Sampling a Light Probe Image to a Light Source List

This tutorial explains how to sample a lighting environment image (eg light probe) into a chosen number of representative light sources using an area subdivision approach. Choice of areas and light source positions and intensities are computed based on the distribution of energy in parts of the image. In this way, the light source position and intensity most representative of each particular area is obtained. This is useful in rendering lighting environments where efficiency is paramount and a good approximation to the full resolution environment will suffice.

The first thing to do is to make sure that the light probe image is already in the correct panoramic format, in this case we require it to be in lat-long format. See this tutorial for how to convert images between different panoramic formats. Now we need to run the sampling algorithm on the lat-long image. Activate the dialog via the menu 'Image->Spherical Transformations->Sample Light Probe Image to Light Source List'.

Here's what the various dialog parameters mean:

Now, let's load up an image and look at the output generated by sampling using the variance minimization technique. We use the familiar 'Grace Cathedral' High-Dynamic Range light probe image, already represented as a lat-long image:

We then sample the image into 64 representative light sources:

For this example we have obtained both the light source and 2D graph output images and made a visualization of a combination of all three in order to show the placement and color of all available outputs. Note how the sample positions correspond to the most representative position in each rectangular area in terms of energy distribution.