCount, WebPDecoder};
pub use self::encoder::{ColorType, EncodingError, WebPEncoder};
pub use self::encoder::{ColorType, EncoderParams, EncodingError, WebPEncoder};

mod decoder;
mod encoder;
Expand Down
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