Decoding animated WebP is 4x slower than libwebp-sys + webp-animation

[Shnatsel]

In image v0.25.4 and image-webp v0.2.0, decoding the attached animated WebP is 4x slower than using libwebp-sys + webp-animation: sample.zip

image

use std::error::Error;
use image::{codecs::webp::WebPDecoder, AnimationDecoder, ImageReader};

fn main() -> Result<(), Box<dyn Error>> {
    let input = std::env::args().nth(1).unwrap();    
    let reader = ImageReader::open(input)?.into_inner();
    let decoder = WebPDecoder::new(reader)?;
    let mut iter = decoder.into_frames();
    while let Some(_frame) = iter.next() {}
    Ok(())
}

hyperfine results:

  Time (mean ± σ):     411.9 ms ±   3.9 ms    [User: 372.2 ms, System: 38.8 ms]
  Range (min … max):   407.6 ms … 416.8 ms    10 runs

libwebp-sys + webp-animation

use std::error::Error;
use webp_animation::prelude::*;

fn main() -> Result<(), Box<dyn Error>> {
    let input = std::env::args().nth(1).unwrap();    

    let buffer = std::fs::read(input).unwrap();
    let decoder = Decoder::new(&buffer).unwrap();
    let mut iter = decoder.into_iter();
    while let Some(_frame) = iter.next() {}
    Ok(())
}

hyperfine results:

  Time (mean ± σ):      95.9 ms ±   0.4 ms    [User: 128.7 ms, System: 7.4 ms]
  Range (min … max):    95.3 ms …  96.7 ms    30 runs

Analysis

webp-animation shows a bit of multi-threading happening on the profile, with user time being longer than the total execution time, but even accounting for that image-webp is 3x slower.

Breakdown of where the time is spent in image, recorded by samply: https://share.firefox.dev/4fc3utg

The greatest contributors seem to be image_webp::vp8::Vp8Decoder::decode_frame (48%), image_webp::extended::do_alpha_blending (20%), image_webp::vp8::Frame::fill_rgba (16%).

Within decode_frame the biggest contributor is image_webp::vp8::Vp8Decoder::read_coefficients (12% self time, 32% total time), and the code of that function looks like it could be optimized further to reduce bounds checks, etc. #71 is also relevant, but only accounts for 20% of the total time.