Final gathering
Final Gathering (also Final Gather) is an algorithm of image synthesis in computer graphics that is based on ray tracing .
It is often combined with global illumination ( global illumination used) to produce optimal lighting and shadow effects. This algorithm is used especially for diffuse shading and realistic outdoor scenes.
origin
Final gathering was originally intended as a performance- saving caching technique that was supposed to improve and complement global lighting in terms of speed and quality. By determining the approximate local radiation intensity according to photon tracing , the lighting can be calculated more quickly in connection with global lighting, since the global lighting does not have to be calculated so precisely. Furthermore, the “dark corner effect” and the low-frequency noise are reduced.
functionality
The calculation of final gathering is roughly based on the following scheme and depends on various settings:
- Photon tracing phase (the calculation of global lighting )
- Final gathering phase
- Pre-calculation of final gathering points
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Rendering phase
- Calculation of additional final gathering points and interpolation of all FG points.
- Ray tracing
Pre-calculation (presampling)
So-called eye rays are emitted into the scene from the camera. At the points where there is an intersection with the scene geometry, Final Gathering Rays are sent from this intersection into the hemisphere oriented in the normal direction. This type of grid of final gather points adapts to the scene. Fewer Final Gather Points are needed, for example, on large areas, but more in detailed regions and corners; thus a fast and accurate calculation is guaranteed.
These Final Gathering Rays run until they hit the geometry and then give their information about the color and brightness of that intersection with the geometry.
All this information about the lighting of the scene is saved in a 3D final gathering map in the main memory or on the hard drive for later calculation purposes. For more information on the Final Gathering Map, see "Animations".
Additional calculations and interpolation
The calculation of final gathering points in the pre-sampling phase (pre-calculation) is very computationally expensive, so that neighboring points are used to determine the local radiation intensity. This depends on the settings for the maximum and minimum radius:
- The maximum radius represents the region from which the final gathering points can be used . If not enough information can be obtained from the FG Points in this region, new Final Gathering Points will be issued .
- The minimum radius stands for the region from which FG points and their information about the radiation intensity must be used .
These values are important for the user, as they have a strong influence on the speed of calculation and the quality of the image. An example would be a hollow in the shape of a fine line in the geometry. If the minimum radius is too large, this line would appear gaps and artifacts.
Beam path and trace depth
Depending on the setting, the Final Gathering Rays not only cover one distance until they meet geometry, but are also reflected or refracted if necessary. From the first point of intersection with the geometry, the FG Ray then continues to search until it repeatedly encounters geometry or not. How often an FG Ray may be reflected and / or broken is determined by the Trace Depth . Diffuse materials also reflect light!
A trace depth value of 1–3 is normal. It should be noted that the number of permissible reflections and refractions can be controlled separately, but their total cannot be greater than the Trace Depth value. This is to be understood in such a way that, for example, an FG ray is first refracted (of a possible three) and then reflected twice, after which the calculation for this ray stops because the trace depth has been reached from here three.
This value has a significant influence on the calculated image, since the calculation with high trace depth values is far more complex and more precise. A room with only one window as a light source would sometimes appear too dark with a low trace depth (1–2), since only FG rays that directly hit the window illuminate the scene. With higher values (4–8) the room appears brighter because the additional reflections have resulted in much stronger indirect lighting. Similar to global lighting.
Falloff
Usually the FG Rays run until they hit geometry. The falloff value determines this distance. That means, if an FG Ray has reached its "end of life" (maximum distance covered - the falloff value) and has not found any geometry, it stops the search and displays a previously set background color. This speeds up the final gathering process - but it can also become (significantly) less precise if the value is too low.
use
Final Gathering is supported by all large 3D packages , as Mental Ray is integrated into them as a renderer. In the film industry , final gathering partially replaces global lighting. Final Gathering can be used alone or in combination with global lighting. With this combination, both techniques complement each other, so that the quality settings of the individual process can be partially halved. This combination offers the highest level of visual quality such as realism - in productive render times.
As with rendering with Final Gathering alone, effective use requires a comprehensive understanding of the setting options and experience.
Animations
A disadvantage of final gathering is that it can be used for animations without problems. This can cause flickering (e.g. fluctuations in brightness in the image). To prevent this, the Final Gathering 3D map is not renewed for each image (as is the case by default), but is frozen or only calculated areas that have not yet been covered by the Final Gathering Map.
Another possibility is to use global lighting in combination with final gathering. This avoids flickering.
Web links
- Mental Ray User Group - In-depth information on Mental Ray and Final Gathering (en) ( Memento from July 2, 2012 in the Internet Archive )