Physically based lighting

Seeing as we’re aiming for a bleeding edge engine, there is no need to skip out on anything. A little bird whispered in my ear that there are other ways of performing lighting than the standardized simple blinn-phong method commonly used, and since we’re on a pretty flexible budget when it comes to graphics performance, I thought I should give it a good looksie.

Physically based lighting basically takes more into account than regular lighting. It also provides a more ‘real’ representation of the world in terms of reflective light (albedo) and surface roughness/gloss. Couple that with the original cheat called normalmaps and you got yourself some pretty good-looking effects. Basically, all  materials have been added with a new roughness map which allows a graphics artist to author the surface complexity of a model. This allows lighting to properly respond to the surface instead of just applying a uniform specular reflectiveness. The shader code (mostly taken and translated from http://www.altdevblogaday.com/2011/08/23/shader-code-for-physically-based-lighting/) looks like this:

float normalizationTerm = (roughness + 2.0f) / 8.0f;
float blinnPhong = pow(NH, roughness);
float specularTerm = normalizationTerm * blinnPhong;
float cosineTerm = NL;
float base = 1.0f - HL;
float exponent = pow(base, 5.0f);
vec3 fresnelTerm = specColor.rgb + ( 1.0f - specColor.rgb ) * exponent;
float alpha = 1.0f / ( sqrt ( (PI / 4) * roughness + (PI / 2)) );
float visibilityTerm = (NL * (1.0f - alpha) + alpha ) * ( NV * ( 1.0f - alpha ) + alpha );
visibilityTerm = 1.0f / visibilityTerm;
float3 spec = saturate(specularTerm * cosineTerm * fresnelTerm * visibilityTerm) * lightColor.xyz;

As you can see, this code is way more complex than the standard formulae. What you can see here is that instead of using a constant value for specular power, we instead use the roughness. This allows us to have a per-pixel roughness authored by a graphics artist. The only downside to this is that roughness is somewhat unintuitive in terms of encoding/decoding. To decode roughness, which is a value in the range [0..1], I use this formula (taken from  Physically-based Lighting in Call Of Duty: Black Ops):

float specPower = exp2(10 * specColor.a + 1);

This allows our specular power to be in the range [1, 8192]. Since our method uses the Blinn-Phong algorithm for distribution, our specular power is much greater than the range [0..1], however easier to compute than the more advanced yet more intuitive Beckmann algorithm (which actually operates in the range [0..1]). The result can be seen in the picture below:

A part of performing physically based lighting is to also use reflections and proper ‘roughing’ of the reflections. Reflections affects both specular light (since it’s actually a reflection, go figure) and the final color of the surface. To account for this in our completely deferred renderer, the environment maps on reflective objects take roughness into account, and selects a specific mip-level in the environment map based on the roughness. The awesome tool (https://code.google.com/p/cubemapgen/) can take an ordinary cube map and generate mips where each mip is a BRDF approximation (actually there are several different algorithms, but for the sake of clarity we’ll stick to Blinn-Phong BRDFs). We can also say to generate a new mip using a glossness falloff, resulting in a very good-looking mip-chain for our cube maps.

You have have come across this image http://seblagarde.files.wordpress.com/2011/07/reference_top_ref_bottom_mipchain.jpg showing a series of cube maps with different levels of reflectiveness, which is exactly what we are doing and what we want. Just to clarify, this is all precomputed using an original cube map and is not done in real-time! The more interesting part is that what is visible in your environment cube map is irrelevant. What is relevant is that the average color of the cube map fits your scene in terms of colors and lighting. In the pictures below, we have the same model ranging in roughness from 0-1.

As you can see, the roughness changes the surface look of the object dramatically, although still uses the exact same shader. Also note that using the skydome cube map as the reflective cube map is a bit ugly since it’s half bright half dark. That’s all!

 

// Gustav