Question

I am making an Android game app and am using openGL to load 3D obj models into the app. I would like to know if anyone can help me on how to make these objects interactive. All I really need is to make an object clickable, but it would be cool to learn more such as dragging it around the screen and such.

Thanks for any help.

Answer 1

What you want is called picking.

This isn’t a easy task an depending on want you want to do exaclty you have mutliple options to do it.

The Problem: You want to select an Object in 3D space based on a 2D coordinate (mouse/touch position). Because of the 2D mouse coordinates you miss one coordinate to determine the exact click position in the 3D space.

One possible option would be:

• Render your object with a specific color (e.g. completely red)
• After that save the current display buffer to a variable
• Clear the display buffer and render your model again with the standard settings (this is the screen that is displayed to the user)
• determine the click/touch position translate it to a coordinate on your display area
• check the color at this coordinate in your saved display buffer. If the color at this position is red, the user clicked/touch an object

This approach isn’t that flexible but the implementation is very simple compared to other solutions. It is limited because you can only detect if the used clicked/touched a certain object or not but you cannot determine the exact position on the object.

Another possible option is to compute a ray on the 3D world based on the 2D click position and then determine all objects in 3D space that collide with this ray. This is called ray picking.
You can find a OpenGl tutorial for ray picking here

The example uses glRenderMode, glLoadName, etc. which maybe isn’t the best choice if you are not using the fixed function pipeline (e.g. you are using custom shaders, etc.).

Another option whould be to do the math and compute the ray vector yourself based on click position, viewport and projection matrices. If you want to do this the documentation of gluUnproject can help you.