Posts Tagged ‘BOM’


As a final step for this project I tested the performance of the three algorithms I implemented throughout the summer: Opacity Shadow Maps (OSM), Deep Opacity Maps (DOM) and Bounding Opacity Maps (BOM).

I measured the performance in frames-per-second (FPS) using the average FPS provided by Fraps benchmarking tool for a period of 60 seconds. The GPU card (Nvidia GT 335M) had the most important contribution in the measures taken, because all three algorithms are GPU bound (they involve rendering to texture multiple times and no significantly computational task is done on the CPU).

The performance is shown in Figure 1

Figure 1 Plot generated using gnuplot. The variance between the number of layers and the FPS for the three algorithms implemented for this project. Even thought Bounding Opacity Maps have the worst performance they produce realistic renderings even when using the minimum number of layers tested because they follow the real-world light’s distribution (BOM).


Bounding Opacity Maps

Because Deep Opacity Maps (DOM) give information only about the starting position for the splitting points, two major issues appear:

  1. The position of the splitting points can’t be precisely determined for any type / shape of object because the end splitting position is not specified.
  2. The layers only follow the light’s distribution at the beginning of the object, where the start points are known via the depth map (Figure 1).

Figure 1 A translucent full sphere as seen in real-life (a), the distribution of layers when using DOM (b) and the way the light is distributed in real-life (c).

Furthermore, the example from Figure 1 is not a particular case, the light distribution following the shape of the object for other translucent real world objects, such as blonde hair or trees, as can be seen in Figure 2 and Figure 3.

Figure 2 Real-life lighting of blonde hair (a) and the corresponding layers and light distribution (b). It can be observed that the layers and the light distribution follow the shape of the object.

Figure 3 Real-life lighting of a tree (a) and the corresponding layers and light distribution (b). It can be observed that the layers and the light distribution follow the shape of the object.

By computing an extra depth map, in which depth information about the furthest away points is given, instead of the closest ones, the limitation of DOM regarding the lack of information for the end splitting points is solved, and more important the layers follow the light’s distribution in real-life.

This is achieved by Bounding Opacity Maps (BOM) where the layering follows the light distribution in real-life by interpolating the values from the two depth maps when choosing the splitting points.

The difference in splitting between DOM and BOM in Crystal Space can be seen in Figure 4 and the difference in rendering in Figure 5.

Figure 4 Difference in splitting between DOM (a) and BOM (b) when using 16 layers – first layer corresponds to light green and the last layer to black. It can be seen that because the end splitting points are not specified in DOM the layers don’t cover the whole length of the object (the final color is not black as in (b)).

Figure 5 Difference in rendering between DOM (a) and BOM (b) when using 16 layers. Because DOM don’t specify the end splitting points some grass strands (from the red circle), corresponding to the last layer, are given false shadow information.