[whatwg] Canvas 2D memory management

[Ashley Gullen]

Some developers are starting to design large scale games using our HTML5
game engine, and we're finding we're running in to memory management
issues.  Consider a device with 50mb of texture memory available.  A game
might contain 100mb of texture assets, but only use a maximum of 30mb of
them at a time (e.g. if there are three levels each using 30mb of different
assets, and a menu that uses 10mb of assets).  This game ought to fit in
memory at all times, but if a user agent is not smart about how image
loading is handled, it could run out of memory.

We have a WebGL renderer which solves this by explicitly creating and
deleting textures as necessary when switching levels, which guarantees that
memory is managed efficiently.  It also has the additional benefit that all
necessary textures are pre-loaded, so there's no janking during the game as
the first drawImage() of a particular asset in the level uploads a texture.

I would like to suggest memory management features for the canvas 2D
rendering context.  By explicitly pre-loading images and releasing them at
the end of the level we can guarantee that devices will not run out of
memory, as well as making gameplay smoother.

Some ideas:
1) add new functions to the canvas 2D context, such as:
ctx.load(image): cache an image in memory so it can be immediately drawn
when drawImage() is first used
ctx.unload(image): release the image from memory

2) we can drawImage() every image on startup to force lazy-loading browsers
to load everything that will be used, but there's still no way to indicate
which images should be released at the end of a level.  This could be left
for the browser to determine (perhaps releasing by least-recently-used),
but perhaps this should be required in the specification?

3) leave current behavior as it is and suggest WebGL for this type of
application

My preference is option 1, but I don't know if this works for all use cases
and will work nicely with implementations.  Any thoughts?

Ashley Gullen
Scirra.com