On Tue, 04 Sep 2012 19:15:46 +0200, Boris Zbarsky <bzbar...@mit.edu> wrote:
On 9/4/12 1:02 PM, David Geary wrote:
Sure, but those use cases will be in the minority
What makes you say that?
Outside of games, I think they're a majority of the canvas-using things
I've seen.
I think it makes the most sense to add a context lost handler to the
spec and leave it up to developers to redraw the canvas.
OK, yes, let's call that option 9. And I'll add option 10: do nothing.
So now our list is:
1) Have a way for pages to opt in to software rendering.
2) Opt canvases in to software rendering via some sort of heuristic
(e.g. software by default until there has been drawing to it for
several event loop iterations, or whatever).
3) Have a way for pages to opt in to having snapshots taken.
4) Auto-snapshot based on some heuristics.
5) Save command stream.
6) Have a way for pages to explicitly snapshot a canvas.
7) Require opt in for hardware accelerated rendering.
8) Authors use toDataURL() when they want their data to stick around.
9) Context lost event that lets authors regenerate the canvas.
10) Do nothing, assume users will hit reload if their canvas goes blank.
Any other options, before we start trying to actually decide which if
any of these might be workable?
-Boris
It's important to discuss implementation details so we don't spec
something that's not implementable on all platforms. That said we
obviously should try to stay clear of specifying how things should be
implemented and instead spec what end result we're after. There's little
distinction from the end users point of view for example between
snapshotting an saving the command stream.
Can we live with a weaker statement than a guarantee that the canvas
content will be retained? Perhaps a "best effort" may be enough?
It's obviously in the vendors interests to do as good a job as possible
with retaining canvas content, and I believe for example on Android it's
possible to get notified before a power-save event will occur. That would
enable us to do a read-back and properly restore the canvas (investigation
needed). For applications that just cannot loose any data then canvas
obviously isn't the best choice for storage. If we do have the context
lost event then at least new versions of those applications can be sure to
render out all their data to a canvas and move it over to an img while
listening for the context lost event. Existing applications that cannot
loose data... well, maybe that's a loss we'll have to accept, but by all
means dazzle me with a brilliant solution if you have one.
JFYI I'd say by far the most common lost context scenario on desktop would
be browsing to your driver manufacturers page, downloading and installing
a new driver.
--
Erik Möller
Core Gfx Lead
Opera Software
twitter.com/erikjmoller
