Re: [matplotlib-devel] Canvas HTML5 backend

[Jae-Joon L. <lee...@gm...>]

On Tue, Sep 1, 2009 at 12:57 PM, Michael Thompson<mic...@gm...> wrote:
> 2009/9/1 John Hunter <jd...@gm...>:
>> On Tue, Sep 1, 2009 at 10:32 AM, Michael Thompson<mic...@gm...> wrote:
>>> Hi,
>>>  I'm trying to work on the canvas javascript backend I found here
>>> [1]. I'm trying to add text but the canvas origin is at the top left,
>>> how can I transform the co-ordinates from the matplotlib to canvas?
>>>
>>>    def draw_text(self, gc, x, y, s, prop, angle, ismath=False):
>>>        ctx = self.ctx
>>>        ctx.font = "12px Times New Roman";
>>>        ctx.fillStyle = "Black";
>>>        ctx.fillText("%r" % s, x, y)
>>>
>>> [1] http://bitbucket.org/sanxiyn/matplotlib-canvas/src/80e9abf6d251/backend_canvas.py
>>
>> The backend canvas should know its height, so height-y should
>> transform from bottom to top
>
> Thanks, turns out to be a problem setting the size of the canvas
> element that the javascript is rendered into. If self.flipy is set
> then the text.py takes care of subtracting y from the height.
>
> Next problem is the text alignment, look OK on the right axis but
> wrong on the left I presume it's the alignment.
>
> The documentation says that s should be a matplotlib.text.Text
> instance and I can use s.get_horizontalalignment() but it seems that s
> is a unicode string. How can I find the alignment I should set on the
> text?
>
> Michael
>


My understanding is that all the backends should use left-bottom
alignment. Text alignment in matplotlib is handled by mpl itself (not
by the backend), and for this to work, you have to define
get_text_width_height_descent method correctly.

The real question is how we know the metric of the font that will be
used for rendering. I have little knowledge about the html canvas
specification, but I presume that the situation is very similar to the
svg case. Unless we embed the fonts (the svg backend has an option to
embed the fonts as paths), I don't think it is possible to get it
right.

Again, I have little knowledge about html5 canvas thing, and I hope
any expert out ther clarify this issue.

-JJ

ps. gnuplot seems to use embedded fonts for their html5 canvas backend
(I haven't checked carefully but their demo output uses canvastext.js,
originally from  http://jim.studt.net/canvastext/)



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