Shipping the png crate in Chromium

[anforowicz]

Hello!

I really appreciate all the work that has been put into the png crate (and fdeflate crate as well as the Rust toolchain and other Rust crates). As a way of saying "thank you", let me share the status (and excitement) of what is going on with shipping the png crate in Chromium.

History of edits of this post:

• 2024-12-30: initial post
• 2025-02-21: updated to reflect newly started Canary+Dev field trials
• 2025-03-13: updated to reflect newly started Beta field trials

We started Chrome Canary experiments in the 2nd half of December 2024, Chrome Dev experiments in the 2nd half of February 2025, and Chrome Beta experiments in the middle of March 2023. If all goes well, then I hope to go to Stable in April and then try shipping in M135.

Below are some additional details in the form of questions and answers. Let me know if you have any other questions.

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Q: What Chrome versions participate in the experiments?

• Chrome 133.0.6891.0 is the first version with Rust PNG built into the binary. Rust PNG is disabled by default in this version, but can be force-enabled using chrome://flags/#rusty-png.
• Chrome 133.0.6892.0 was the minimum version in the current Canary 50%-50% experiments in December 2024.
• Chrome 135.0.7027.0 is the minimum version for participating in the current Canary+Dev+Beta 50%-50% experiments.

Depending on the experiment results on earlier channels, we plan to expand to M135 Stable during Q2 2025. For a tentative Chromium release schedule please see https://chromiumdash.appspot.com/schedule.

Q: How can I tell which experiment group I am in?

To find out which experiment group you are in, go to chrome://version and search for “977e3a5f” in the list of “Active Variations”:

• Control groups:
  • 977e3a5f-89078435 maps to RustyPng-Control_20241212 (Canary experiments in Dec 2024 - Jan 2025)
  • 977e3a5f-05a8121a maps to RustyPng-Control_20250220 (Canary+Dev experiments in Feb-... 2025)
  • 977e3a5f-dd8cc695 maps to RustyPng-Control_20250312 (Canary+Dev+Beta experiments in March-... 2025)
• Enabled groups:
  • 977e3a5f-2a45d995 maps to RustyPng-Enabled_20241212
  • 977e3a5f-c1958267 maps to RustyPng-Enabled_20250220
  • 977e3a5f-af2a6a20 maps to RustyPng-Enabled_20250312

Note that getting assigned to one of the experiment groups may depend on getting a new field trials “seed” and therefore may require rebooting a browser. Also note that your local ChromeVariations enterprise policy may limit which field trials you may participate in.

Q: Can I switch to a different experiment group?

You can't pick which experiment group you will be assigned to. Experiment group assignment happens randomly, and is “sticky” / “permanent” for the given Chrome client.

If you want to force Rust png on or off (e.g. to triage if the field trial may be responsible for a bug), then navigate to chrome://flags/#rusty-png and force the flag to be enabled or disabled. Note that this will exclude you from the A/B comparisons (i.e. you will be assigned to “ForcedOn_…” or “ForcedOff_…” group, rather than to the control or the experiment group).

Q: What versions of the libraries are covered?

• Control group: libpng/zlib-based decoders and encoders that normally ship in Chromium
  • blink::PNGImageDecoder
  • SkPngDecoder (used mostly through //ui/gfx/codec/png_codec.h API)
  • SkPngEncoder (used mostly through //ui/gfx/codec/png_codec.h API)
• Experiment group: Chromium/Skia’s decoders and encoders based on Rust png crate
  • SkPngRustDecoder and SkPngRustEncoder
  • 135.0.7027.0+: png version: 0.17.16; fdeflate version: 0.3.7; see also Chromium’s Cargo.lock. Chromium carries local patches that corresponds to Add support for parsing eXIf chunk. #568 and Treat most auxiliary chunk errors as benign. #569

Q: Can you share performance results from the field trials?

Yes, I plan to share aggregate performance numbers. OTOH, I want to wait until we have results from the Stable channel, because:

• The bigger user population of the Stable channel helps to get more statistically significant results (e.g. tighter confidence intervals).
• The numbers need to go through an internal review before being approved for sharing externally. (Mostly to double-check that I won’t accidentally share any secret or competitive data like relative user populations on different platforms, and will only share relative rather than absolute counts or times.)

Preliminary M135/Canary+Dev data shows decoding runtime improvement on Desktop, and a regression on Mobile/ARM. Encoding performance results are similarily mixed, but in the other direction. These 2 aspects should get a bit better in M136 thanks to http://review.skia.org/962878, https://crrev.com/c/6333255, and https://crrev.com/c/6354419.

Disclaimer: Let me also note various factors that may impact the performance results (and therefore explain if/why Chromium numbers may not match other measurements):

• Chromium uses a patched version of zlib that uses SIMD for better performance (and therefore other results comparing across libpng and Rust PNG may not necessarily carry over to this A/B experiment in Chromium)
• Palette expansion optimizations in libpng or png crate are not used by SkPngDecoder nor SkPngRustDecoder, because (unlike blink::PNGImageDecoder) Skia applies alpha premultiplication and color space transformations to <=256 pixels of the palette, rather than to N pixels of the image.
• Chromium ships a single binary per platform, which means that the compiler can only target the baseline instruction set. For example, auto-vectorization in Chromium binaries cannot use AVX-512 SIMD.
• Performance measurements are highly sensitive to the data corpus.

[yisibl]

I'm reporting a Chrome bug #391914245 which will cause our site to not be able to decode this apng.

[fintelia]

Running the image through pngcheck reports "tRNS not allowed in RGB+alpha image". So the image is corrupt, but other decoders suppress the error. We've got #525 tracking changes to match that behavior, but it should probably be prioritized. Thanks for flagging this!

[anforowicz]

@fintelia, would it be possible to release a new version of the png crate to https://crates.io/crates/png? This would help me absorb the most recent fixes (including #569) into Chromium. I'd be happy with either a minor version update (e.g. 0.17.17) or with a new major version (e.g. 0.18.0). Thank you in advance for your help!

(We are restarting field trials of using Rust png in Chrome's Canary and Dev channels - I've edited the top-level post to reflect this. It would be great if a new png version can be published within a week or so to avoid impacting Dev users with known issues. Please shout if this schedule won't work - in that case I'll try to crate a local patch that Chromium can carry in the meantime. FWIW https://chromiumdash.appspot.com/schedule says that Chrome M135 will branch on Monday, March 3rd.)

[fintelia]

We've already taken in a bunch of API breaking changes, so it'll have to be an 0.18 release. I'll see about figuring out what's left for that and try to get a release prepared

[fintelia]

@anforowicz just opened #577

[anforowicz]

Thanks! I'll track Chromium updates in https://crbug.com/400455848

[anforowicz]

I just wanted to drop a quick note that we've expanded the field trials to 50% of M135/Beta. I've edited the top-level post accordingly.