[whatwg] Enabling LCD Text and antialiasing in canvas

[Stephen White]

On Tue, Feb 12, 2013 at 6:14 PM, Rik Cabanier <cabanier at gmail.com> wrote:

>
>
> On Tue, Feb 12, 2013 at 2:56 PM, Stephen White <senorblanco at chromium.org>wrote:
>
>> On Fri, Nov 23, 2012 at 6:04 PM, Ian Hickson <ian at hixie.ch> wrote:
>>
>> > On Thu, 29 Mar 2012, Jeremy Apthorp wrote:
>> > > On Thu, Mar 29, 2012 at 10:25 AM, Jeremy Apthorp <
>> jeremya at chromium.org
>> > >wrote:
>> > > > On Thu, Mar 29, 2012 at 8:41 AM, Ian Hickson <ian at hixie.ch> wrote:
>> > > >> On Fri, 13 Jan 2012, Jeremy Apthorp wrote:
>> > > >> >
>> > > >> > I'd like to draw non-antialiased lines in a <canvas>. Currently
>> it
>> > > >> > seems that the only way to do this is to directly access the
>> pixel
>> > > >> > data.
>> > > >> >
>> > > >> > Is there a reason there's no way to turn off antialiasing?
>> > > >>
>> > > >> What's the use case?
>> > > >
>> > > > Pixel-art style games.
>> > >
>> > > Specifically: even with the new image smoothing stuff in place for
>> > > drawImage, a 1:2 diagonal line will still be anti-aliased (only the
>> > > antialiasing will look silly scaled up to 2x).
>> >
>> > Do you have an example of a game where lines are drawn using a line API
>> > without antialiasing, then scaled up? Most "pixel art" games I've seen
>> > tend to use bitmaps for that kind of thing.
>> >
>> >
>> > On Mon, 12 Nov 2012, Justin Novosad wrote:
>> > >
>> > > For many types of apps, DOM-based rendering is uncompetitively slow
>> > > [so we should make text rendering in canvas more controllable]
>> >
>> > This seems like something we should fix, not something we should work
>> > around by having people use canvas instead. Using canvas has all kinds
>> of
>> > terrible side-effects, like reducing the likely accessibility of the
>> page,
>> > making searcheability much worse, etc.
>> >
>> > Also, do you have any metrics showing the magnitude of this problem on
>> > real-world sites that might consider using canvas instead?
>> >
>> >
>> > > If LCD text were enable-able, authors would have to be mindful of a
>> > > number of caveats in order to avoid rendering artifacts.
>> >
>> > Do we have any reason to believe the majority of authors would make the
>> > right decisions here?
>> >
>> > (The main reason we haven't provided control over things like
>> antialiasing
>> > is that many authors tend to make terribly bad decisions.) (Before
>> anyone
>> > gets offended, by the way: that you are reading this almost guarantees
>> > that you are above average in terms of authoring ability.)
>> >
>> >
>> > On Tue, Nov 13, 2012 at 9:37 PM, Robert O'Callahan wrote:
>> > >
>> > > We'd have to define what happens when you use subpixel antialiasing
>> > > "incorrectly", because we can be pretty sure authors will use it
>> > > incorrectly and expect to get interoperable behavior.
>> >
>> > That's certainly true.
>> >
>> >
>> > > Mozilla supports a "mozOpaque" attribute which makes the canvas buffer
>> > > RGBX (initialized to solid black) and enables subpixel antialiasing
>> for
>> > > most text drawing. That might be enough to address your use-cases.
>> >
>> > I haven't specified this; if other vendors intend to implement this let
>> me
>> > know and I can spec it. I'm not sure it's worth it though.
>> >
>>
>> [blowing the dust off this thread]
>>
>> Folks on the Chrome team are looking into implementing this attribute, and
>> would be interested in seeing it spec'ed.
>
>
> What are you implementing? Initializing the canvas to black or subpixel
> antialiasing?
>

We're interested in both aspects:  the opportunity for culling and blending
optimizations at composite time, as well as enabling subpixel AA.

Stephen

>
>
>>
>> >
>> > On Wed, 14 Nov 2012, Robert O'Callahan wrote:
>> > > On Wed, Nov 14, 2012 at 8:09 AM, Justin Novosad <junov at chromium.org>
>> > wrote:
>> > > >
>> > > > Are there precedents for exposing features with documented caveats?
>> > > > (excluding caveats that were discovered after the fact)
>> > >
>> > > Yes, and many of them have been extremely problematic, because Web
>> > > authors will ignore the caveats.
>> >
>> > Right. I'd really like to avoid adding more if we can help it.
>> >
>> >
>> > On Wed, 14 Nov 2012, Justin Novosad wrote:
>> > >
>> > > There is a recent improvement in Chrome called "deferred 2D canvas
>> > > rendering" (enabled by default as of Chrome 23).  It is a mechanism
>> that
>> > > records 2d canvas commands during JS execution, and only executes them
>> > > for real when the render buffer needs to be resolved (draw to screen,
>> > > getImageData, toDataURL, etc.).  If you want to check it out, the guts
>> > > are in Skia: SkGPipe is a sort of FIFO for graphics commands,
>> > > SkDeferredCanvas is a wrapper that manages the GPipe and automatically
>> > > flushes it and applies some command culling optimizations.
>> > >
>> > > So to come back to the problem of with and without subpixel AA
>> buffers:
>> > > if rendering is deferred, the non-AA buffer would never get rasterized
>> > > (and possibly never even allocated), unless it needs to be.  Obviously
>> > > there are practical limitations, for example we cannot store an
>> > > unlimited stream of recorded commands, so if the canvas draws
>> > > indefinitely without ever being cleared, at some point we have to
>> > > rasterize the non-AA buffer just so that we can safely discard the
>> > > recording data. Also, if at record time the necessary conditions for
>> > > subpixel AA are not met, perhaps we just forget about it.
>> > >
>> > > I admit this is a complex solution for implementors, but it makes the
>> > > management of subpixel-AA safety transparent to web authors.
>> >
>> > I think it'd be reasonable (for some definition of reasonable that
>> > relates to whether it's compatible with the spec, anyway) for
>> implementors
>> > to do this today, without having to expose any control to the author.
>> >
>> >
>> > On Thu, 15 Nov 2012, Fred Andrews wrote:
>> > >
>> > > The canvas that scripts draw into could be over-sized with the UA down
>> > > sampling this to fit the target size and taking into account the
>> > > sub-pixel screen layout when doing so.
>> >
>> > On Thu, 15 Nov 2012, Justin Novosad wrote:
>> > >
>> > > Obviously, that would be costly (x3 pixels), but I think it is a very
>> > > realistic solution and relatively low hanging fruit. The over-sizing
>> of
>> > > the canvas would have to be handled under the hood by the UA though,
>> > > because it depends on LCD component ordering and orientation, which
>> > > means querying the OS/display driver. A lot of the kinks with the
>> > > over-sized canvas approach have already been ironed out for solving
>> the
>> > > problem of High-DPI support ( put/getImageDataHD ), and I like the
>> idea
>> > > of unifying the two. Implementing this would mostly be a matter of
>> > > adding per color component compositing of canvases. Also, the pixel
>> > > aspect ratio would have to be taken into account for line drawing.
>> >
>> > getImageDataHD() requires that the pixels be square, but so long as that
>> > is taken into consideration (e.g. by dropping down to square pixels if
>> the
>> > author calls putImageDataHD()) I think this could probably be made to
>> work
>> > within the spec's current requirements.
>> >
>> >
>> > > Regarding the concerns about accessibility, I think the problem can be
>> > > solved by using HitRegions with labels.
>> >
>> > Oh it _can_ be solved. That's not the problem. Accessibility is not
>> about
>> > what is _possible_, it's about what actually _is_. Most authors,
>> > realistically speaking, aren't goin to be using hit regions sufficiently
>> > for us to declare victory here.
>> >
>> >
>> > > Come to think of it, there should be an option to make the UA do this
>> > > automatically: create a HitRegion with a label every time text is
>> drawn
>> > > to a canvas.
>> >
>> > I considered doing that, but it gets really fiddly when you're doing
>> > things like text that fades over multiple frames. In the end I decided
>> > that the magic wasn't worth it, as it would likely screw up more often
>> > than it would actually help.
>> >
>> > --
>> > Ian Hickson               U+1047E                )\._.,--....,'``.    fL
>> > http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._
>> ,.
>> > Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
>> >
>>
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