On 12/17/13 10:55 PM, Rik Cabanier wrote:

On Tue, Dec 17, 2013 at 9:36 PM, David Flanagan <dflana...@mozilla.com <mailto:dflana...@mozilla.com>> wrote:

    On 12/17/13 8:36 PM, Rik Cabanier wrote:
    Hi David,

    is there a reason why you are completely decoding the image when
    you create the imageBitmap? [1]
    I assume that that is the intent of calling createImageBitmap() on
    a blob. Since JPEG decoding probably takes significantly longer
    than blocking on memory access, I assume that lazy decoding is not
    really allowed.

No, nothing in the spec says that you *must* decode the bits:

    The exact judgement of what is undue latency of this is left up to
    the implementer, but in general if making use of the bitmap
    requires network I/O, or even local disk I/O, then the latency is
    probably undue; whereas if it only requires a blocking read from a
    GPU or system RAM, the latency is probably acceptable.

In your case, things are reversed. Allocating system ram will kill performance and cause undue latency. Reading the JPEG image on the fly from a Flash disk will be less disruptive and faster.

    But that misses my point. On the devices I'm concerned with I can
never completely decode the image whether it is deferred or not. If I decode at full size, apps running in the background are
    likely to be killed because of low memory. I need the ability to
    do the downsampling during the decoding process, so that there is
    never the memory impact of holding the entire full-size image in

    If you detect a situation where this operation causes excessive
    memory consumption, you could hold on to the compressed data URL
    and defer decoding until the point where it is actually needed.
    Since exhausting VM will create "undue latency", this workaround
    follows the spirit of the spec.

    If you really want to have the downsampled bits in memory, you
    could create a canvas and draw your image into it.
    I can't do that because I don't have (and cannot have) a full-size
    decoded image.  I've got a blob that is a JPEG encoded 5 megapixel
    image.  And I want to end up with a decoded 320x480 image.  And I
    want to get from A to B without ever allocating 20mb and decoding
    the image at full size

The downsampling happens *during* the drawimage of the imageBitmap into the canvas. At no point do you have to allocate 20mb.

Ah. I see what you're saying now. My first reaction was "that's brilliant!". Unfortunately, my second reaction was that drawImage() would then block on the image decoding, and unless this was being done in a Worker, I'm almost certain that would be an unacceptable performance hit. (One of my use cases is scanning an SD card for hundreds of images and generating thumbnails for each of them. If doing this used drawImage() calls that blocked the main thread, my UI would be completely non-responsive.)

I also suspect that adding an async version of drawImage() to the canvas API is a non-starter because that API is pretty fundamentally synchronous.


Reply via email to