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Re: [css3-images] `image-resolution: span` and transformations.

From: Simon Sapin <simon.sapin@exyr.org>
Date: Fri, 26 Jul 2013 18:21:38 +0100
Message-ID: <51F2B022.80509@exyr.org>
To: www-style@w3.org
Le 26/07/2013 17:56, Tab Atkins Jr. a écrit :
> On Fri, Feb 1, 2013 at 5:56 AM, Simon Sapin <simon.sapin@kozea.fr> wrote:
>> The snap keyword for image-resolution is defined as:
>>
>>> If the "snap" keyword is provided, the computed ‘<resolution>’ (if
>>> any) is the specified resolution rounded to the nearest value that
>>> would map one image pixel to an integer number of device pixels. If
>>> the resolution is taken from the image, then the used intrinsic
>>> resolution is the image's native resolution similarly adjusted.
>>
>> <resolution> values of image-resolution count image pixels per CSS pixels.
>> snap is about image pixel per device pixels. CSS transforms can change the
>> ratio between CSS pixels and device pixels[1].
>>
>> Therefore, should `snap` be affected by transforms? If this is done at
>> computed value time, do implementations need to have accumulated
>> transformation matrices that early?
>>
>>
>> [1] Although I’m not sure that ratio still makes sense if the transformation
>> has any skew component, a rotation component that is not a multiple of 90°
>> around the Z axis, or a scale component that does not preserve aspect
>> ratios.
>
> I don't think it should.  For one, this would mean that the intrinsic
> size of an image changes as you transform it, which is clearly not a
> good result.
>
> For two, as you note, only a limited set of transforms can reasonably
> affect this - what happens if you're just using "scale(x)", and then
> change to "scale(x) skew(y)" on hover?  Ignore the entire transform
> effect?  Filter the transform for things we can affect?  What if the
> skew() is instead before the scale()?

Agreed for transforms.

Your first point also applies to user zoom, especially with mobile-style 
panning zoom. What does "snap" mean in this context?


> Plus, for the purpose of drawing into a <canvas>, we're choosing to
> report the native screen resolution * zoom level, but aren't paying
> attention to transforms.  This should work similarly.

I don’t understand, can you expand on that? I’m not very familiar with 
canvas, how does it expose the device resolution?

-- 
Simon Sapin
Received on Friday, 26 July 2013 17:22:01 UTC

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