Draw multiple views of scene on single screen - c++

Question

I have a class for projection in OpenGL, which a user can use as follows:
//inside the draw method
customCam1.begin();
//draw various things here
customCam1.end();
The begin and end methods in my class are simple methods right now as follows:
void CustomCam::begin(){
saveGlobalMatrices();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-hParam,hParam,-tParam,tParam,near,far);//hParam and tParam are supplied by the user of the class
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void CustomCam::end(){
loadGlobalMatrices();
};
I want the user to be able to create multiple instances of the above class (supply different params for lParam and tParam for each of these classes) and then draw all the three on the screen. In essence, this is like three different cameras for the scene which are two be drawn on the screen. (Consider for example top, right, bottom view to be draw on the screen with the screen divided in three columns).
Now since there's only one projection matrix, how do I achieve three different custom cam views at the same time?

Answer 1

You just have to draw the scene three times using a different projection matrix (camera object in your case) each time. And in each of those three passes you set a different viewport for your renderings to appear in different parts of the overall framebuffer:
glViewport(0, 0, width/3, height); //first column
customCam1.begin();
//draw scene
customCam1.end();
glViewport(width/3, 0, width/3, height); //second column
customCam2.begin();
//draw scene
customCam2.end();
glViewport(2*width/3, 0, width/3, height); //third column
customCam3.begin();
//draw scene
customCam3.end();
But you cannot draw the whole scene using three different projection matrices and three different viewports all in one go.
EDIT: For the sake of completeness you can indeed do this with a single pass, using geometry shaders and the GL_ARB_viewport_array extension (core since 4.1). In this case the vertex shader would just do the modelview transformation and you would have all three projection matrices as uniforms and in the geometry shader generate three different triangles (projected by the respective matrices) for each input triangle, and each with a different gl_ViewportIndex:
layout(triangles) in;
layout(triangle_strip, max_vertices=9) out;
uniform mat4 projection[3];
void main()
{
for(int i=0; i<projection.length(); ++i)
{
gl_ViewportIndex = i;
for(int j=0; j<gl_in.length(); ++j)
{
gl_Position = projection[i] * gl_in[j].gl_Position;
EmitVertex();
}
EndPrimitive();
}
}
But given your use of depracted old functionality, I'd say geometry shaders and OpenGL 4.1 functionality are not yet an option for you (or at least not the first thing to change in your current framework).