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Sponsored Feature: Updated Tools Spice Up New Ghostbusters Game
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Sponsored Feature: Updated Tools Spice Up New Ghostbusters Game

January 14, 2010 Article Start Previous Page 3 of 4 Next

Step 6: A Third Solution is Created

When the Intel team shared their findings with the developers at Terminal Reality, Mark Randel suggested-and implemented-a third solution: a "pixel height test."

Figure 6 shows the idea behind the pixel height test. The bounding sphere of an object is shown as circles in Figure 6 and indicates the pixel coverage on the screen required for that object either when the object is close to the camera or when it is farther away.

Using the pixel height test on the objects in a scene, the test can determine which objects contribute less than one full pixel to the displayed frame. To approximate the pixel coverage, the test determines the object height in screen space in pixels. This testing code is executed on the processor. As a result of the pixel height test, if the pixel height of an object is less than a pixel, the object is not submitted for rendering.

In the troublesome Library scene, the fact that the objects (books) all had identical dimensions-because they are instantiations of a single object-made the test easier and faster to run because the bounding spheres for all tested objects (books) were identical.

Step 7: The Results of the Pixel Height Test

Figure 7 shows the result of implementing the pixel height test on the Library scene in Ghostbusters. Using the software switch created by Randel, developers were able to turn the test on and off. When the pixel height test is running, objects (books) that are less than one pixel in height in the scene, are not rendered. As shown by the data in the green oval in Figure 7, where the test was turned on, the frame rate of this scene doubled when the books less than one pixel in height were not rendered.

The data in Figure 7 also shows that the overall usage of the graphics resources went up, with the test indicating that the game was now using resources more optimally.

Figures 8 and 9 are the screen captures of the scene before and after the test was enabled. There is no visual difference between the two renderings, because no visible object was affected by the change.

When the team first started this analysis, the scene was rendering so slowly that it was considered the major issue preventing the game from being highly playable. Based on a thorough analysis and the implementation of the pixel height test that followed, the scene ended up rendering at double the original frame rate. Other scenes in the game enjoy even higher frame rates.

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