Bug 157629 – Implement more of the canvas TextMetrics API

• Status: NEW
• Component: Canvas

[Simon Fraser (smfr)]

We should implement more of https://html.spec.whatwg.org/multipage/scripting.html#textmetrics

We currently just have 'width'.

[Fujii Hironori]

Arno's WIP patch is in Bug 82798.

[Fernando Serboncini]

New spec: https://github.com/whatwg/html/pull/3931

[Radar WebKit Bug Importer]

<rdar://problem/44101655>

[Justin Michaud]

https://github.com/whatwg/html/pull/3931 adds advances, and moves the baselines into a dictionary

https://github.com/whatwg/html/issues/3995 has some more discussion about Safari's existing implementation

https://github.com/whatwg/html/issues/4026 has some interesting discussion about a number of edge cases. 

This patch adds advances and moves the baselines. When multiple code points are shaped into one glyph, it outputs the same "advance" for both (note that "advances" in the spec are actually distances from the left of the text box to the left of the glyph). If one code point becomes multiple glyphs, it breaks completely. If a code point does not get shaped into a glyph, it will have the same "advance" as the previous character, but I think it will break if the first character in the text does not become a glyph. I am going to need some help to understand how implement these edge cases.

With this patch, it passes http://w3c-test.org/2dcontext/drawing-text-to-the-canvas/2d.text.measure.advances.html (although you need to run it locally. I think it has something to do with the fonts). This test isn't very thorough though.

Chromium has this feature behind a flag. To enable it, run 
```
/Applications/Google\ Chrome\ Canary.app/Contents/MacOS/Google\ Chrome\ Canary --enable-blink-features=ExtendedTextMetrics
```

Note that Chromium's implementation is broken for rtl text. 

I have attached a manual test case to play around with. I also attached a screenshot of it in action ("Screenshot 1").

One of the advances is missing in this implementation compared to chrome, as seen in the image. I am not sure what the correct behavior is.

[Justin Michaud]

Created attachment 349683 [details]
Screenshot 1 of the patch vs. chrome

[Justin Michaud]

Created attachment 349684 [details]
Patch

[Justin Michaud]

Created attachment 349685 [details]
Manual test case (updated from last patch on this issue)

[EWS Watchlist]

Attachment 349684 [details] did not pass style-queue:


ERROR: Source/WebCore/ChangeLog:12:  You should remove the 'No new tests' and either add and list tests, or explain why no new tests were possible.  [changelog/nonewtests] [5]
Total errors found: 1 in 17 files


If any of these errors are false positives, please file a bug against check-webkit-style.

[Justin Michaud]

I think there should be a separate issue for handling baselines correctly. Right now, we don't consider the font at all. You can see this in the screenshot above as well.

[EWS Watchlist]

Comment on attachment 349684 [details]
Patch

Attachment 349684 [details] did not pass win-ews (win):
Output: https://webkit-queues.webkit.org/results/9205634

New failing tests:
imported/blink/fast/canvas/canvas-measure-text-rtl.html
imported/blink/fast/canvas/canvas-normalize-string.html
imported/blink/fast/canvas/canvas-measure-bidi-text.html
fast/canvas/canvas-measureText-2.html

[EWS Watchlist]

Created attachment 349699 [details]
Archive of layout-test-results from ews202 for win-future

The attached test failures were seen while running run-webkit-tests on the win-ews.
Bot: ews202  Port: win-future  Platform: CYGWIN_NT-6.1-2.10.0-0.325-5-3-x86_64-64bit

[EWS Watchlist]

Comment on attachment 349684 [details]
Patch

Attachment 349684 [details] did not pass ios-sim-ews (ios-simulator-wk2):
Output: https://webkit-queues.webkit.org/results/9205557

New failing tests:
imported/w3c/web-platform-tests/workers/semantics/interface-objects/002.worker.html

[EWS Watchlist]

Created attachment 349702 [details]
Archive of layout-test-results from ews123 for ios-simulator-wk2

The attached test failures were seen while running run-webkit-tests on the ios-sim-ews.
Bot: ews123  Port: ios-simulator-wk2  Platform: Mac OS X 10.13.4

[Myles C. Maxfield]

Comment on attachment 349684 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=349684&action=review

I'm not sure this API is mature enough to implement. I don't quite understand how other browsers have implemented it given how many standardization discussions we're having about it.

> Source/WebCore/ChangeLog:12
> +        No new tests (OOPS!).

Please update this to say the name of the test that you updated.

> Source/WebCore/platform/graphics/FontCascade.cpp:294
> +    advances.append(glyphBuffer.initialAdvance().width());

The Web API right now should probably ignore the initial advance. https://github.com/whatwg/html/issues/4030#issuecomment-420945138

> Source/WebCore/platform/graphics/FontCascade.cpp:329
> +    while (nextGlyph < glyphBuffer.size()) {
> +        const Font* nextFontData = glyphBuffer.fontAt(nextGlyph);
> +
> +        if (nextFontData != fontData) {
> +            fontData = nextFontData;
> +            currentOffset = lastOffset + 1;
> +        }
> +
> +        lastOffset = glyphBuffer.offsetInString(nextGlyph) + currentOffset;
> +        offsets.append(lastOffset);
> +        nextGlyph++;
> +    }

We have logic in ComplexTextController::offsetForPosition() that does this. We should investigate pulling that code out into a shared function that we can call here instead.

Specifically, look for // FIXME: Instead of dividing the glyph's advance equally between the characters, this

[Myles C. Maxfield]

I think this API is too nascent to implement right now.