It would indeed be nice to have better approximation for the integral. The MRP method results this strange two-tail drop shaped highlight for sphere/disc lights at grazing angles for GGX (also apparent in Dropot’s article in GPU Pro 5).

]]>about the specular area light we are not satisfy of any solution we have try currently, including the one from Brian. That is why we have chose to not talk about it.

You are right that finding the closest point from reflection ray is hard and we have try to follow the papers you mention in the past. But it is way too costly. We end up with a coarse approximation (i.e we do not find the closest point of reflection ray). But other than the shape, the major trouble is the energy conservation part. After many fails, we have reuse the values that Brian use for the Sphere.

In practice (depends on the games because we have game that use them, other that doesn’t) our area lights are use sparingly and our lighters try to hide the artifacts. It still a benefit compare to punctual light but it have a cost.

In the future I want to get away from Brian’s method and try to follow something more mathematically correct.

]]>I got a question regarding your disk light specular calculation. You mentioned that you use Karis’ method for area light specular in general. For disk lights this would require finding closest point on the disk to the ray (reflection / dominant BRDF vector). However, this is quite involved calculation to do correctly in a shader as explained in this paper by Eberly: http://www.geometrictools.com/Documentation/DistanceToCircle3.pdf

Are you using Eberly’s method or some kind of approximation? I tried couple of approximations myself which unfortunately gave strange specular shape for tilted disk lights. ]]>