[dojo-contributors] SVG/VML/Canvas API discussion

Tom Trenka ttrenka at gmail.com
Tue Apr 18 01:07:15 EDT 2006


It's not so much of whether we should or shouldn't support Canvas, per se.
I know that some feel that canvas is very limited but I think that's because
it really is just a canvas upon which you can draw raster primitives.  Both
SVG and VML are vector languages (think the difference between a JPG and an
EPS file).

But we'd start off script-based, and so the *commands* currently implemented
with canvas would probably be the best place to start--even though
underneath they'd be rendering to SVG or VML.  That's essentially what I
meant.  For instance, dojo.draw.line() would map to <svg:line /> and
<vml:line />, etc.

Anyways, that's the thought.

trt

On 4/17/06, Bill Keese <bill at dojotoolkit.org> wrote:
>
> > I know one of the things Alex has asked me for is that any primitive
> > drawn be able to fire and respond to events.  Not sure how we'll pull
> > that off with actual Canvas drawings, but I'll cross that bridge when
> > we get to it (probably it will involve looking for a mouse click on
> > the canvas element itself--if that's supported--and then analyzing the
> > current rendering to see what the top most element is that one clicked
> > on, what fun).
>
> PS: I realized my last message wasn't clear.  What I meant was, maybe we
> shouldn't support Canvas.  If we just support SVG/VML then it's easier
> to support events.  I'm not sure of all the pros/cons though.
>
> Bill
>
> Bill Keese wrote:
> > Hi Tom,
> >
> > Thanks for handling this, and also for doing the bug triage.
> >
> > There's a guy named Gavin that promised to do this API but I haven't
> > heard anything from him for a while; maybe he lost interest.  Here's the
> > link:
> >
> > http://thread.gmane.org/gmane.comp.web.dojo.user/5504/focus=5726
> >
> > (this is why I made that InProgressProjects page on the wiki)
> >
> > As for my comments: I'm not a graphics expert, but I wonder whether we
> > should support the canvas API model (like you suggested) or the SVG
> > model.  The difference being that in SVG there's a handle to every
> > object you draw, so you can attach event handlers / erase previously
> > drawn objects, etc.  Presumably canvas is faster but SVG/VML are more
> > powerful.
> >
> > Bill
> >
> > Tom Trenka wrote:
> >> Hi all,
> >>
> >> Since the Iterator discussion went *so* well (&lt;sarcasm/&gt; but not
> >> really), I figure it's time to start asking questions about an
> >> SVG/VML/Canvas library, since it would seem that some think this would
> >> be a very useful thing.  So let me kick off the discussion (since I'll
> >> probably write it)...
> >>
> >> The nominal purpose of an SVG+VML+Canvas library would be to add a
> >> single layer of abstraction over the two vector (and one bitmap)
> >> drawing languages so that a developer can write one set of code, and
> >> have it render properly on the major browser platforms.  However, I
> >> have some issues with this, so let me lay this out first before diving
> >> into a request for purpose+proposal.
> >>
> >> 1.  My personal feeling is that widgets (one of the core uses for
> >> something like this) should not be limited to an encompassing API for
> >> drawing, and that there's no real difference (aside from platform)
> >> between writing markup in VML and SVG and writing markup in HTML.  All
> >> three are valid, existing markup languages; to force an API over two
> >> of these languages would be a detriment to the system.
> >>
> >> Note the use of the term "force".  I'm not suggesting the API isn't
> >> needed, just that we shouldn't force things to rely on it that we
> >> release ourselves.
> >>
> >> 2.  In the same vein, we shouldn't ignore the need for core files in
> >> the same vein as style.js and html.js for the other markup languages.
> >> svg.js is already a part of the library (even though a good portion of
> >> the functionality in that file is stub functionality that isn't
> >> working yet).  I might expect there to be a vml.js at some point as
> >> well; I will certainly add it when I get a better sense of what kind
> >> of helper functions are needed (particularly when we finally address
> >> DnD with vector markup).
> >> ---
> >> All of which to say is that I'd like it to be clear that any vector
> >> API could (and probably will) rely on some core files but it should
> >> not be the other way around.
> >>
> >> Now that that disclaimer is out of the way...what do we think we want
> >> out of such an API?  What kind of opinions are there as to the shape
> >> of such an API?  What does Dojo really need in these terms?
> >>
> >> I'll put forth a suggestion, and then let's discuss:  I would say that
> >> we would start with a script-based API, modeled on the Canvas API,
> >> upon which we could write general purpose widgets to represent drawing
> >> primitives (and eventually more complex ones).  Before anyone objects,
> >> let me try to be clear:  when I say "modeled" on the Canvas API, I'm
> >> referring to methods and the general results of those methods.  The
> >> Canvas API is based on OpenGL, which makes sense from a scripting
> >> standpoint; I don't see any reason why we shouldn't use the same
> >> approach.
> >>
> >> I know one of the things Alex has asked me for is that any primitive
> >> drawn be able to fire and respond to events.  Not sure how we'll pull
> >> that off with actual Canvas drawings, but I'll cross that bridge when
> >> we get to it (probably it will involve looking for a mouse click on
> >> the canvas element itself--if that's supported--and then analyzing the
> >> current rendering to see what the top most element is that one clicked
> >> on, what fun).  What other things would you want out of such an API?
> >>
> >> trt
> >>
> >> (for the record, I will certainly be looking at the Google
> >> ExplorerCanvas code but I'm pretty sure we won't incorporate it
> >> because of licensing...unless, Alex, you want to take a close look at
> >> it and see?  My inclination is to learn from it but not use it, just
> >> to be safe, plus I'd hate for Dojo to be reliant on other
> >> projects--since it seems like one of the goals here is to be the other
> >> way around).
> >>
> >>
> >>
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