If it's computing theta/2, perhaps the trig formulas are for converting from a quaternion to a matrix? Does it look like the matrix in the quaternion section of this page?
http://en.wikipedia.org/wiki/Rotation_operator_(vector_space) On Mon, May 14, 2012 at 10:30 PM, Shawn Singh <[email protected]>wrote: > > Hi all, > > I'm looking at TransformationMatrix::rotate3d(rx, ry, rz). This code does > something indirect, and I don't understand why. Instead of initializing > each rotation using sin(theta), cos(theta), the code computes theta/2, and > then uses trig identities to initialize the rotation matrix. > > I checked really quickly with fprintf, and it seems like we could actually > gain 1-2 bits of precision if we avoid doing this, and use sin(theta) and > cos(theta) directly. In the current code, more error seems to accumulate > due to sin^2 (theta / 2). Squaring that value instantly increases the > error inherent in the computation. I cannot think of any valid reason that > this code uses those trig identities instead of directly using sin and cos. > Does anyone else know why? Is this worth changing to gain some precision? > > On a secondary note, its also fishy that we are freely mixing floats and > doubles in the rotation code. But, I don't think that is as significant > error accumulation as the sin^2. > > Thanks, > ~Shawn > > _______________________________________________ > webkit-dev mailing list > [email protected] > http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev > >
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