[whatwg] addPath and CanvasPathMethods

[Robert O'Callahan]

On Fri, Mar 21, 2014 at 8:15 AM, Justin Novosad <junov at google.com> wrote:

> Sorry for the confusion, the point I was trying to make was unrelated to
> the CTM question (almost). My point is that the tesselation of a path is
> something that can be cached in a Path2D object.
>
> If you do this, you can take advantage of the cached tessellation:
> (apply tranform 1 to ctx)
> ctx.fill(path1)
> (apply tranform 2 to ctx)
> ctx.fill(path2)
>
> If you do it this way, the aggregated path needs to be re-tesselated each
> time because the winding rule would need to be re-applied:
> (apply tranform 1 to ctx)
> ctx.addPath(path1)
> (apply tranform 2 to ctx)
> ctx.addPath(path2)
> ctx.fill();
>
> Technically, these two ways of drawing are not equivalent (depends on
> compositing mode, transparency, and winding rule, overlaps between paths),
> but they can be used to achieve similar things.  Nonetheless the second way
> is detrimental to performance, and we'd be encouraging it by providing an
> addPath method on the context.  Besides, if the dev really needs to add
> paths together, it can be done inside an intermediate path object.
>

An implementation can turn #2 into #1 if the paths obviously don't overlap.
If they might overlap, the author probably shouldn't be doing the latter!
TBH I don't see why authors would choose the latter approach.

Rob
-- 
Jtehsauts  tshaei dS,o n" Wohfy  Mdaon  yhoaus  eanuttehrotraiitny  eovni
le atrhtohu gthot sf oirng iyvoeu rs ihnesa.r"t sS?o  Whhei csha iids  teoa
stiheer :p atroa lsyazye,d  'mYaonu,r  "sGients  uapr,e  tfaokreg iyvoeunr,
'm aotr  atnod  sgaoy ,h o'mGee.t"  uTph eann dt hwea lmka'n?  gBoutt  uIp
waanndt  wyeonut  thoo mken.o w