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[css-transforms] SVG WG Review of CSS Transform Proposal

From: Doug Schepers <schepers@w3.org>
Date: Tue, 03 Mar 2009 02:59:05 -0500
Message-ID: <49ACE349.9020004@w3.org>
To: www-style CSS <www-style@w3.org>, www-svg <www-svg@w3.org>
Hi, CSS Working Group-

The SVG Working Group recently discussed the proposals for CSS 2D and 3D 
Transform modules [1,2]. We appreciate and support the desire to add 
transformation functionality to CSS, and we would like to work with the 
CSS WG to ensure compatibility between the CSS WG and SVG WG work in 
this area.  We look forward to the CSS WG review of our corresponding 
specification.  We understand that this review comes early, before FPWD, 
but hope that helps move everyone forward faster.

As you know, the SVG WG has also been working on a 3D/2.5D/Perspective 
Transforms specification to extend the existing SVG transforms, based on 
the implementation experience some of the SVG implementers have reported 
in making their own custom extensions.  This seems like an area of much 
interest and value to authors.  We have modified our own proposal to 
match the CSS 3D Transfrom spec where possible, and have prepared it for 
publication; to make sure that content authors are not confused, we 
would like to publish the FPWD of our spec on the same day as the CSS 
Transforms specs, with a news item explaining that we are working 
together to ensure mutual compatibility.  We propose March 11, 2009.

Below are some questions and comments about the proposed CSS Transform 
specifications.  Please feel free to break them down into individual 
emails for discussion.

We would like to know if there are any published use cases and 
requirements for the proposed modules.  It would help us if we could 
review the proposals from a better understanding of the particular use 
cases and requirements you wish to support.  In particular, as discussed 
below, we would like to know what are the use cases for supporting 
general 4x4 3D transformation matrices, as opposed to affine 3D plus 
perspective projection.

Are translateX/Y/Z and scaleX/Y/Z necessary, given that the proposal 
includes translate3d and scale3d? There seems to be little benefit to 
this redundancy.  In response to minutes from our F2F where we were 
discussing CSS Transforms, Dean said[3]:

> While these might shortcuts look like syntactic sugar, they allow for
> something important - which is the ability to break a transform list into
> components that can manipulated individually. This is especially important
> when you're animating between transforms (not necessarily with CSS
> Animations, even JS gets the benefit). Flattening all the transform
> operations is a lossy process (not in the final matrix result, but you lose
> the list).

This seems to be an argument against removing translateX/Y/Z etc. and 
requiring authors to use matrix3d(), rather than an argument against 
'translate' etc, taking optional arguments.  We understand that 
flattening the matrix is a lossy process, however isn't translateZ(40) = 
translate3d(0 0 40)?  You can still get that individual translation 
component out of the list.

> In general I'm not sure there is benefit in minimising the syntax. What is
> the cost?

The cost is minimal by itself, but these things add up, and it seems the 
benefit is minimal too.  To do y-translation in SVG, you just do 
translate(0 40); it doesn't seem particularly onerous to specify the 0.

The SVG Working Group appreciates that the CSS 3D Transforms proposal 
tries to use naming consistent with SVG Transforms. Regarding this, has 
the CSS Working Group considered extending the 'scale', 'translate' and 
'rotate' SVG transform items to allow for both 2D and 3D transforms to 
be applied based on the values passed in (as opposed to creating extra 
scale3d et. al. properties)?

For example:
  * scale(<sx> [<sy> [<sz>]])
  * translate(<tx> [<ty> [<tz>]])
  * rotate(<rotate-angle> <nx> <ny> <nz> [<cx> <cy> <cx>])

Since backward compatibility is important we tested current browsers. 
In most, the entire transform attribute is ignored if it contains a 
transform item that is invalid under the limitations of SVG 1.1 
transforms (such as a scale() with three values). In Opera, all of the 
transform items up to the invalid transform item are applied, and only 
the invalid item and those that follow are ignored.

In SVG and the CSS 2D Transforms proposal, 'translate' and 'scale' both 
contain optional parameters so that both symmetric and asymmetric 
transformation of all axes are possible. Has the CSS Working Group 
considered optional parameters for scale3d, translate3d and rotate3d as 

For example:
  * scale3d(<sx> [<sy> <sz>])
  * translate3d(<tx> [<ty> <tz>])
  * rotate3d(<angle> <nx> <ny> <nz> [<cx> <cy> <cz>])

Is there any reason a 4x4 transformation model was chosen? OpenVG uses a 
3x3 matrix interface and SVG devices that implement CSS 3D Transforms as 
currently specified will not be able to take advantage of the OpenVG 
API. There exist implementations of SVG on top of OpenVG (such as 
AlexVG) which otherwise would need to implement their own transform 
pipeline.  One of the advantages of using OpenVG is existing 
implementations get hardware acceleration.  Note that the current 
proposed SVG Transforms Module model is compatible with OpenVG and 
OpenGL.  We would like to see use cases for specifying 4x4 matrices 
which aren't simply affine 3D transformations plus a single point 
projection transformation.

Dean in his mail said:
> I suggest you stick with 4x4 matrices over 3x3. I wouldn't let the fact
> that OpenVG is 3x3 (is it? I read this in the minutes but didn't check)
> impact the decision.

We believe so.  On http://www.khronos.org/openvg/ it states "Image 
drawing uses a 3x3 perspective transformation matrix".

> Cameron says that "general 3d effects are not useful" - maybe I
> misunderstood that?

The intent of Cameron's comment was more along the lines of wondering 
what effects that you can achieve with a general 4x4 matrix that aren't 
supported by affine 3D plus perspective projection.

Is there a reason for allowing perspective to be part of the transform 
list as well as being a separate property?  Authors could specify 
multiple perspective transformations (and possibly also the perspective 
property), which would produce odd results.

For example:
transform="translate(...) perspective(...) scale(...) rotate3d(...) 

The SVG Working Group thinks that 'transform-origin' is a really good 
property; however, it has the following issues:

* Its initial value is "50% 50% 0", which is incompatible with how SVG 
transforms work in the absence of transform-origin (i.e., SVG behaves as 
if "0 0 0" is the default value).  Anne suggested on IRC[4] that SVG 
elements could have an initial value of "0 0 0" while other elements 
have "50% 50% 0", but this might be confusing for authors.

* The property appears to be overloaded in that it seems that the 
coordinate space that the property operates in depends on the values. Is 
this correct?

* If lengths are specified for transform-origin is the length 
measurement taken from the origin of the current coordinate system or 
from the top left hand corner of the object's bounding box?  What if the 
property was broken up into transform-origin-bbox and 
transform-origin-userspace or something similar?

We discussed 'transform-style' which automatically orders rendering of 
objects based on Z position of the object when set to "preserve-3d". As 
Dean mentions, rendering of the content may be difficult for SVG UAs 
given that SVG uses the painter's algorithm. One possibility is to only 
allow re-ordering of objects based on their Z position when inside a 
container with the transform-style property set. A similar option is to 
introduce a container element <layeredG>, <g3d>, or <layer> where the 
order of rendering the child elements is based on their Z position and 
not the document order. This would help restrict the complexity when 
rendering objects with 3D transforms in SVG. The SVG Working Group 
believes this is one possible solution which helps reduce the 
complexities involved in implementing SVG Transforms. Any comments the 
CSS Working Group has regarding this would be greatly appreciated.

Objects with depth may not always be parallel to the screen (as a result 
of rotations). Two problems can occur as a result of this. Firstly, what 
is the Z position when an object is not parallel to the screen (e.g. if 
it has a rotateY applied to it)? Is it the "closest" appearing Z point, 
the "furthest" appearing Z point, the center of the object, the top left 
point, or the 4th column 3rd row of the transform matrix?  Secondly, if 
two objects have the same Z position, are both not parallel to the 
screen and intersect, are the objects clipped or is one rendered on top 
of the other in document order?

The CSS Transform proposal says in the introduction
Any value other than ‘none’ for the transform results in the creation of 
both a stacking context and a containing block. The object acts as 
though position: relative has been specified, but also acts as a 
containing block for fixed positioned descendants. The position on the Z 
axis of a transformed element does not affect the order within a 
stacking context. With elements at the same z-index, objects are drawn 
in order of increasing z position.

z-index is only mentioned once in the proposal. We assume that this is 
z-index as defined in the CSS 2.1 specification[5]   and uses a stacking 
context as described in Appendix E. How would this work with documents 
that do not establish a stacking context (or alternatively, put 
everything in one stacking context) such as SVG?

In CSS Transforms, rotate() takes an <angle> value, which must have a 
unit.  In SVG, this takes a unitless number (assumed to be in degrees). 
  The SVG WG is open to exploring the addition of units in transform 
values, including angle units, into SVG.  We would expect to allow 
'deg', 'grad', and 'rad' as units, as already supported in some other 
SVG attributes, and perhaps to allow for the 'turn' unit also.  For 
backwards compatibility, we would want both SVG and CSS Transforms to 
allow unitless values as well, with degrees as the default if no unit is 

Finally, the CSS list syntax states that "If a function takes more than 
one argument, the arguments are separated by a comma (‘,’) with optional 
whitespace before and after the comma."  The SVG specification allows as 
a list-delimiter token a non-zero-length string consisting of an 
optional comma and any number of whitespace characters, (\s+|\s*,\s*) in 
perl regex syntax.  The SVG WG intends to allow this as a general rule 
in any SVG attributes, to make authoring and implementing easier and 
more consistent.  We would like for CSS Transforms to allow this as well.

[3] http://www.w3.org/mid/DE386490-A480-4D9F-939C-F17C554925B6@apple.com
[4] http://krijnhoetmer.nl/irc-logs/whatwg/20090225#l-201
[5] http://www.w3.org/TR/CSS21/visuren.html#propdef-z-index
[6] http://www.w3.org/TR/CSS21/zindex.html

-Doug Schepers, on behalf of the SVG WG
Received on Tuesday, 3 March 2009 07:59:19 UTC

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