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Clarification of empty filter tag behavior

From: Felix Pahl <fpahl@web.de>
Date: Mon, 09 Feb 2004 16:05:44 +0100
Message-ID: <4027A1C8.24630C72@web.de>
To: www-svg@w3.org

The spec does not seem to define the behavior of an empty filter tag,
i.e. a filter tag containing no filter primitives. Batik and the
Adobe SVG viewer display different behavior, neither of which is
the behavior I would have expected.

Batik treats the empty filter tag as a no-op and renders the
filtered element as if it didn't have a filter property. It
doesn't even clip the element to the filter effects region.

The Adobe Viewer treats the empty filter as resulting in a
transparent canvas, i.e. it doesn't render the filtered
element at all.

What I would have expected, since it seems to be the most
natural extension of the behavior to this special case, is
that the element is rendered unfiltered, but clipped to the
filter effects region. By default (i.e. if no "in" attributes
are specified), each filter primitive takes an image and
produces a new image, with the original image being the
source graphics and the last image being the overall result
of the filter. (By the way, the first part (that the first
image is the source graphics) is explicitly stated in the
spec, but I couldn't find the second part (that the last
image is the overall result) -- this is of course sort of
obvious, but should be stated explicitly nonetheless.)
It seems natural to assume that if there is no filter
primitive to operate on it, the original image, i.e. the
source graphics, is also the last image, i.e. the
result of the filter. Certainly no fully transparent
"initial image" ever appears in the process (as it does
when the elements of a group are composited).
(Item 2 in the description of the example in Section
15.6 says "that the offscreen for the filter is
initialized to transparent black", but I believe this
is just a misleading way of expressing what is better
expressed in the last paragraph of Section 15.1, which
is a statement about filter *primitives*, not about the
filter as a whole. If I am wrong on this and the sentence
in Item 2 is right as it stands, it needs to be put in a
far more prominent place.)

Regarding clipping to the filter effects region:
Section 15.5 is slightly unclear on this -- it says:
"A 'filter' element can define a region on the canvas
to which a given filter effect applies". This could
mean either that the result of the filter effect
is clipped to the filter effects region, or it could
mean that the filter applies only inside the region,
whereas the filtered element is passed through unchanged
elsewhere. However, Section 3.7 (though it refers twice
to Chapter 15 where I believe the first reference is
meant to be to Section 15.5) is a bit clearer in implying
that the parts of the unfiltered element lying outside
the filter effects region are clipped. Batik and the
Adobe Viewer in general seem to agree on this, but in
the case of an empty filter tag, Batik doesn't clip the
unfiltered element to the filter effects region.

felix.
Received on Monday, 9 February 2004 10:33:09 GMT

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