On May 31, 2009, at 6:55 PM, Boris Zbarsky wrote:
3) It's not clear to me why imagedata actually exposes device
pixels,
nor is it clear to me how this is supposed to work if the same
document is being rendered to multiple devices. Is a UA allowed
to have a higher internal resolution for a canvas (in device
pixels)
and then sample when painting to the device? This might well be
desirable if the UA expects the canvas to be scaled; it can well
reduce scaling artifacts in that situation. It doesn't seem
reasonable, to me, to expose such super-sampling via imageData;
it's entirely an optimization detail.
Worse yet, the current setup means that a script that tries
createImageData, fill in the pixels, and then paint it to the
canvas, needs to fill different numbers of pixels depending on the
output device. I fully expect script authors to get this very very
wrong, since it's such non-intuitive behavior. It would make more
sense to just have the script work entirely in CSS pixels; if it
wishes to create a higher-resolution image it can create a canvas
with bigger dimensions (and scale its actual display via setting
its width and height CSS properties).
In some environments, a CSS pixel may be more than one device pixel.
In this case, getImageData followed by putImageData will lose
resolution. In other cases, a CSS pixel may be a non-integer number of
device pixels.
To see an example of a browser running in this environment, on Mac OS
X launch Quartz Debug, open the User Interface Resolution window, and
try setting the scale factor to 1.5 or 2.0. In this case in Safari,
the CSS px unit will respect the scale factor, but the canvas backing
store will still be in device pixels so users get the benefit of the
higher resolution. (Currently Apple doesn't ship any systems that use
a non-1.0 UI scale factor by default.)
The current design makes it possible to write code that will do the
right thing in a scaled UI. It also makes it easy to do the wrong
thing and copy only a fraction of the area intended. The alternate
design of using CSS pixels would make it impossible to get right, but
the failure mode would just be to lose resolution.
Regards,
Maciej
