Merge pull request w3c#342 from mfreed7/master

Clip backdrop root (input) image to border box of element

Author: chrishtr

filter-effects-2/Overview.bs

@@ -79,6 +79,10 @@ output as its SourceGraphic.
 
 Filter functions must operate in the sRGB color space.
 
+If the filter functions list includes a <a href="https://drafts.fxtf.org/filter-effects-1/#blurEquivalent">blur()</a>
+filter, the filter will be applied with edgeMode="duplicate", with the edge defined by the clipped, transformed border
+box of the element. See [[#BackdropRoot]].
+
 A computed value of other than ''backdrop-filter/none'' results in the
 creation of both a <a href="https://www.w3.org/TR/CSS21/zindex.html">stacking
 context</a> [[!CSS21]] and a <a
@@ -94,13 +98,14 @@ be seen, you can use "background-color: transparent;".
 
 # Backdrop Root # {#BackdropRoot}
 
-The <dfn>Backdrop Root Image</dfn> is defined as the entire painted output produced by
-drawing, in <a
-href="https://www.w3.org/TR/CSS2/zindex.html#painting-order">painting order</a>,
-all content between (and including) the <a>Backdrop Root</a> element and the
-child element that needs access to the "backdrop". No content that is a DOM
-ancestor of the Backdrop Root element should contribute to or affect the
-Backdrop Root Image.
+The <dfn>Backdrop Root Image</dfn> for an element E is the final image that would be produced by the following steps:
+1. Start at the <a>Backdrop Root</a> element that is the nearest ancestor of E.
+2. Paint all content, in <a href="https://www.w3.org/TR/CSS2/zindex.html#painting-order">painting order</a>, between (and including) the ancestor Backdrop Root element and element E.
+3. Flatten the painted content into a 2D screen-space buffer.
+4. Transform the border box of element E to 2D screen-space, and clip the final painted output to this border quad.
+
+Note: No content that is a DOM ancestor of the Backdrop Root element should
+contribute to or affect the Backdrop Root Image.
 
 A <dfn>Backdrop Root</dfn> is formed, anywhere in the document, by an element in any of the following scenarios. See [[#BackdropRootTriggers]] for more details on each:
  * The root element of the document (HTML).