Compression filesize changes between v0.17.5 - v0.17.9

[JamesMcGuigan]

Recently run cargo update on my project and noticed differences in png file compression size between png versions v0.17.5-v0.17.9 for an otherwise identically rendered fractal.

I started with png="=0.17.5" in Cargo.toml and ran cargo update; ./fractal.sh. This would update Cargo.lock, perform a clean release build and regenerate the fractal images. This did not result in a git diff.

I incrementally version bumped png from v0.17.5 -> v0.17.9 and observed the following changes in filesize

CHANGE: fractal.png
21189 bytes (=0.17.5)
29596 bytes (=0.17.6) +40%
29596 bytes (=0.17.7) SAME
24981 bytes (=0.17.8) -20%
24981 bytes (=0.17.9) SAME == +20% overall

CHANGE: fractal.zoom.png
38284 bytes (=0.17.5)
50080 bytes (=0.17.6) +30%
50080 bytes (=0.17.7) SAME
45547 bytes (=0.17.8) -10%
45547 bytes (=0.17.9) SAME == +20% overall

Unsure if my fractal images count as a representative image sample, but it appears something in the png compression algorithm was refactored in versions 0.17.6 resulting in 30-40% filesize increase, following a second refactor in 0.17.8 that reduced filesize 10-20%, whilst still remaining 20% larger than 0.17.5

I have documented this testcase as a series of git commits, in this branch:

• https://github.com/JamesMcGuigan/fractals/tree/png-bug-report

JamesMcGuigan/fractals@331dd24

fractal.sh | png="=0.17.5" 

JamesMcGuigan/fractals@15f12fb

fractal.sh | png="=0.17.6"
cargo update
    Updating crates.io index
    Updating git repository `[https://github.com/yewstack/yew/`](https://github.com/yewstack/yew/%60)
    Removing adler32 v1.2.0
    Removing deflate v1.0.0
    Updating png v0.17.5 -> v0.17.6

CHANGE: fractal.png      | 21189 -> 29596 bytes (+40%)
CHANGE: fractal.zoom.png | 38284 -> 50080 bytes (+30%)

JamesMcGuigan/fractals@a596fd3

fractal.sh | png="=0.17.7"
cargo update
    Updating crates.io index
    Updating git repository `[https://github.com/yewstack/yew/`](https://github.com/yewstack/yew/%60)
    Updating miniz_oxide v0.5.4 -> v0.6.2
    Updating png v0.17.6 -> v0.17.7

SAME: fractal.png      | 29596 bytes
SAME: fractal.zoom.png | 50080 bytes

JamesMcGuigan/fractals@760699d

fractal.sh | png="=0.17.8"
cargo update
    Updating crates.io index
    Updating git repository `[https://github.com/yewstack/yew/`](https://github.com/yewstack/yew/%60)
      Adding fdeflate v0.3.0
    Removing miniz_oxide v0.6.2
    Updating png v0.17.7 -> v0.17.8

CHANGE: fractal.png      | 21189 -> 29596 -> 24981 bytes (+20% overall)
CHANGE: fractal.zoom.png | 38284 -> 50080 -> 45547 bytes (+20% overall)

JamesMcGuigan/fractals@9070746

fractal.sh | png="=0.17.9"
cargo update
    Updating crates.io index
    Updating git repository `[https://github.com/yewstack/yew/`](https://github.com/yewstack/yew/%60)
    Updating png v0.17.8 -> v0.17.9

SAME: fractal.png      | 24981 bytes
SAME: fractal.zoom.png | 45547 bytes

[fintelia]

We completely changed the compression libraries in 0.17.6 and again in 0.17.8!

As of 0.17.8, we use an extremely fast approach from fdeflate if you use Compression::Fast (which the image crate uses by default unless you override it) and use flate2 which is much slower but gets better compression ratios otherwise.