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so does the speed of world objects. We show how to dissociate the
simulation rate and the frame rate when we review our demo game in
Section 8.4.1.
TIP : Keep the simulation rate and frame rate independent of
game-loop frequency.
8.1.4 Mobile Graphics
Games are essentially about polygons and pixels. In many 3D games,
objects are represented by a mesh. This is usually a set of vertices (points)
in 3D space which define a set of adjacent polygons (usually triangles)
describing the surface of an object.
It should be no surprise that an object made up of a lot of triangles is
more expensive to render than one made up of relatively few triangles.
This has been the case for years and high-end graphics cards usually
address this using a variety of techniques including caching geometry
directly in graphics memory on PC video cards to avoid unnecessary
traffic over the bus.
In addition, performance is affected by the actual number of pixels
that need to be updated between one frame and the next. The amount of
pixels an object needs to occupy in screen space is a direct function of the
size of the polygon, its distance from the observer and the angle it makes
to the line of sight. This is why polygon-fill rates are key performance
indicators of graphics cards for PCs. On mobile phones, processors are
very limited and can easily form a performance bottleneck for a game.
Consequently, it is quite possible for a complex object made up of many
small polygons to render faster than an object built with a smaller number
of large polygons, because it requires fewer pixels. This directly affects
the frame rate and game performance.
TIP : On mobile phones, the number of pixels to be updated has a large
effect on resultant frame rate due to slower processors.
Lastly, a technique used in mainstream game development is the
concept of varying levels of detail (LOD). The idea leverages the fact that
an object in the distance is not as clear and sharp as one that is closer
to us. Imagine we want to render a tank that moves around in a 3D
world. We could have three models for the tank - one to use when it is
off in the distance, which would consist of the fewest triangles, a model
of intermediate complexity for intermediate distances, and a high-quality
(or high triangle count) mesh when the tank is close to the point of view.
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