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5. Complexity and Time Issues

When talking about mass scenes the computational complexity is very important. The most relevant is the time complexity. The time necessary to create a nice image can be divided into three exclusive parts: the time to prepare and to design the scene, time for the parser to read the source and to build the internal structures, and the time that the rendering engine needs to render the image.

The time needed for the scene composition has been reduced significantly. The scene composer does not have to mark for each of the generated objects the place where to put it. The boring long lasting work surely belongs to the computer.

Execution of all proposed commands takes place in the parse time. Although the parse time is much smaller that the other two times, the shortening of that time is still not a worthless job. During the scene preparation the designer needs to preview the image (at low resolutions) many times, where a longer parse time is very annoying. To accelerate the layout minimal distance comparison, some types of hash-tables are applicable. As the reader has certainly noticed, all the layout techniques work this way: generate a random entity (a vertex or a ray) and try to use the entity to find a reference point. If the reference point has been found, it is good, but if not (the vertex has not been inside or the ray did not hit anything), the quest for a reference point has to be repeated. To avoid these unlucky choices, some space partition trees could be made that would assign correct probabilities to the tree nodes. Thereby a random entity (the vertex or the ray) will have lower probabilities to be generated in the regions where the chance to find a reference point is lower.

The render time acceleration is a hard task and out of the scope of this paper. There is one suggestion how to save some render time (and quite a lot of memory): to count the distance from the camera to the object being generated in the object's macro to reduce the model quality according to this distance. It is sufficient if the models in the front (near the camera) have higher detail, and the others at the back have lower detail. However POV-Ray makes for the rendering some sort of scene object trees, thereby the render time rises by the logarithm of the scene complexity - the number of objects in the scene. In other words it almost does not matter if there is one thousand of objects in the scene or ten thousands.

As we have chosen POV-Ray, it works on almost any platform and it is possible to accelerate the parse and render times by a migration to a faster machine.

# of elements
Parse Time
Render Time
The letter soup
The characters on a terrain
The hairy monster

Table 1: The parse and render time comparison, executed on an AMD K6-2 350 with a preview resolution (320 x 200 pixels) without the antialiasing.

Mass Scenes Rendering Framework, Dušan Bezák, 1999-2001,