Theory: Relief Mapping

It's time to check out the actual relief mapping technique, as we now know how parallax occlusion mapping works, this will be pretty straight forward.

In parallax occlusion mapping the illusion of depth was created by stepping fixed values, which gives a pretty nice result, but not as nice as we want it to be. What we want to do with relief mapping is to find the exact displacement for every fragment. In other words: the surfaces depth will be an exact match with the values that the height map gives us. If we can achieve that, then we could generate a very accurate illusion of depth in our surfaces.
How can we be so exact without making the ray tracer run forever? We use binary search. Say, we needed a step size of 1048576 to get a good result with our ray tracer, that isn't really practical. With binary search that could be achieved with only 20 steps (2^20 = 2048). Binary search in the ray tracer works like this: The currentHeight is initially set to 0.5, then we step either up or down. That depends on what our height shows: if the height map value is greater then our currentHeight we step up with currentHeight/2. If it is smaller, we step down with currentHeight/2 and stores this as a possible match. This procedure continues for the specified number of binary search steps.

Now, we got a match, but it isn't really exact yet. If an object occludes another object, using binary search, we will not get the correct silhouettes. It's pretty easy to understand because of what we stated above: we start height / 2, we step up, finds an intersection. Then we never will be stepping down, but the correct match could still be down there, because of this occlusion.
This can easily be solved though. If we use a simple linear search with a big step size before our binary search, we can find an intersection which is fairly good. This intersection is then refined with the binary search, just searching in the appropriate section, and there we have our exact match.

As you easily could understand, this shader is quite more hardware consuming then the parallax occlusion mapping shader. But when finding a sweet spot with the binary search step size and the linear search step size, we could use this in real time applications. On fairly powerful machines, this shader would run with pretty good frames per second. These days, it would be quite possible to have it as an enthusiast option in video games. As Intel Skylake, AMD Zen and the Radeon 300 series is upcoming, I'm sure we will see some popular game engines implementing this in the future.

Thats about it for the relief mapping shader. Gonna post some details and updates about the project next.  

  A pretty cool result of using relief mapping with shadowing that I found.