Fix blendToIdentity case in PerspectiveTransformOperation::blend

Source/platform/transforms/PerspectiveTransformOperation.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 22 matching lines @@
 
 namespace WebCore {
 
 PassRefPtr<TransformOperation> PerspectiveTransformOperation::blend(const TransformOperation* from, double progress, bool blendToIdentity)
 {
     if (from && !from->isSameType(*this))
         return this;
 
     if (blendToIdentity) {
         double p = floatValueForLength(m_p, 1);
-        p = WebCore::blend(p, 1.0, progress); // FIXME: this seems wrong. https://bugs.webkit.org/show_bug.cgi?id=52700
+        p = 1.0 / WebCore::blend(1.0 / p, 0.0, progress);

[Julien - ping for review, 2013/12/05 01:18:45]

This formula is overly complicated and it took me a while to understand what was
going on. It can be simplified to:

p = p / (1 - progress)

This uncovers an issue in the formula: if progress == 1, we are going to do a
division by 0.

         return PerspectiveTransformOperation::create(Length(clampToPositiveInteger(p), Fixed));
     }
 
     const PerspectiveTransformOperation* fromOp = static_cast<const PerspectiveTransformOperation*>(from);
     Length fromP = fromOp ? fromOp->m_p : Length(m_p.type());
     Length toP = m_p;
 
     TransformationMatrix fromT;
     TransformationMatrix toT;
     fromT.applyPerspective(floatValueForLength(fromP, 1));
     toT.applyPerspective(floatValueForLength(toP, 1));
     toT.blend(fromT, progress);
     TransformationMatrix::DecomposedType decomp;
     toT.decompose(decomp);
 
     if (decomp.perspectiveZ) {
         double val = -1.0 / decomp.perspectiveZ;
         return PerspectiveTransformOperation::create(Length(clampToPositiveInteger(val), Fixed));
     }
     return PerspectiveTransformOperation::create(Length(0, Fixed));
 }
 
 } // namespace WebCore