gradient proposal

[Michael Brauer <michael.brauer@sun.com>]

Dear TC members,

my action item is a proposal for gradients in the Open Office XML format.

First of all, I would like to start with a comparison between the 
gradients in SVG and in the  OpenOffice.org XML format, by going along 
the SVG specification

Comparison
==========

<linearGradient>
----------------
This element is equivalent to <draw:gradient>/<draw:transparency> with 
draw:style="linear|axial" in the OpenOffice.org (OOo) XML spec.

gradientUnits="userSpaceOnUse|objectBoundingBox": This attribute does 
not exist in the OOo XML spec, but the behavior is like 
gradientUnits="objectBoundingBox". This equals to the default value of SVG.

gradientTransform="<transform-list>" : This attribute does not exist in 
the OOo XML spec. However, very simple transformations can be achieved 
by the draw:angle attribute.

x1, x2, y1, y2="<coordinate>": These attributes are not supported by the 
OOo XML spec. Instead of this, draw:style="linear|axial" and draw:angle 
can be used to specify the gradient direction.

spreadMethod="pad|reflect|repeat" : This attribute is not supported by 
the OOo XML spec, but the behavior is like spreadMethod = "pad", that is 
also the SVG default.

xlink:href="<uri>" : This attribute is not supported by the OOo XML 
spec. If inheritance of gradients would be supported, then most likely 
by a style:parent-style-name attribute that is used for other kind of 
styles already.


<radialGradient>
----------------
This element is equivalent to <draw:gradient>/<draw:transparency> with 
draw:style="radial".

gradientUnits, gradientTransform, spreadMethod, xlink:href: See 
<linearGradient>

cx, cy="<coordinate>": equivalent to draw:cx and draw:cy

r="<length>: This attribute is not supported by the OOo XML spec. The 
radius is calculated based on the target objects width and height, and 
the gradient's center. The details need to be clarified. However, the 
SVG specification also seems not to specify clearly how for instance 
r="50%" is interpreted if applied to a real rectangle.

fx, fy="<coordinate>": These attributes are not supported, but the 
behavior is like fx=cx and fy=cy. This equals to SVG's defaults.


<stop>
------

This element is not supported, but the OOo XML gradient specification 
equals a SVG gradient specification with exactly two stops.

offset="<number>|<percentage>": For the first stop, this attribute 
equals the draw:border attribute. For the second stop, this attribute's 
value is "100%".

stop-color: For the first stop, this attribute equals the 
draw:start-color of <gradient>. For the second stop, this attribute 
equals draw:end-color of <gradient>.

stop-opacity: For the first stop, this attribute is the opposite of the 
draw:start transparency of <transparency>. For the second stop, this 
attribute is the opposite of the draw:end transparency of <transparency>.

Missing Features
================

The following features exist in the OpenOffice.org XML specification, 
but do not exist in SVG:

- OOo XML supports ellipsoid, square and rectangular gradients (it might 
be that ellipsoid gradients can be archived in SVG by applying 
transformations).

- OOo XML supports an intensity for the start and end color values .

- OOo XML supports specifying a gradient and a transparency 
simultaneously to one target object.

- All OOo XML gradients have a name. This name is required to be 
referencable from a graphics style.


Proposal
========
Based on this comparison, I would like to make the following proposals, 
that are in fact more or less independent of each other:

a) Since the SVG gradient specification is more powerful than the OOo 
XML gradient specification, the Open Office XML format supports SVG's 
gradient definition in addition to the ones already existing. If a SVG 
gradient definition is used within the Open Office XML format, it must 
have a draw:name attribute attached. For simplicity, the gradientUnits 
attribute will be ignored. Parent gradients are only evaluated, if they 
are specified by an style:parent-style-name attribute (xlink:href might 
be specified additionally for SVG compatibility). SVG gradients are only 
evaluated if they are referenced by a draw:gradient attribute, but not 
if they are referenced by a draw:opacity attribute. The result of 
referencing a SVG gradient with draw:gradient and an opacity gradient 
additionally (with draw:opacity) is unspecified.

b) The <draw:transparency> element will be renamed to <draw:opacity>, 
draw:start and draw:end to draw:start-opacity and draw:end-opacity, and 
their meaning will be reversed. The draw:transparency attribute that can 
occur in <style:properties> will be renamed to <draw:opacity>.

c) Additional gradient definitions that are neither supported by OOo XML 
nor by SVG might be added if they are supported by existing applications.

d) We get in contact with the SVG working group to check whether the 
missing features (except the name that is office specific) can be added 
to SVG and to coordinate future gradient extensions with SVG.

Best regards

Michael