[Webkit-unassigned] [Bug 52700] Transition from perspective(500px) to 'none' is probably wrong

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=52700





--- Comment #2 from Chris Marrin <cmarrin at apple.com>  2011-01-19 11:43:09 PST ---
The formula for blendToIdentity is right:

     if (blendToIdentity)
        return PerspectiveTransformOperation::create(m_p + (1. - m_p) * progress);

Because it's blending down to the identity perspective value, which is 1. 

The other case is:

        return PerspectiveTransformOperation::create(decomp.perspectiveZ ? -1.0 / decomp.perspectiveZ : 0.0);

In the other case we construct from and to matrices and then let TransformationMatrix blend them. TransformationMatrix decomposes both matrices and then linearly interpolates all the decomposed values. Then composes the result into a new matrix and returns the value. PerspectiveTransformOperation::blend then decomposes that matrix and picks out the perspective Z value, which is the one we want.

I did it this crazy way because perspective is not linear, so applying a linear interpolation would give us the wrong results.

So I believe the code is correct, although we could certainly optimize the interpolation!

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