- From: Philip Jägenstedt <philipj@opera.com>
- Date: Mon, 31 Aug 2009 10:55:04 +0200
On Mon, 31 Aug 2009 08:08:05 +0200, Ian Hickson <ian at hixie.ch> wrote: > On Mon, 24 Aug 2009, Philip J?genstedt 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 J?genstedt Core Developer Opera Software
Received on Monday, 31 August 2009 01:55:04 UTC