Detect if a quad is actually visible 2D in OpenGL

Question

I currently have 16 tiles, with individual images that make up 1 big map. I pan by transforming right at the beginning before any actual drawing with this:

GL.Translate(G_.Pan(0), G_.Pan(1), 0)

Then I zoom by doing this:

GL.Ortho(-G_.Size * 1.5 ^ G_.ZoomFactor, G_.Size * 1.5 ^ G_.ZoomFactor, G_.Size * 1.5 ^ G_.ZoomFactor, -G_.Size * 1.5 ^ G_.ZoomFactor, -1, 1)

G_.Size is a constant that only varies on startup depending on parameters, zoom factor ranges from -1 to -13

What I want to be able to do is check if 1 of the 16 tiles is within the visible area, so then I stop them drawing when they are not on screen. I had found some quite complex methods for doing it, but it was 3D and seemed like a lot of work for something that should be simple. I would of thought it would of been something like just checking if a point is within the bounds of visible area, but I have no idea on how to get the visible area.

Answer 1

Andon M Coleman already suggested you to implement projection volume culling (a generalized form of frustum culling). This is however outside the scope of OpenGL. You must understand that OpenGL is not a "magical" scene graph that does scene management and the likes. It's mere drawing API; what it does is putting shaded, textured points, lines or triangles on the screen and that's it. The rest is up to you, or the libraries you choose to implement it.

In the case of projection volume culling you're testing if a given piece of geometry intersects with the volume defined by the planes that form the borders of the volume. Your projection matrix defines such planes, specifically it transform the view space vertex position volume into the range [-1;1]×[-1;1]×[0;1] of perspective divided clip space. So by inverting the projection matrix and unprojection the corners of the [-1;1]×[-1;1]×[0;1] cube through that you determine the limiting planes of the projection volume in view space.

You then use that information to intersect your quads with the volume to see if they cross it, i.e. are in any way visible.