[whatwg] Enabling LCD Text and antialiasing in canvas

[Stephen White]

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()
fillRect()?


> 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.

Stephen