Estimate Text Layout using an OffscreenCanvas

[jwagner]

I tried to use plotters in a web worker and ran into a panic in estimate_layout. The reason is fairly obvious, there is no window or document inside of a worker context.

To get plotters to work inside of a web worker I hacked up some code to measure the text size using an OffscreenCanvas.

There are some benefits and drawbacks to this.

I think it should be more resilient than creating a span in the dom and measuring offsetWidth/Height because it is not affected by CSS.

It should be more performant than adding a dom node and measuring the offset properties which cause layout reflows. This is just an educated guess. Didn't benchmark it - takes this claim with a pinch of salt.

I think the main drawback is compatibility. While it is considered to be baseline now, browser support isn't ideal.

There are several ways I see forward. This approach could be used as a fallback when window isn't available or visa versa.

I think it's also an option to just fall back to a really crude approximation along the lines of height = size, width = size * text.len().

Anyway would be glad to get some feedback on this from the maintainers.

[AaronErhardt]

Sounds reasonable to me, but I'm not proficient enough to tell which approach is preferable. In the end, both solution seem like a hack to me, and the question is which hack has the better trade-offs. I'd like to hear another opinion on this matter, but unfortunately I'm the only somewhat active maintainer currently.

[jwagner]

Thanks a lot for getting back to me and keeping the project afloat @AaronErhardt.

If browser compatibility is the main concern I could also just add a fallback to a regular (non-offscreen) canvas. That way there is (essentially) only one code path and the dom hackery can go. Canvas is supported in practically every browser that supports wasm.

[philocalyst]

@jwagner Sounds like the stable option... If you would go forward with adding the fallback, we could go forward with merging :)

[jwagner]

@philocalyst I'm happy to add the fallback as soon as someone with merge permissions confirms that this is what they want.

[philocalyst]

Sure, man, I'm just helping out around the repo and know this would make it more maintainable and more likely to be considered. Do with that what you will... ▰▰▰▰▰ Miles Wirht 🙃

…

From: Jonas Wagner ***@***.***> Sent: 27 January 2026 14:35 To: plotters-rs/plotters ***@***.***> Cc: Miles Wirht ***@***.***>, Mention ***@***.***> Subject: Re: [plotters-rs/plotters] Estimate Text Layout using an OffscreenCanvas (PR #664) jwagner left a comment (plotters-rs/plotters#664) @philocalyst I'm happy to add the fallback as soon as someone with merge permissions confirms that this is what they want. -- Reply to this email directly or view it on GitHub: #664 (comment) You are receiving this because you were mentioned. Message ID: ***@***.***>

[AaronErhardt]

@philocalyst I'm happy to add the fallback as soon as someone with merge permissions confirms that this is what they want.

I would merge it :)