[whatwg] Requesting TextMetrics baseline property

Tab Atkins Jr. jackalmage at gmail.com
Tue Aug 30 16:33:01 PDT 2011

On Tue, Aug 30, 2011 at 3:56 PM, Charles Pritchard <chuck at jumis.com> wrote:
> 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.

Okay, that's reasonable.


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