[webkit-dev] Canvas backing resolution

Oliver Hunt oliver at apple.com
Fri Mar 4 11:11:52 PST 2011

WebKit's canvas implementation has always scaled the backing buffer according to actual screen resolution,  i suspect it doesn't pay any attention to zoom however :-/

Arguably that's a bug.


On Mar 4, 2011, at 10:08 AM, Charles Pritchard wrote:

> In the future?
> On Mar 4, 2011, at 8:51 AM, Oliver Hunt <oliver at apple.com> wrote:
>> For reference desktop webkit implementations will automatically increase the canvas backing store resolution as the device:css pixel ratio increases.
>> --Oliver
>> On Mar 4, 2011, at 1:44 AM, Charles Pritchard wrote:
>>> On 3/4/2011 1:35 AM, Kenneth Rohde Christiansen wrote:
>>>> Hi again,
>>>>> I'm trying to keep the canvas bitmap at the same pixel resolution as the
>>>>> device,
>>>>> otherwise it is blurry.
>>>> OK, I see.
>>>>> This, for example, works if the pixel ratio is 2.
>>>>> <canvas style="width: 100px; height: 100px;" width="200"
>>>>> height="200"></canvas>
>>>>> Yes, when the user has a zoom level set, I'd like to paint to match the
>>>>> pixel ratio,
>>>>> so that, a zoomed in user still receives crisp output, instead of blurry
>>>>> output.
>>>> -webkit-device-pixel-ratio does not change when the user scaled the
>>>> content; it is a device value.
>>>> On old iPhone it is 1.0, on Android it can be 0.5, 1.0 and 1.5
>>>> depending on it being a low, medium (160) or high dpi (240) device. On
>>>> the iPhone 4 and the new iPod Touch it is 2.0.
>>>> On Android devices, with for instance, a device-pixel-ratio of 1.5,
>>>> you can disable the upscaling by adding target-densitydpi=device-dpi
>>>> to the viewport meta tag.
>>>> What you can do is to disable user scaling (add user-scaling=no to the
>>>> viewport meta tag) and then handle zooming in your app using the touch
>>>> events. This only works on mobile devices though.
>>> I haven't debugged with enough tablet devices, such as the color kindle, to know how many webkit distros it works with.
>>> What should I do about user zoom on the desktop (and possibly, the kindle) ?
>>> Drawing in high res, just-in-case, makes for a poorer experience for users at a 1.0 ratio,
>>> and asking users to slide a "sharpen" slider is a little awkward.
>>> Mozilla, in their implementation, has setup device-pixel-ratio to scale when the user scales content;
>>> they directly expose window css pixel ratio value to trusted scripts, but not to web content; requiring
>>> web content to use css device-pixel-ratio.
>>> Microsoft has exposed the values through window.screen.
>>> -Charles
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