[Bug 27638] New: FEATURE REQUEST: Selectively enable metrics in measureText using optional second argument

https://www.w3.org/Bugs/Public/show_bug.cgi?id=27638

            Bug ID: 27638
           Summary: FEATURE REQUEST: Selectively enable metrics in
                    measureText using optional second argument
           Product: HTML WG
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: HTML Canvas 2D Context
          Assignee: jaymunro@microsoft.com
          Reporter: motiz88@gmail.com
        QA Contact: public-html-bugzilla@w3.org
                CC: mike@w3.org, public-html-admin@w3.org,
                    public-html-wg-issue-tracking@w3.org

Hello,

I am the author of https://github.com/motiz88/canvas-text-metrics-polyfill .

I'd like to make the case for a small enhancement following up on the
resolution of https://www.w3.org/Bugs/Public/show_bug.cgi?id=7798 , relating to
the properties added to TextMetrics (
https://html.spec.whatwg.org/#drawing-text-to-the-bitmap ).

Background: Browser support for this useful and simple API is still
surprisingly scarce (to my knowledge only Chrome Canary has it, behind an
experimental flag, with a buggy implementation), so I have written a polyfill
(which I believe is the first conforming to this API) based mainly on
off-screen drawing and scanning. This approach is rather slow in the general
case, but results in a robust and (at least, largely) conforming polyfill.

My suggested enhancement below, already implemented in my polyfill, would
enable such polyfills (and possibly native implementations?) to be lighter and
faster, in cases where e.g. only one or two metrics are needed out of the whole
set. The availability of a fully conforming and performant polyfill may finally
spur adoption of this API by developers and browser makers.

<suggestion>

interface CanvasRenderingContext2D {
...
TextMetrics measureText(DOMString text, optional CanvasMeasureTextOptions
options);
...
};

interface CanvasMeasureTextOptions {
  // x-direction
  attribute boolean width; // advance width
  attribute boolean actualBoundingBoxLeft;
  attribute boolean actualBoundingBoxRight;

  // y-direction
  attribute boolean fontBoundingBoxAscent;
  attribute boolean fontBoundingBoxDescent;
  attribute boolean actualBoundingBoxAscent;
  attribute boolean actualBoundingBoxDescent;
  attribute boolean emHeightAscent;
  attribute boolean emHeightDescent;
  attribute boolean hangingBaseline;
  attribute boolean alphabeticBaseline;
  attribute boolean ideographicBaseline;
};

Augmented semantics of measureText():

If the options argument is missing or null, measureText() behaves as currently
specified, and must return a TextMetrics object with valid values set on all
its specified properties.

If there is a non-null options argument, then the returned TextMetrics object
is only required to contain the set of properties whose names coincide with the
names of properties on the options object which are both (1) set, and (2) equal
to true. The TextMetrics object MAY additionally contain some properties it is
not required to, in which case their values are not defined.

</suggestion>

I would appreciate any feedback on this.

Thanks,

Moti Zilberman

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Received on Wednesday, 17 December 2014 12:34:33 UTC