Path Tracing is a rendering algorithm similar to raytracing in which rays are thrown from a virtual camera and tracked through a simulated scene for example for a 3D configurator. Path tracing uses random samples to gradually calculate a final image. The random sampling process makes it possible to represent some complex phenomena that are not covered by regular raytracing, but it usually takes longer to produce a high-quality path tracing image.
Random sampling in path tracing causes noise to appear in the rendered image. The noise is removed by the algorithm generating more samples, i.e. color values resulting from a single beam. The Path Tracing algorithm is explained in more detail below.
In Path Tracing, the beams are randomly distributed within each pixel in the camera room and at each intersection with an object in the scene, a new reflection beam is generated pointing in a random direction. After a certain number of bounces, each beam eventually leaves the scene or is absorbed. When a beam finishes rummaging around the scene, a sample value is calculated based on the objects against which the beam bounced. The sample value is added to the average of the source pixel.
The samples in a path-traced image are evenly distributed across all pixels. The color of each pixel is the average of all samples calculated for that pixel.
The random components in Path Tracing make the rendered image appear noisy. The noise decreases over time as more and more samples are calculated.
Samples per Pixel (SPP).
The decisive factor for the render quality is the number of samples per pixel (SPP).
The higher the SPP you have in a rendered image, the less noise you will notice. However, the additional quality per sample decreases the more samples you already have (since each sample only contributes to an average of all samples). For example, the difference in image quality between 20,000 SSP and 21,000 SSP will not be as noticeable as between 1,000 SSP and 2,000 SSP.
Sunlight does not require a high SSP to produce a beautiful image. Outdoor shots can be played back with relatively low SPP when sunlight is activated. Emitters (torches, lava, glow stones, pumpkins, etc.) like SSP need to reduce noise, so indoor shots and similar scenes in low-light environments require a much higher SPP number to look good.
There is no clear answer to how long it will take to render a scene. The general guideline is: the longer you render an image, the better it will be. Consider the declining returns discussed above.
The time it takes to render a beautiful looking image depends on how well lit the scene is, how many samples per second the renderer can produce (depending on how fast your CPU is), and how many pixels the canvas has.
The scaling of the canvas has an effect on the render time proportional to the pixel area of the canvas. An image of 800 x 800 pixels takes four times as long to achieve the same quality as an image of 400 x 400 pixels, because the total number of pixels has quadrupled. If your renderings take too long, you can reduce the canvas size to get faster results.
More about noise.
Small but bright light sources, such as flashlights, bring a lot of noise into the scene. It takes a particularly long time to render a scene, which is usually illuminated by a few torches. This is an unfortunate and inevitable disadvantage of the Path Tracing Rendering method.
The reason for this effect is the low probability that every light path scanned contains the flashlights compared to the high luminance of the object. The end result is the average of all samples, but the average can be “too high” for a long time due to the high luminance. The average decreases over time, but for a while there may be a pixel illuminated by a particular light source near several pixels that clearly stand out from the others that have not yet been illuminated by the same source, hence the bright dots seen above at low sample numbers.
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