(Task 7) Environment Lighting - cmu462/Scotty3D GitHub Wiki

The final task of this assignment will be to implement a new type of light source: an infinite environment light. An environment light is a light that supplies incident radiance (really, the light intensity dPhi/dOmega) from all directions on the sphere. Rather than using a predefined collection of explicit lights, an environment light is a capture of the actual incoming light from some real-world scene; rendering using environment lighting can be quite striking.

The intensity of incoming light from each direction is defined by a texture map parameterized by phi and theta, as shown below.

In this task you need to implement the EnvironmentLight::sample_L() method in static_scene/environment_light.cpp. You'll start with uniform direction sampling to get things working, and then move to a more advanced implementation that uses importance sampling to significantly reduce variance in rendered images.

Step 1: Uniform sampling

To get things working, your first implementation of EnvironmentLight::sample_L() will be quite simple. You should generate a random direction on the sphere (with uniform (1/4pi) probability with respect to solid angle), convert this direction to coordinates (phi, theta) and then look up the appropriate radiance value in the texture map using bilinear interpolation (note: we recommend you begin with bilinear interpolation to keep things simple.)

You an designate rendering to use a particular environment map using the -e commandline parameter: (e.g., -e ../exr/grace.exr )

Since high dynamic range environment maps can be large files, we have not included them in the starter code repo. You can download a set of environment maps from this link.

Tips:

• You must write your own code to uniformly sample the sphere.
• envMap->data contains the pixels of the environment map
• The size of the environment texture is given by envMap->w and envMap->h.

Step 2: Importance sampling the environment map

Much like light in the real world, most of the energy provided by an environment light source is concentrated in the directions toward bright light sources. Therefore, it makes sense to bias selection of sampled directions towards the directions for which incoming radiance is the greatest. In this final task you will implement an importance sampling scheme for environment lights. For environment lights with large variation in incoming light intensities, good importance sampling will significantly improve the quality of renderings.

The basic idea is that you will assign a probability to each pixel in the environment map based on the total flux passing through the solid angle it represents. We've written up a set of notes for you here.

Here are a few tips:

• When computing areas corresponding to a pixel, use the value of theta at the pixel centers.
• We recommend precomputing the joint distributions p(phi, theta) and marginal distributions p(theta) in the constructor of EnvironmentLight and storing the resulting values in fields of the class.
• Spectrum::illum() returns the luminance (brightness) of a Spectrum. The probability of a pixel should be proportional to the product of its luminance and the solid angle it subtends.
• std::binary_search is your friend. Documentation is here.