OpenGL draw partial object in scrollable panel

Question

I am making a GUI in OpenGL (more specifically lwjgl). I have tried hard to research different ways of doing this but I am having a hard time finding exactly what I want. I do not want to use any external libraries (only ones built in OpenGL, even trying to stay away from using GLUT) and I would like to have it work on anything that supports OpenGL (ex. Frame Buffer Objects don't work on older graphic cards).

I am making a 3D GUI with a scrollable panel as a component. The problem is I don't know how to draw a partial GUI component without doing a lot of calculations to only render part of it. I am making the components out of OpenGL primitives, not textures. I was hoping there is an easy way to do this like use multiple viewports. I don't really even understand what viewports are.

In short: I need to have a scrollable panel as a component overlapping other GUI components (since it will be a drop down menu) and not let any of the components in my panel draw outside my panel.

Answer 1

If you just want to prevent drawing pixels that are outside of a rectangular region (and I think that's what you're asking), than glScissor is exactly what you're looking for.

In lwjgl, you can find the function in org.lwjgl.opengl.GL11.