EXTENDED TRACING DEPTH When dealing with transparent objects in our scene, the Tracing Depth in Presto render panel is an important factor to get an artifact-f ree image. A too low value can produce black areas on glass objects or other transparent materials like the leaves of a tree or any material on which we have applied a clip map. A Subsurface Scattering material (SSS) is another case where higher Tracing Depth may be needed to avoid too dark materials. The only solution to avoid these problems, was to increase the Tracing Depth until the artifacts disappear. By doing so we also increase the render times and in some cases quite a lot. In many situations a higher Tracing Depth is needed to fix a small area of a scene that otherwise could have been rendered without such high Tracing Depth. Let's have a look at image 01 to see a typical situation. This kitchen scene looks overall ok, we have nice lighting and reflections but there is a problem at the wine glasses in the background and at the bread (that uses Subsurface Scattering) that is also too dark. The Tracing Depth is too low to travel though all those glasses which makes them turn black and the same goes for the bread that will need more Tracing Depth to show the Subsurface Scattering effect. The solution is to increase our Tr acing Depth until we have nice clean transparent glasses and a nice looking bread. The image was rendered using Tracing Depth of 6. Let's see how much we have to increase it to reach a satisfying result (images 02-05). Image 01. scene by Andrew Price 1
Image 02. Tracing Depth: 6 Time: 6 minutes 52 seconds Image 03. Tracing Depth: 16 Time: 9 minutes 18 seconds 2
Image 04. Tracing Depth: 18 Time: 9 minutes 24 seconds Image 05. Tracing Depth: 21 Time: 9 minutes 30 seconds 3
A Tracing Depth of 21 seems to give nice results; but we also see that we get a render time penalization going from a Tracing Depth of 6 and a render time of 6 minutes and 52 seconds to a render time of 9 minutes and 30 seconds with Tracing Depth 21. Now there is a solution if we want to keep the render time low and this is to make use of the new feature introduced with Thea 1.4 called Extended Tracing Depth. You can enable this feature by clicking the check box on Extended Tracing located under Tracing Depth (image 06). Image 06. By enabling Extended Tracing Depth we can efficiently render scenes like the one presented here with lower Tracing Depth values and have better render times. Looking at the comparison between the render with Tracing Depth 6 and Extended Tracing Depth enabled with the render using Tracing Depth 21, we can see that we get comparable image quality with shorter render time (image 07). The benefit on render times will depend a lot on the scene complexity, light setup and image resolution. The more complex the scene and the more light sources being used, the more you will notice the render time benefit using Extended Tracing Depth. 4
Image 07. Extended Tracing Depth ON Tracing Depth: 6 Time: 7 minutes 39 seconds Extended Tracing Depth OFF Tracing Depth: 21 Time: 9 minutes 30 seconds 5
On exterior scenes we often have a similar problem but this time with trees and other vegetation that use clip maps. In general, for an exterior scene a low Tracing Depth can be enough for Global Illumination. Using a Tracing Depth of 4 to 6 can give satisfying results but here again we can have an issue when objects like trees are present, where we have a lot of clipping maps for the leaves. In this case the clip map is indicating where the texture of the leave should be transparent and a too low Tracing Depth can quickly make the tree show unnatural dark areas in places where the Tracing Depth stops, but more clip maps are present, blocking the light from showing through. By using Extended Tracing Depth we can solve this issue like we did for the kitchen scene (image 08). Image 08. Extended Tracing Depth OFF Tracing Depth: 5 Extended Tracing Depth ON Tracing Depth: 5 6
ADVANCED MODE Extended Tracing Depth will work for most common scenes but in some cases altering the default values may be needed. You can access the Extended Tracing Depth settings by enabling Advanced user mode in the Thea Preferences window (image 09). Image 09. 7
Transparency Depth determines the Extended Tracing Depth for all transparent materials like Thin Film, Glossy Glass and Clip Map. It only affects transparency. Internal Reflection Depth determines the Extended Tracing Depth for transparent materials that have refraction and total internal reflection. These materials are created with the use of Thea Glossy material (for example solid glass or water). If you notice that you get dark areas on solid glass, the cause is often because of too low Internal Reflection Depth and not due to the Transparency Depth. SSS Depth determines the Extended Tracing Depth for Subsurface Scattering (SSS) materials. In some cases increasing this value is needed to increase the brightness of bright colored dense SSS materials. Note that the Extended Tracing Depth values get added to the normal Tracing Depth value, meaning that if we have a Tracing Depth = 6 and a Transparency Depth = 15, the final Transparency Depth is = 21. The same happens also with the other Extended Tracing Depth values. The default values for Extended Tracing Depth are set to 15 and work good for most common scenes. 8