[whatwg] Processing the zoom level - MS extensions to window.screen

Ian Hickson ian at hixie.ch
Thu May 12 20:34:13 PDT 2011

On Fri, 11 Feb 2011, Glenn Maynard wrote:
> On Fri, Feb 11, 2011 at 3:24 PM, Ian Hickson <ian at hixie.ch> wrote:
> > On Wed, 29 Dec 2010, Glenn Maynard wrote:
> > > I hit this problem in a UI I worked on.  It rendered into a canvas 
> > > the size of the window, which can be zoomed and scrolled around.  
> > > At 100% full page zoom this works well.  At 120% zoom, it creates a 
> > > canvas smaller than the window, which is then scaled back up by the 
> > > browser, resulting in a blurry image.  Full page zoom should work on 
> > > the UI around it--I didn't want to disable it entirely--but the 
> > > canvas itself should be created in display pixels, rather than CSS 
> > > pixels.
> > >
> > > I didn't find any reasonable workaround.  All I can do is tell 
> > > people not to use full-page zoom.  Many users probably see a blurry 
> > > image and don't know why, since there's no way to detect full-page 
> > > zoom in most browsers to even hint the user about the problem.
> >
> > That's a bug in the browser. If it knows it's going to be zooming up 
> > the canvas when it creates the backing store, it should be using a 
> > bigger backing store.
> It sounds like you're saying that, if the user's full-page zoom level is 
> 110% and the page requests a 100x100 canvas, the browser should create a 
> 110x110 backing store instead.  There are several problems with that:
> - The full-zoom level can be changed by the user after the canvas is 
> already rendered.  If I load a page at 100%, the canvas renders at that 
> resolution, and then I change the full-zoom level to 110%, there's no 
> way for the browser to know this and use a bigger backing store in 
> advance.

Sure, this is a "best-effort" kind of thing.

> - The data would have to be downscaled to the exposed 100x100 resolution
> when exported with ImageData.

No, ImageData exposes the underlying data, not the data in CSS pixels.

> Similarly, rendering a 100x100 image into a canvas set to 100x100 would 
> upscale the image, blurring it: the developer should be able to expect 
> that blitting a 100x100 image into a 100x100 canvas will be a 1:1 copy.

It would make no difference since the canvas is zoomed 110% anyway.

> - If, rather than displaying it in the document at the full-zoom level, 
> the data is sent to the server, the results would be blurry.  For 
> example, if I create a 1000x1000 canvas (and the browser's backing store 
> is actually 1100x1100), and I send the finished data to the server (at 
> the exposed 1000x1000), the browser has to resample the final image, 
> blurring it.

Yup. If you want to do graphics and know the resolution of the backing 
store, doing it on the client is a poor path. You don't know what 
resolution the image you get back will be in.

> > I went to books.google.com, opened up the first book in my library, 
> > and zoomed in, and it reflowed and rerendered the text to be quite 
> > crisp. I don't see any problem here. Images were similiarly handled 
> > beautifully.
> >
> > Could you elaborate on the steps to reproduce this problem?
> (I tried this, and text was blurry even when I zoomed using only that 
> page's built-in zoom mechanism; it seemed to be scaling the rendered 
> page and not rerendering text at all.  I figured some books might not be 
> OCR'd so I tried another couple books, but it still happened; then it 
> somehow crashed FF3, so I gave up.)


Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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