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Re: [csswg-drafts] [fx-filter-effects] Behavior of feTile is incompatible between browsers

From: Amelia Bellamy-Royds via GitHub <sysbot+gh@w3.org>
Date: Fri, 22 Dec 2017 20:43:30 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-353671675-1513975410-sysbot+gh@w3.org>
Interesting.

The issue is caused by the fact that the filter primitive subregion defined for your `<feImage>` extends outside the boundaries of the filter region defined for your `<filter>` element.

Firefox and WebKit are clipping the "filter primitive region" (aka the pattern tile) to the intersection of the two regions.

Chromium and Edge are clipping the image rendering to the filter region, but they are not clipping the pattern tile dimensions.  I think this is the closest interpretation of the spec text.

As an author, neither is intuitive, nor particularly useful. Instead, I would want to be able to define an infinitely-repeated texture tile in userSpace, without worrying about whether the reference tile overlaps the bounding box of the current shape.  (This is how a `<pattern>` works.)

[feTile definition](https://drafts.fxtf.org/filter-effects/#feTileElement): 

>  feTile replicates the reference tile in both X and Y to completely fill the target rectangle. The top/left corner of each given tile is at location (`x + i*width, y + j*height`), where (x,y) represents the top/left of the input image’s filter primitive subregion, width and height represent the width and height of the input image’s filter primitive subregion, and i and j can be any integer value. In most cases, the input image will have a smaller filter primitive subregion than the feTile in order to achieve a repeated pattern effect.

[filter primitive subregion definition]()

> All filter primitives have attributes x, y, width and height which together identify a filter primitive subregion which restricts calculation and rendering of the given filter primitive. The x, y, width and height attributes are defined according to the same rules as other filter primitives coordinate and length attributes and thus represent values in the coordinate system established by attribute primitiveUnits on the filter element.
> 
> 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 filter primitive subregion has a negative or zero width or height, the effect of the filter primitive is disabled.
>
> The filter region acts as a hard clip clipping rectangle on the filter primitive’s input image(s).
> 
> The filter primitive subregion acts as a hard clip clipping rectangle on the filter primitive result.
>
> All intermediate offscreens are defined to not exceed the intersection of the filter primitive subregion with the filter region. ...

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Received on Friday, 22 December 2017 20:43:35 UTC

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