What is the current state of sub-pixel accuracy in the major browsers?

Question

I\'m working on a drawing application which requires high levels of accuracy, and I\'m wondering which of the major browser platforms (including the HTML Canvas element, and

Answer 1

Short answer:
No. It is NOT possible/documented.
And even if determined experimentally, it is NOT guaranteed to remain the same in future.

Long answer:
At sub-pixel accuracies, there is a lot of variance among Browsers/OS/HW about how the input is captured/rendered. With h/w acceleration being enabled on most modern browsers, there are a large number variations in rendering across different PCs running different browsers on different operating systems. So much so that, it is possible to even identify every unique user by the slightly different variations in the rendered output of a common sample.

Rather than worrying about the discrepancies in the underlying frameworks, How about designing the UI of your drawing application to be independent of those problems. Couple of methods i can think of right now are:

1. Allow editing the image at zoomed/magnified levels.

2. Design a snap-to-grid method for elements.

Update:
The "zoom" operation would your custom implementation and NOT a feature of the underlying frameworks. So if you need sub-pixel accuracy to the order of 1/10th of a pixel, one would need to have a 10x_zoom() implemented as part of you web-app which would render the data from

1st pixel --> 10x10pixels at (0,0),
2nd pixel --> 10x10pixels starting from (11,11).

This way one would have a very magnified view of the data, but the framework is blissfully unaware of all this and renders accurate to the onscreen-pixel(which in our case now is 1/10th of the image pixel).

Also an important thing to note that this operation would consume a lot of memory if done for the entire image at once. Hence doing this for ONLY the visible part of the image in a "zoom-window" would be faster and a less memory intensive process.

Once implemented in your drawing web-app the sub-pixel inaccuracies in the frameworks might not turn out to be a problem for the user as he can always switch into these modes and provide accurate input.

Answer 2

Currently, you can expect the best rounding and sub-pixel support to come from Mozilla with IE as the runner up. IE might end up being more fine tuned, but their release cycles are so long that Mozilla is likely to stay ahead of them.

As far as doing sub-pixel layout, you may be chasing a wisp, because the sub-pixel advantage improves anti-aliasing issues, not screen location accuracy. Your image will never be more accurate than 1 pixel from the true position, regardless of sub-pixel support.

The reason why some browsers don't zoom properly has nothing to do with sub-pixel support, it is because they are not remembering the exact position and rounding correctly. In other words, they are prematurely rounding the position and that causes the image to be mis-aligned.