What exactly is a buffer in OpenGL, and how can I use multiple ones to my advantage?

Question

Not long ago, I tried out a program from an OpenGL guidebook that was said to be double buffered; it displayed a spinning rectangle on the screen. Unfortunately, I don't have the book anymore, and I haven't found a clear, straightforward definition of what a buffer is in general. My guess is that it is a "place" to draw things, where using a lot could be like layering?

If that is the case, I am wondering if I can use multiple buffers to my advantage for a polygon clipping program. I have a nice little window that allows the user to draw polygons on the screen, plus a utility to drag and draw a selection box over the polygons. When the user has drawn the selection rectangle and lets go of the mouse, the polygons will be clipped based on the rectangle boundaries.

That is doable enough, but I also want the user to be able to start over: when the escape key is pressed, the clip box should disappear, and the original polygons should be restored. Since I am doing things pixel-by-pixel, it seems very difficult to figure out how to change the rectangle pixel colors back to either black like the background or the color of a particular polygon, depending on where they were drawn (unless I find a way to save the colors when each polygon pixel is drawn, but that seems overboard). I was wondering if it would help to give the rectangle its own buffer, in the hopes that it would act like a sort of transparent layer that could easily be cleared off (?) Is this the way buffers can be used, or do I need to find another solution?

Answer 1

OpenGL does know multiple kinds of buffers:

• Framebuffers: Portions of memory to which drawing operations are directed changing pixel values in the buffer. OpenGL by default has on-screen buffers, which can be split into a front and a backbuffer, where drawing operations happen invisible on the backbuffer and are swapped to the front when finishes. In addition to that OpenGL uses a depth buffer for depth testing Z sort implementation, a stencil buffer used to limit rendering to cut-out (=stencil) like selected portions of the framebuffer. There used to be auxiliary and accumulation buffers. However those have been superseeded by so called framebuffer objects, which are user created object, combining several textures or renderbuffers into new framebuffers which can be rendered to.

• Renderbuffers: User created render targets, to be attached to framebuffer objects.

• Buffer Objects (Vertex and Pixel): User defined data storage. Used for geometry and image data.

• Textures: Textures are sort of buffers, i.e. they hold data, which can be sources in drawing operations

The usual approach with OpenGL is to rerender the whole scene whenever something changes. If you want to save those drawing operations you can copy the contents of the framebuffer to a texture and then just draw that texture to a single quad and overdraw it with your selection rubberband rectangle.