[whatwg] Requesting TextMetrics baseline property

Charles Pritchard chuck at jumis.com
Tue Aug 30 15:56:04 PDT 2011

On Aug 30, 2011, at 3:26 PM, "Tab Atkins Jr." <jackalmage at gmail.com> wrote:

> On Tue, Aug 30, 2011 at 3:19 PM, Charles Pritchard <chuck at jumis.com> wrote:
>> I'd like to see a new property added to the TextMetrics object from Canvas
>> 2d.
>> var t = ctx.measureText('text span');
>> t == { width: int, baseline: int };
>> The baseline corresponds to the vertical offset of the anchor point.
>> A tiny number. It's not the same as height, but it has a similar use.
>> It allows allows the author to manage style markers on spans of text
>> presenting different baselines.
>> The following textBaseline image can not be generated in canvas:
>> https://developer.mozilla.org/en/drawing_text_using_a_canvas
>> http://www.whatwg.org/specs/web-apps/current-work/images/baselines.png
>> It can, but we have to do css hacks/edge detection, as measureText is
>> insufficient.
>> By exposing the baseline offset in the TextMetrics method, we would be able
>> to generate
>> the lines in that image, and appropriately position the text captions.
>> It seems appropriate that the illustration for textBaseline be something
>> that can be easily generated from within canvas. The same standard is held
>> for SVG documentation.
> Agreed, though there are multiple baselines that could be requested.
> The diagram you link to points out three of them.  The alphabetic
> baseline is what you want most of the time, but all of them can be
> useful depending on the type of text you're formatting.

The author would have to set ctx.textBaseline, and re-run TextMetrics for each baseline they are incorporating.

The enumeration of supported baselines is already in the canvas spec.

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