OpenGL is inherently a 3D API.  However, sometimes it is convenient to switch to “2D mode”, based on the screen size in pixels.

This can be achieved by changing the projection matrix using
glm::ortho(0, w, 0, h);
where [w, h]T is the window size, and using an identity matrix for the modelview matrix.

In the template code, there is an orthographic projection matrix based on the window size already defined.  Use it, and an identity matrix for the modelview to enter “2D mode”:

fontProgram->SetUniform("matrices.projMatrix", pCamera->GetOrthographicProjectionMatrix());
fontProgram->SetUniform("matrices.modelViewMatrix", glm::mat4(1));

Typically one does 2D rendering last, after all the 3D rendering has occurred. 

Example:  in the template, the text is rendered after the 3D graphics.



Once we’re in “2D mode”, units are in pixels.  The OpenGL coordinate system will be:



To render a full-screen quad, simply draw a triangle strip to fill the screen (can be done with four vertices v0 v1 v2 v3).  z values can be 0.  For example:

modelViewMatrixStack.Push(); 
    modelViewMatrixStack.SetIdentity();
    pMainProgram->SetUniform("matrices.modelViewMatrix", modelViewMatrixStack.Top());
    pMainProgram->SetUniform("matrices.projMatrix", m_pCamera->GetOrthographicProjectionMatrix());
    m_pQuad->Render(); // Render quad in 2D mode
modelViewMatrixStack.Pop();

High