Index Making Pictures

## Bounding plane

For outdoor scenes, it is often necessary to have a "far" bounding plane to limit the number of triangles needing to be rendered. Without this the game would slow down too much.

In this first example, we show the effect of the viewing volume, a truncated rectangular cone. The viewing direction reveals something in the distance, on the lefthand extreme of the field of view. This only partly intersects the viewing volume, so only part of it is rendered. In particular the upper and lower parts of the structure can be seen but nothing in between to connect them.

An obvious reaction to seeing the above is to turn your view directly at the object, to try to see more. Instead, it vanishes entirely. This is because the "far" viewing plane no longer intersects the object and is now nearer than the object, as a result of the rotation:

If we go back to the original viewing direction and use the game set-up controls to adjust the "far" plane to be a little further away, we see more of the object:

With the viewing plane set as far away as possible, we see all of the object. This however makes the maximum demand on the PC graphics card, because the scene contains many more polygons, textures etc. If it is too heavily loaded, the frame-rate drops and it becomes hard to move around. It is for this reason that games with complex environments sometimes allow you to adjust the "far" plane (as in this example), so you can balance the appearance with the interaction that your PC and graphics card can sustain. If you look carefully at the pictures above and below, you will see that the mist is different too. Compare the mist on the ground to the right of both pictures to spot this. Again this is a detail-hiding technique.

In this second sequence, the character moves steadily closer to the trees and then to a castle-like building, which gradually becomes more detailed as parts of it come within the bounding plane. A mist calculation is also used to reduce the visibility of detail. Thus features first appear as white in the distance, then toned down, then reveal their texture. You can also observe a vertical plane of mist in the pictures. (I have boosted the contrast, to make the effects more visible. This results in the colour balance varying slightly from picture to picture: this is not the case when playing the game.) This sequence also shows you that the sky is treated differently: it is certainly beyond the "far" plane and so would be invisible unless treated as a special case. Of course the mist should make the sky white, when the misty white shapes would not be so obvious against the skyline. It does not, again because the sky is simply a background on which the main scene is rendered. Finally, you can see a nice feature of this game: the time of day changes, so dusk is arriving and the stars appear (final two frames).