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Thread: how do I pass color info to vertex shader?

  1. #1
    Junior Member Newbie
    Join Date
    Dec 2011

    how do I pass color info to vertex shader?

    Ok, so I'm making fireworks and it works rather well.
    As a basis for this I am using sparks, which are rendered as follows:
    Code :
    void Fireworks::render()
    	//set the size of the rendering area
    	glViewport(0, 0, width, height);
    	//modelview transform matrix
    	Matrix4 modelviewmatrix = Matrix4::lookAtMatrix(viewpoint,
    	                                                Vector3(0.0f, 0.0f, 1.0f));
    	//projection transform matrix
    	Matrix4 projectionmatrix = Matrix4::perspectiveMatrix((float)M_PI_4, (float)width / height, 1.0f, 10000.0f);
    	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    	//use the terrain shader
    	//set our matrices as "uniform" variables
    	glUniformMatrix4fv(glGetUniformLocation(terrainshader.getProgram(), "matmodelview"), 1, GL_TRUE, modelviewmatrix.elements());
    	glUniformMatrix4fv(glGetUniformLocation(terrainshader.getProgram(), "matprojection"), 1, GL_TRUE, projectionmatrix.elements());
    	spark* p = sparks;
    	while(p != NULL){
    	  for(int i = 0; i < 3; i++){
    	    glVertex3f(p->position.x(), p->position.y(), p->position.z());
    	    glVertex3f(20.0f*sin(rand())+p->position.x(), 20.0f*cos(rand())+p->position.y(), 20.0f*cos(rand())+p->position.z());
     	  p = p->next;

    These go through the vertex shader which is

    Code :
    out vec3 _color;
    void main()
    	// Get a copy of the normal vector
    	_color = ?
    	// Transform the vertex position to screencoordinates
    	gl_Position = matprojection * (matmodelview * in_position);
    and the fragment shader which is

    Code :
    in vec3 _color; //normal vector
    void main()
    	//severely simple lighting model
     	fragcolor = vec4(_color , 1.0);

    now I think I got the passing from the vertex to the fragment shader right, but I want to know how I can pass the color information from my program to the vertex shader. I can't seem to find how to do that...

    I could post the rest of the code, but I don't think it'll matter (and it's probably bad anyway). I currently set fragcolor to a fixed color and it works.

  2. #2
    Senior Member Frequent Contributor
    Join Date
    Mar 2009
    Karachi, Pakistan

    Re: how do I pass color info to vertex shader?

    From the snippet you have posted, it appears you want to assign the per-vertex normal as color. To do that, you would have to pass the per-vertex normals (glNormal3f) along with positions (glVertex3f). Assuming that the normal is passed in to vertex shader as in_normal, then you can simply assign it to the color output variable like this,
    Code :
    in vec3 in_normal;
    in vec4 in_position;
    out vec3 _color;
    uniform mat4 MVP; //combined modelview/projection matrix
    void main()
       // Assign the normal vector
       _color = in_normal;
       // Transform the vertex position to clip space
       gl_Position = MVP * in_position;
    See if this helps.

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