[whatwg] Requesting TextMetrics baseline property

[Tab Atkins Jr.]

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.

~TJ