SSE 4-Ray vs. Sphere?

Practical and theoretical implementation discussion.
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ironwallaby
Posts: 8
Joined: Sun Dec 30, 2012 9:48 pm

SSE 4-Ray vs. Sphere?

Post by ironwallaby » Thu Aug 08, 2013 8:48 pm

I'm finally getting with the program and working on learning SSE, but I'm having trouble trying to build a 4 ray vs. 1 sphere intersection function. While it's trivial in single-ray land, when working with SIMD instructions I'm getting pretty confused about how one goes about handling branching.

Does anybody have any pointers to where I can read more about the topic, or (better yet) have any example code that I could dig into?

Thanks!

graphicsMan
Posts: 164
Joined: Mon Nov 28, 2011 7:28 pm

Re: SSE 4-Ray vs. Sphere?

Post by graphicsMan » Thu Aug 08, 2013 8:52 pm

This appears to show how to do this:

http://felix.abecassis.me/2012/08/sse-v ... onal-code/

ironwallaby
Posts: 8
Joined: Sun Dec 30, 2012 9:48 pm

Re: SSE 4-Ray vs. Sphere?

Post by ironwallaby » Fri Aug 09, 2013 7:33 pm

Thanks a ton, that was exactly the nudge I needed.

For example, here's a 4-ray vs. sphere:

Code: Select all

static v4sf intersect(const ray4_t *ray) {
  /* Compute discriminant. */
  const v4sf b = -dot(&(ray->origin), &(ray->direction)),
             d = b * b - sq(&(ray->origin)) + V4SF_ONE,
             t = b - __builtin_ia32_sqrtps(d);

  /* Which rays hit the sphere? (Note that this works since any rays that
   * didn't hit the sphere will be NaN, because we had to sqrt a negative
   * number. Comparing that to zero will be false as a result.) */
  const v4si mask = (v4si) __builtin_ia32_cmpgeps(t, V4SF_ZERO);

  /* Return the rays that hit the sphere, or infinity otherwise. */
  return __builtin_ia32_orps(
    __builtin_ia32_andps((v4sf) mask, t),
    __builtin_ia32_andnps((v4sf) mask, V4SF_INFINITY)
  );
}
(Yeah, yeah, the sphere is always a unit sphere centered on the origin and we only check against the outside, but one thing at a time. :) I also don't know if it's faster to bail early if all of the discriminants are negative or not. I'll have to test.)

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