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Re: [fxtf-drafts] [transforms-2] Dubious definition of perspective(0)

From: Amelia Bellamy-Royds via GitHub <sysbot+gh@w3.org>
Date: Fri, 07 Apr 2017 19:02:16 +0000
To: public-fxtf-archive@w3.org
Message-ID: <issue_comment.created-292624418-1491591734-sysbot+gh@w3.org>
On the specific issue of the "neutral elements addition": that entire section needs to be re-written, as tracked in https://github.com/w3c/csswg-drafts/issues/932.  The "neutral" perspective value is actually `perspective(Infinity)` (the same as no perspective modification at all), although I don't think we currently allow `perspective(Infinity)` to be specified directly.

`perspective(0)` in general, however, suffers from the problem that it is logically sensible, but mathematically unfeasible as currently defined.  The perspective distance is relative to the X-Y plane, so it is really a position, not an absolute distance.  Even negative values are logical so long as the objects being viewed are also at (more) negative _z_ coordinates.  In order to create useful matrices, however, you would need to normalize the entire scene relative to the viewer/perspective distance.

Practical use cases of allowing zero and negative perspective distances would be to create a "zoom-in" effect of the viewer passing through a 3D scene.  Currently, to make it work you have to instead treat the viewer's perspective as a fixed, finite, positive _z_ position, and then translate the entire scene along the _z_-axis towards the viewer.

GitHub Notification of comment by AmeliaBR
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Received on Friday, 7 April 2017 19:02:22 UTC

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