[whatwg] Enabling LCD Text and antialiasing in canvas

Stephen White senorblanco at chromium.org
Wed Feb 13 17:39:24 PST 2013

On Wed, Feb 13, 2013 at 5:31 PM, Rik Cabanier <cabanier at gmail.com> wrote:

> On Wed, Feb 13, 2013 at 11:25 AM, Stephen White <senorblanco at chromium.org
> >wrote:
> > On Wed, Feb 13, 2013 at 12:22 PM, Rik Cabanier <cabanier at gmail.com>
> wrote:
> >
> >> For blending optimizations, it might be better to introduce a function
> >> instead of a boolean attribute like 'opaque'.
> >> What you really want, is to matte [1] the canvas with a solid color so
> >> you can optimize compositing.
> >>
> >> How about this API:
> >>
> >> void applyMatte(DOMString color); // color is a CSS rgb color value
> >> (alpha is ignored)
> >>
> >>
> >> When you call this function, the canvas is matted with that color. If
> >> it's the first drawing call, you can just fill the canvas with that
> color
> >> (no compositing needed)
> >> After matting, you no longer have to read or update the alpha channel
> >> since it's always 1 which should speed up drawing.
> >>
> >
> > Just to be sure we're on the same page, when I mentioned compositing
> > optimizations, I was referring to compositing the canvas backing store
> into
> > the page, not compositing operations within the canvas itself.
> >
> sorry, I didn't mean to say blending. This is for optimizing compositing
> within the canvas and of the canvas into the page.
> >
> > One advantage of using an element attribute is that it could be used at
> > backing store allocation time, to allocate RGB instead of RGBA.  Forcing
> a
> > reallocation of the backing store on attribute change would be consistent
> > with changing width and height of the canvas, which have the same effect.
> >  Doing so on a context operation would not.
> >
> why not? There is no reason for you to allocate the backing store until
> it's needed.

True.  I was just trying to understand when you would use this function
more than once during canvas lifetime.  Presumably, that's the reason for
having a function, no?  If so, and you don't maintain any special state
about the opacity of the canvas, how is this different from an rgb()

> The strange thing with an element attribute is that you can't change it
> back and it's also detached from the JS code that does the drawing.
You can change it back programmatically through the DOM, as you can with
the element width and height.

> > If we did use a context function approach as you suggest, how would
> > subpixel AA be handled?  Would it be enabled on first call of the
> function
> > and never disabled?
> >
> I did not think about subpixel AA.
> If I read the mozilla proposal correctly, they do AA if they know that the
> canvas is opaque.
> So, with my proposal, there would be no AA until you call 'applyMatte'
> (assuming you follow the mozilla way of doing AA).

OK, so there would have to be a bit of state saved at that point, in order
to know if subpixel AA is allowable?

At any rate, I don't think this works if you subsequently modify the
canvas's alpha (see below).

> > Is there a way to query if the canvas is opaque once it's called?
> >
> Wouldn't the user know that he called 'applyMatte'? Also why do you want to
> query it?

> >  (I'm assuming that all changes to canvas alpha after the first call
> would
> > have to be ignored, since otherwise you'd have to sniff every operation
> to
> > see if it affected alpha, and "reset the bit", although perhaps I'm
> > misunderstanding your proposal.)
> >
> No, you don't have to do that and can still use alpha.
> Simple alpha compositing is defined as follows [1] (in premultiplied
> alpha):
> co = cs + cb x (1 - αs)
> αo = αs + αb x (1 - αs)
> If you know that the backdrop is matted (αb = 1), the second formula always
> resolves to 1 so you can skip it.

The problem is, canvas supports more than simple (source-over) compositing,
so there are ways of modifying the destination alpha back to non-1, e.g., a
primitive with non-1 alpha drawn with source-copy, or via putImageData().
 At that point, the state of the canvas's alpha is unknown, so the page
compositor can make no assumptions about its opacity.


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