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# Screeny hold the resolution pixels the screen

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Now that we have a global understanding of what a particle system is, let's take a closer look at how we can implement it. Consider the following class definition for a basic particle system:

A variety of emitters have frequently been used. The most well known is the point emitter, which generates all particles at a point in space, as in an explosion. But explosions are not born in a single point in space. In fact, only the Big Bang happened this way, and we're not even sure about that. Explosions in the real world have volume and shape, as particles of fire emerge from the exploding object. Thus, sometimes we will perturb the point, so particles are born in an area surrounding it, but not quite at the point. This is a very common approach in particle system creation: adding an amount of randomness so results do not look too regular and algorithmic. For example, for our point emitter, we would use something like this:

point pos(3,5,4); // particles are born in 3,5,4
pos.x += ((float)rand()%2000-1000)/1000;
pos.y += ((float)rand()%2000-1000)/1000;
pos.z += ((float)rand()%2000-1000)/1000;

A third type of emitter is referenced with regard to the player's position. Imagine that you need to simulate rain. Quite likely, you will not fill the whole game level with hundreds of thousands of individual raindrops. It simply makes no sense because distant raindrops will not be visible. In this scenario, it would be great to generate rain right where the user can notice it most, which is directly in front of him. Thus, we generate rain in our regular coordinates, and then translate and rotate to the particle system's final position and rotation. Here is an example of how to generate these particles. I assume fov is the horizontal aperture of the camera, and distnear and distfar are the range of distances we want to fill with particles:

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This code assumes that SCREENX, SCREENY hold the resolution in pixels on the screen. Obviously, most of the complexity of these systems is not in the spawning of particles, but in the actual rendering. You can see different types of emitters in Figure 19.1.

Figure 19.1. Emitters for different effects.

Particle Behavior

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