Advanced Programming with OpenGL ES 3.0 - Praise for OpenGL ES 30 Programming Guide - page 376

explosion. The next input to the vertex shader is the uniform variable
u_centerPosition . This variable is set to the center location of the
explosion at the start of a new explosion. The setup code for u_time
and u_centerPosition appears in the Update function in the C code
of the example program, which is provided in Example 14-6.

Example 14-6 Update Function for Particle System Sample

void Update (ESContext *esContext, float deltaTime)
{
UserData *userData = esContext−>userData;

userData−>time += deltaTime;

glUseProgram ( userData−>programObject );

if(userData−>time >= l.Of)
{
float centerPos[3];
float color[4] ;

userData−>time = O.Of;

// Pick a new start location and color
centerPos[0] = ((float)(rand() % 10000)/10000.0f)−0.5f;
centerPos[l] = ((float)(rand() % 10000)/10000.0f)−0.5f;
centerPos[2] = ((float)(rand() % 10000)/10000.0f)−0.5f;

glUniform3fv(userData−>centerPositionLoc, 1,
&centerPos[0]);

// Random color
color[0] = ((float)(rand() % 10000) / 20000.Of) + 0.5f;
color[l] = ((float)(rand() % 10000) / 20000.Of) + 0.5f;
color[2] = ((float)(rand() % 10000) / 20000.Of) + 0.5f;
color[3] = 0.5;

glUniform4fv(userData−>colorLoc, 1, &color[0]);
}
// Load uniform time variable
glUniformlf(userData−>timeLoc, userData−>time);
}

As you can see, the Update function resets the time after 1 second elapses
and then sets up a new center location and time for another explosion.
The function also keeps the u_time variable up-to-date in each frame.

The vertex inputs to the vertex shader are the particle lifetime, particle start
position, and end position. These variables are all initialized to randomly