Sampling

Sampling describes how many measurements a render engine takes to work out the color of each pixel. In path tracing, every sample is a random path that leaves the camera, bounces around the scene and eventually reaches a light. Averaging those paths gives the pixel its final value, and the more samples there are, the closer the average gets to the true answer and the less grain remains.

Samples versus time

Sampling is usually expressed as samples per pixel (spp). Doubling the samples does not halve the noise; the improvement slows down as you go. Past a certain point, extra samples add a lot of time for very little visible gain. In practice artists set a fixed sample count, a time limit or a target noise threshold. Adaptive sampling goes a step further: it stops working on areas that are already clean and spends the effort where noise lingers, such as behind glass, in corners lit only by bounced light and in glossy reflections.

Sampling in architectural rendering

A facade in direct sunlight clears up with relatively few samples. A room with the curtains half drawn, lit only by light bouncing in from a window, needs many more. Sampling and denoising are best planned together: a reasonable sample count followed by a light denoising pass. If you want the physics behind it, our article What Is Path Tracing? walks through how light paths are traced.