What's the best way to draw a fullscreen quad in OpenGL 3.2?

Question

I'm doing ray casting in the fragment shader. I can think of a couple ways to draw a fullscreen quad for this purpose. Either draw a quad in clip space with the projection matrix set to the identity matrix, or use the geometry shader to turn a point into a triangle strip. The former uses immediate mode, deprecated in OpenGL 3.2. The latter I use out of novelty, but it still uses immediate mode to draw a point.

Answer 1

You can send two triangles creating a quad, with their vertex attributes set to -1/1 respectively.

You do not need to multiply them with any matrix in the vertex/fragment shader.

Here are some code samples, simple as it is :)

Vertex Shader:

const vec2 madd=vec2(0.5,0.5);
attribute vec2 vertexIn;
varying vec2 textureCoord;
void main() {
   textureCoord = vertexIn.xy*madd+madd; // scale vertex attribute to [0-1] range
   gl_Position = vec4(vertexIn.xy,0.0,1.0);
}

Fragment Shader :

varying vec2 textureCoord;
void main() {
   vec4 color1 = texture2D(t,textureCoord);
   gl_FragColor = color1;
}