OpenGL color transform

Question

I'm using OpenGL to draw a large array of 2D points with their colors. Each point (vertex) has also defined it's alpha channel in MX.c array. I'd like to be able to increase or decrease the alpha value of whole array (of every vertex displayed). Is there a clever way to do it, using OpenGL functions? Here's my drawing method:

void PointsMX::drawMX()
{
    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);

    glColorPointer(4, GL_UNSIGNED_BYTE, 0, MX.c);
    glVertexPointer(2, GL_DOUBLE, 0, MX.p);

    glPushMatrix();
    glTranslated(position[X], position[Y], 0.0);
    glScaled(scale, scale, 1.0);
    glDrawArrays(GL_POINTS, 0, MX.size);
    glPopMatrix();

    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);
}

Answer 1

As datenwolf points out in his comments, you can do this pretty simply using a shader, but not using the fixed function pipeline (which is what you're using if you never call glUseProgram().

If you're not using lighting, reproducing the fixed function shaders isn't very hard, and a little googling will help you get up to that point.

The key here is that you want to change something that is normally a vertex attribute (the alpha channel of the color) to a configurable value for the entire drawing operation. In shader terms this means overriding the vertex attribute with a uniform. A uniform is simply a value you pass into an OpenGL program which then has the same value for every vertex or fragment processed (depending on whether you put it into the vertex or fragment shader).

Here's an example of a very basic vertex shader:

#version 330
uniform mat4 Projection = mat4(1);
uniform mat4 ModelView = mat4(1);
layout(location = 0) in vec3 Position;
layout(location = 3) in vec4 Color;

out vec4 vColor;

void main() {
  gl_Position = Projection * ModelView * vec4(Position, 1);
  vColor = Color;
} 

And a corresponding fragment shader

#version 330
in vec4 vColor;
out vec4 FragColor;

void main()
{
  FragColor = vColor;
}

In order to accomplish what you're trying to do, you'd want to change the vertex shader to add an additional uniform representing your alpha override:

#version 330
uniform mat4 Projection = mat4(1);
uniform mat4 ModelView = mat4(1);
uniform float AlphaOverride = -1.0;
layout(location = 0) in vec3 Position;
layout(location = 3) in vec4 Color;

out vec4 vColor;

void main() {
  gl_Position = Projection * ModelView * vec4(Position, 1);
  vColor = Color;
  if (AlphaOverride > 0.0) {
    vColor.a = AlphaOverride;
  }
} 

If you fail to set the AlphaOverride uniform it will be -1, and will therefore be ignored by the vertex shader. But if you set it to a value between 0 and 1, then it will be applied to the alpha channel of your vertex.