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Re: Meaning of CSS units

From: Jon Ferraiolo <jferraio@Adobe.COM>
Date: Mon, 09 Aug 1999 09:47:07 -0700
Message-Id: <199908091643.JAA07030@mail-345.corp.Adobe.COM>
To: Ben Hutchings <benh@lsl.co.uk>
Cc: www-svg@w3.org
At 02:47 PM 8/2/99 +0100, Ben Hutchings wrote:
>Is the meaning of CSS units defined by the user space or the viewport
>space?
>
>I think they are defined by the viewport space.  7.7 says "the various
>absolute unit specifiers...represent lengths within the initial user
>coordinate system".  The initial user coordinate system is not
>necessarily the same as the initial viewport coordinate system, is it?

We need to clean up the wording in the spec about this. I just re-read
section 7.7 and I can see why people might be confused.

As background:

* At the start of an SVG document, there is an implicit "initial viewport
coordinate system" whose units match pixel units of the parent user agent.
Also, unless there is a fitBoxToViewport attribute on the <svg> element,
the "initial user coordinate system" is set to exactly match the initial
viewport coordinate system.

* The transform attribute modifies the current user coordinate system only;
it does not change the current viewport coordinate system

* An <svg> element embedded within an ancestor <svg> element establishes
both a new viewport coordinate system and a new user coordinate system.
Thus, the current viewport coordinate system does not necessarily match the
initial viewport coordinate system.

The answer is that CSS units represent values in the current viewport
coordinate system. The rules for handling CSS units are described in
section 7.9 of the July 30 spec.

>The introduction seems to shy away from making an explicit statement
>about this.

We will improve the descriptions in one of next drafts.

Jon Ferraiolo
SVG Editor
Adobe Systems Incorporated
Received on Monday, 9 August 1999 12:45:02 GMT

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