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Re: box-shadow offset when transform:rotate is used

From: fantasai <fantasai.lists@inkedblade.net>
Date: Fri, 19 Feb 2016 21:38:42 -0500
To: mail@ChristianMayer.de, www-style@w3.org
Message-ID: <56C7D1B2.4030700@inkedblade.net>
On 02/18/2016 04:55 PM, Christian Mayer wrote:
> Hello all,
>
> currently the the box-shadow offset is calculated with regard to the
> element and then the transform is applied to both, the element and it's
> shadow.
>
> For a transform:translate() this makes mostly sense, but for a
> transform:rotate() it usually doesn't:
> The box-shadow offset should fake a lighting of the design so it should
> always go in the same direction for all elements (ignoring special uses
> for special effects). So when a element is rotated by a
> transform:rotate() the box-shadow is currently also rotated.
>
> The only way to show a consistently lighted design is to do the maths
> for the inverted rotation yourself and apply that to the box-shadow
> offset to compensate the transform:rotate().
> But this will only work on a static design.
>
> Once the design is e.g. animated by a transition things are getting very
> ugly. As
> http://stackoverflow.com/questions/35102216/keep-box-shadow-direction-consistent-while-rotating
> shows, some non obvious tricks must be used to do the compensation.
>
> So is there a way (or can it be added?) to make the offset always work
> in screen coordinates and not in element coordinates?
> E.g. by adding an additional keyword?
>
> Like
>
>    box-shadow: screen 2px 3px 4px #000
>    transform: translate(10px, 20px) rotate(40deg)
>
> to have the element translated and rotated and then on the final place
> of the screen a box shadow added that is painted 2px to the right and
> 3px to the bottom from exactly that place.
>
> This also applies to the text-shadow as well, of course.

Box shadows are intentionally kindof dumb, as graphical effects go.
They're used for a lot of different effects, many of which are actual
shadows, but not all. I'm not sure changing behavior here is a good
idea therefore.

It might be worth looking into applying filters post-transform. That
should handle shadows, and other things besides.

~fantasai
Received on Saturday, 20 February 2016 02:39:11 UTC

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