[whatwg] Canvas 2D memory management

Paul Bakaus pbakaus at zynga.com
Mon Jan 14 08:39:55 PST 2013

On 11.01.13 18:41, "Rik Cabanier" <cabanier at gmail.com> wrote:

>On Wed, Jan 9, 2013 at 8:00 AM, Ashley Gullen <ashley at scirra.com> wrote:
>> 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
>> 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
>> 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
>> memory is managed efficiently.  It also has the additional benefit that
>> 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
>> I would like to suggest memory management features for the canvas 2D
>> rendering context.  By explicitly pre-loading images and releasing them
>> 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
>Is this what you're looking for:

Very interesting! I wasn't aware of this part of the spec. Sounds like
this could become extremely useful. Looking forward to play with an
implementation to see if it fixes our issues.

>> ctx.unload(image): release the image from memory
>Releasing all reference to ImageBitmap should release it from memory. Are
>you looking for a scheme that does not involve garbage collection?
>Some of you concerns with memory management could be addressed with
>Basically, you can put all your images in a WeakMap and during the draw
>cycle, you pull them out and use them. If they're no longer there, it
>that the garbage collector has kicked in to free up memory and you need to
>reload (which will unfortunately cause a jank). I'm unsure how smart the
>garbage collector is because you probably want the items in the WeakMaps
>be deleted last.
>> 2) we can drawImage() every image on startup to force lazy-loading
>> to load everything that will be used, but there's still no way to
>> which images should be released at the end of a level.  This could be
>> 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
>> and will work nicely with implementations.  Any thoughts?
>> Ashley Gullen
>> Scirra.com

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