Re: Filter Effects: Filter primitive subregion clarification

On Fri, Aug 9, 2013 at 1:41 PM, Dirk Schulze <dschulze@adobe.com> wrote:

> Hi Stephen,
>
> The spec currently says:
>
> ""
> The filter primitive subregion act as a hard clip clipping rectangle on
> both the filter primitive's input image(s) and the filter primitive result.
> ""
>
> Which means the input and the output is clipped.
>
> As you noticed, implementations have different behavior. With slightly
> different tests you'll see that some implementations are not consistent
> with them self. I would very much prefer to remove the requirement of input
> AND output clipping. Reading the spec, it sounds like the original text
> intended just output clipping. I think this is more preferable then
> clipping both, input and output.
>

Thanks, Dirk.

I suppose you could make the argument that if you had only output clipping,
you could always achieve input-and-output clipping by inserting a no-op
node (e.g., FEOffset) with the filter primitive subregion set to the same
values as the child node, e.g. http://jsfiddle.net/C6zSD/3/. Whereas if you
have input-and-input clipping, there's no easy way to achieve output
clipping only.

However, the above example points out another difference: it looks like the
Firefox implementation of a clipped feGaussianBlur clamps the blur values
to the edge pixels ("clamp mode"), resulting in a solid black square,
rather than clamping to transparent black ("decal mode"), giving the soft
edges seen on Chrome, Safari and IE. It's not clear from the spec who is
correct here.

On Aug 9, 2013, at 6:52 PM, Stephen White <senorblanco@chromium.org> wrote:
>
> > On a related note, the subsequent paragraph is a bit confusing:
> > All intermediate offscreens are defined to not exceed the intersection
> of ‘x’, ‘y’, ‘width’ and ‘height’ with the filter region. The filter region
> and any of the ‘x’, ‘y’, ‘width’and ‘height’ subregions are to be set up
> such that all offscreens are made big enough to accommodate any pixels
> which even partly intersect with either the filter region or the
> x,y,width,height subregions.
> >
> > The last sentence here seems to contradict the first: if the offscreen
> is not to exceed the intersection of the subregion and filter region, how
> can it accommodate pixels which "intersect with either the filter region or
> the subregion" (sounds more like a union)?
>
> First, the easiest behavior is if you have x, y, width, height defined by
> the author:
>
> ""
> All filter primitives have attributes ‘x’, ‘y’, ‘width’ and ‘height’ which
> together identify a subregion which restricts calculation and rendering of
> the given filter primitive.
> ""
>
> I think it should be clear what the subregion is in this case.
>
> Now to your question above if the subregion is not completely defined by
> the author. You actually can not just copy paste one sentence:
>
> ""
> ‘x’, ‘y’, ‘width’ and ‘height’ default to the union (i.e., tightest
> fitting bounding box) of the subregions defined for all referenced nodes.
> If there are no referenced nodes (e.g., for <feImage> or <feTurbulence>),
> or one or more of the referenced nodes is a standard input (one of
> SourceGraphic, SourceAlpha, BackgroundImage,BackgroundAlpha, FillPaint or
> StrokePaint), or for <feTile> (which is special because its principal
> function is to replicate the referenced node in X and Y and thereby produce
> a usually larger result), the default subregion is 0%, 0%, 100%, 100%,
> where as a special-case the percentages are relative to the dimensions of
> the filter region, thus making the default filter primitive subregion equal
> to the filter region.
> ""
>
> If the subregion is not specified, then there are two conditions:
> 1) filter primitive has input filter primitives
> 2) filter primitive does not have inputs.
>
>
OK, so in this sentence:

"The filter region and any of the ‘x’, ‘y’, ‘width’ and ‘height’ subregions
are to be set up such that all offscreens are made big enough to
accommodate any pixels which even partly intersect with either the filter
region or the x,y,width,height subregions."

the "either/or" refers to the filter primitive subregion components (if
specified) or the filter region components (if not)? That seems confusing
to me, since it seems to be specifying the union of two rectangles, rather
than the fallback of each component of the filter primitive subregion to
the filter region, which is already taken care of by the other paragraph.

JMHO,

Stephen


> To 1) If there are input filter primitives, then the sub regions is the
> union of all subregions of each input filter primitive. This union is
> clipped with the filter region.
> To 2) If there is no input filter primitive (like for the pseudo filter
> primitives), then you use 0%, 0%, 100% and 100% of the filter region.
>
> I hope that clarified your question.
>
> Greetings,
> Dirk