How do you determine when an object is drawn on-screen in OpenGL?

Question

I'm extremely new to OpenGL. I'm writing a program that displays flying 3D text on screen. I need to know when certain text string appears (drawn) onto the screen and are visible to the user. The program needs to identify which text strings are displayed. (Note: although my problem deals with text, it could be generalized to any OpenGL object).

At first, I started to think that I could use OpenGL's picking mechanism, but so far I've only seen examples where the selection area is focused on some sort of user interaction. I want to know what objects are displayed on the entire window area. This leads me to think I'm on the wrong track... Am I missing something?

Any suggestions are welcome.

Answer 1

hmm, is it actually in 3D? or is it just 2D text on the screen in 2D space? in that case I would just keep track of it manually. how exactly are you drawing your text?

generally the way you do this is with a "frustum check" where you basically just make a volume for the camera and test whether you're 3d objects are inside it or not.

Answer 2

You can try OpenGL's feedback mechanism. In this mode, OpenGL generates fragments and passes them to a feedback buffer. If something is clipped, no fragments will be generated. When the text becomes visible, you will find the corresponding fragment in the fragment buffer.

This link should get you started.

Here is another link, the Question 10.010 seems particularly relevant to what you want.

Answer 3

Run your object coordinates through your projection and modelview matrices to get screen-space coordinates. Compare the X/Y output against your screen extents to figure out if the text is on-screen.

Answer 4

You can use the query objects (specifically those object created using GL_ARB_occlusion_query extension Specification). Those object are used to query how many fragments are rendered using a sequence of OpenGL operations (begin/end, etc...).

Another scheme (software only), is to determine a bounding box for your rendered text, then compute mathematically whether the bounding box is inside the view frustrum (derived from the current perspective used for rendering.

A note: using OpenGL picking doesn't necessary imply the use of gluPickMatrix. You can render you scene "as is", and the query the rendered names (altought picking is deprecated from OpenGL 3).

Query objects are easy to use, and they are lightweight. Picking is another good solution for most hardware, but more schematic than query objects.