--- a/layout/svg/nsFilterInstance.cpp
+++ b/layout/svg/nsFilterInstance.cpp
@@ -22,16 +22,33 @@
 #include "SVGContentUtils.h"
 #include "FilterSupport.h"
 #include "gfx2DGlue.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
 using namespace mozilla::gfx;
 
+FilterDescription
+nsFilterInstance::GetFilterDescription(nsIContent* aFilteredElement,
+                                       const nsTArray<nsStyleFilter>& aFilterChain,
+                                       const UserSpaceMetrics& aMetrics,
+                                       const gfxRect& aBBox,
+                                       nsTArray<mozilla::RefPtr<SourceSurface>>& aOutAdditionalImages)
+{
+  gfxMatrix unused; // aPaintTransform arg not used since we're not painting
+  nsFilterInstance instance(nullptr, aFilteredElement, aMetrics,
+                            aFilterChain, nullptr, unused,
+                            nullptr, nullptr, nullptr, &aBBox);
+  if (!instance.IsInitialized()) {
+    return FilterDescription();
+  }
+  return instance.ExtractDescriptionAndAdditionalImages(aOutAdditionalImages);
+}
+
 static UniquePtr<UserSpaceMetrics>
 UserSpaceMetricsForFrame(nsIFrame* aFrame)
 {
   if (aFrame->GetContent()->IsSVG()) {
     nsSVGElement* element = static_cast<nsSVGElement*>(aFrame->GetContent());
     return MakeUnique<SVGElementMetrics>(element);
   }
   return MakeUnique<NonSVGFrameUserSpaceMetrics>(aFrame);

--- a/layout/svg/nsFilterInstance.h
+++ b/layout/svg/nsFilterInstance.h
@@ -53,16 +53,29 @@ class nsFilterInstance
   typedef mozilla::gfx::SourceSurface SourceSurface;
   typedef mozilla::gfx::DrawTarget DrawTarget;
   typedef mozilla::gfx::FilterPrimitiveDescription FilterPrimitiveDescription;
   typedef mozilla::gfx::FilterDescription FilterDescription;
   typedef mozilla::dom::UserSpaceMetrics UserSpaceMetrics;
 
 public:
   /**
+   * Create a FilterDescription for the supplied filter. All coordinates in
+   * the description are in filter space.
+   * @param aOutAdditionalImages Will contain additional images needed to
+   *   render the filter (from feImage primitives).
+   * @return A FilterDescription describing the filter.
+   */
+  static FilterDescription GetFilterDescription(nsIContent* aFilteredElement,
+                                                const nsTArray<nsStyleFilter>& aFilterChain,
+                                                const UserSpaceMetrics& aMetrics,
+                                                const gfxRect& aBBox,
+                                                nsTArray<mozilla::RefPtr<SourceSurface>>& aOutAdditionalImages);
+
+  /**
    * Paint the given filtered frame.
    * @param aDirtyArea The area than needs to be painted, in aFilteredFrame's
    *   frame space (i.e. relative to its origin, the top-left corner of its
    *   border box).
    */
   static nsresult PaintFilteredFrame(nsIFrame *aFilteredFrame,
                                      nsRenderingContext *aContext,
                                      const gfxMatrix& aTransform,
@@ -139,16 +152,22 @@ public:
   /**
    * Draws the filter output into aContext. The area that
    * needs to be painted must have been specified before calling this method
    * by passing it as the aPostFilterDirtyRegion argument to the
    * nsFilterInstance constructor.
    */
   nsresult Render(gfxContext* aContext);
 
+  const FilterDescription& ExtractDescriptionAndAdditionalImages(nsTArray<mozilla::RefPtr<SourceSurface>>& aOutAdditionalImages)
+  {
+    mInputImages.SwapElements(aOutAdditionalImages);
+    return mFilterDescription;
+  }
+
   /**
    * Sets the aPostFilterDirtyRegion outparam to the post-filter area in frame
    * space that would be dirtied by mTargetFrame when a given
    * pre-filter area of mTargetFrame is dirtied. The pre-filter area must have
    * been specified before calling this method by passing it as the
    * aPreFilterDirtyRegion argument to the nsFilterInstance constructor.
    */
   nsRegion ComputePostFilterDirtyRegion();