shader cos/sin precision affecting rendering

[fstoner]

Low precision in the implementation of the glsl cos/sin functions causes some distortions as seen in the following 2D capture. The coastline shows a discontinuity around the middle (top-to-bottom) of the image.

Specifically, the use of the "sin" function in czm_latitudeToWebMercatorFraction seems to be the problem.

If you zoom in to a sufficient level, the transformation is assumed to be linear and the discontinuity does not appear.

[fstoner]

For testing, the above image is of http://en.wikipedia.org/wiki/Iturup island to the North-East of Hokkaido.

[kring]

@fstoner can you tell me any more about how to reproduce this? What hardware/browser it happened on for example? Even with DISABLE_CORDIC I can't seem to reproduce it.

[kring]

Nevermind, I can reproduce it on an Intel HD4600 GPU in Chrome on Windows.

[kring]

So it appears to me - correct me if I'm wrong - that the sin function on this Intel GPU is just completely broken. I understand that the standard does not guarantee any particular precision, and I would expect that to manifest as "flat spots" where small changes in x don't impact the output of sin(x). But this is much worse than that. In a region where sin(x) should be monotonically increasing, small changes in x result in sin(x) increasing for a bit, then decreasing just for fun, and then getting back to increasing.

GPUs make me very sad.

Given this kind of garbage from the GPU, the only solution I see is to compute the texture coordinates on the CPU, at least on GPUs with instruction limits that are too low to handle the cordic approach (and I still have no idea how to detect those GPUs).