On Mon, 31 Aug 2009 08:08:05 +0200, Ian Hickson <i...@hixie.ch> wrote:
On Mon, 24 Aug 2009, Philip Jägenstedt wrote:
As far as I can see there's no good reason why createImageData should
take a float as input rather than unsigned long. Having it as float
creates the odd situation where (0.1, 0.1) gives a 1x1 ImageData while
(10.1, 10.1) gives a 10x10 or 11x11 depening on if you ceil or round the
input (not defined). Unless there's a compelling reason to allow
something like (0.1, 0.1) I suggest changing the type and leaving the
float->unsigned conversion to WebIDL.
Twenty years from now, when we're using 960dpi screens, 1 CSS pixel might
well map to ten device pixels reliably, such that people will want
sub-CSS-pixel-level accuracy in their calls to createImageData().
I get the impression this has all been discussed before. Still, it seems
unlikely that any browser will ever be able to switch to anything but a
1:1 CSS pixel:device pixel ratio, as that would break all existing pages
assuming that getImageData(0, 0, 100, 100) returns a 100x100 bitmap
(because assuming that is much easier, unless you read the spec carefully
you're unlikely to know it could ever be any different). I don't doubt
that high DPI screens will happen, but when it does browsers are more
likely to provide an extra flag like getImageData(..., useDevicePixels) or
another opt-in method in order to stay compatible with existing content.
Another option for the page author is simply creating a 1000x1000 canvas
and setting its CSS width/height to 100x100 (assuming the CSS pixel:device
pixel ratio can be found via script).
In any event, judging by existing implementations, the behavior of
createImageData(w, h) isn't as clear as it needs to be:
http://software.hixie.ch/utilities/js/live-dom-viewer/saved/223
Firefox:
log: ctx.createImageData(-1.1,1) => [Exception...
log: ctx.createImageData(-1,1) => [Exception...
log: ctx.createImageData(-0.1,1) => [Exception...
log: ctx.createImageData(0,1) => [Exception...
log: ctx.createImageData(0.1,1) => [Exception...
log: ctx.createImageData(1,1) => 1x1
log: ctx.createImageData(1.1,1) => 1x1
Safari/Chrome:
log: ctx.createImageData(-1.1,1) => 1x1
log: ctx.createImageData(-1,1) => 1x1
log: ctx.createImageData(-0.1,1) => 1x1
log: ctx.createImageData(0,1) => 1x1
log: ctx.createImageData(0.1,1) => 1x1
log: ctx.createImageData(1,1) => 1x1
log: ctx.createImageData(1.1,1) => 2x1
My interpretation of the spec:
log: ctx.createImageData(-1.1,1) => 1x1
log: ctx.createImageData(-1,1) => 1x1
log: ctx.createImageData(-0.1,1) => 1x1
log: ctx.createImageData(0,1) => INDEX_SIZE_ERR
log: ctx.createImageData(0.1,1) => 1x1
log: ctx.createImageData(1,1) => 1x1
log: ctx.createImageData(1.1,1) => 1x1
If the spec doesn't say to round rather than ceil, we're bound to have
subtle compatibility bugs on this.
--
Philip Jägenstedt
Core Developer
Opera Software
