Adding projection matrix to opengl es point sprites particle effect vertex shader

Question

I have been learning opengl es from the opengl es 2.0 programming guide. They have a particle effect that looks like an explosion. I am trying to enhance their example code by adding a mat4 projection matrix to the vertex shader, the shader compiles and works, but I am having problems getting the effect to position taking the projection into account. The code I have is as follows

const char* ParticleExplosionVertexShader = STRINGIFY (

uniform float u_time;
uniform vec3 u_centerPosition;
uniform mat4 Projection;
attribute float a_lifetime;
attribute vec3 a_startPosition;
attribute vec3 a_endPosition;
varying float v_lifetime;

void main()
{
    if ( u_time <= a_lifetime )
    {
        gl_Position.xyz = a_startPosition + (u_time * a_endPosition);
        gl_Position.xyz += u_centerPosition;
        gl_Position.w = 1.0;
    }
    else
        gl_Position = vec4( -1000, -1000, 0, 0 );
    v_lifetime = 1.0 - ( u_time / a_lifetime );
    v_lifetime = clamp ( v_lifetime, 0.0, 1.0 );
    gl_PointSize = ( v_lifetime * v_lifetime ) * 40.0;
}

);

I am able to add the projection to the line without any errors, but unfortunately here its not really required as that code is placing the object of d=screen at the end of its lifetime

gl_Position = Projection * vec4( -1000, -1000, 0, 0 );

I have also tried changing the line

gl_Position.xyz += u_centerPosition;

to

gl_Position += Projection * u_centerPosition;

But I have had no luck getting it to place as I want it

Am I doing something wrong? Or is there a reason the book didn't have a projection matrix such as its not something someone should do with point sprites?

Any help or pointers to what I should look into will be appreciated

Thanks

Edit: Please let me know if you need more information from me

the swine the swine · Accepted Answer · 2012-01-16T08:51:38

What about multiplying the whole gl_Position by modelview-projection matrix, as with any normal geometry?

Also, you will probably need to modify the line that calculates gl_PointSize, for example try to divide it by gl_Position.w (after multiplication by modelview-projection), otherwise the sprites will all have the same size (is that what you are trying to fix?).

Adding projection matrix to opengl es point sprites particle effect vertex shader

1 Answers