[Bug 89634] dEQP-GLES3: sin, cos, tan do not have enough precision

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89634

--- Comment #3 from Ian Romanick <idr at freedesktop.org> ---
We've now had a report of an actual user encountering this problem.  See bug
#91213.

Most apps provide well conditioned data to the trig functions, and I'd prefer
to not penalize their performance for the few apps that do not provide well
conditioned data.  I think we should provide a drirc option to disable
"correctness for huge angles."

We also have to be careful about how we reduce the angle.  The quality of the
trig functions decreases as the magnitude of the angle increases.  In some
applications wrapping from 2π to 0 could cause artefacts due to a discontinuity
introduced in the derivative.

For whatever value of Kπ we choose, we want to wrap values on the range (Kπ,
(K+1)π] to [-Kπ, Kπ].  Values on the range [-(K+1)π, -Kπ) should be treated
similarly.  There may still be a discontinuity in the derivative, but that
should minimise it.

So... the wrap function becomes ((a+Kπ) % 2Kπ) - Kπ, right?

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