Drawing "point-like" shapes in OpenGL, indifferent to zoom

Question

I'm working with Qt and QWt3D Plotting tools, and extending them to provide some 3-D and 2-D plotting functionality that I need, so I'm learning some OpenGL in the process.

I am currently able to plot points using OpenGL, but only as circles (or "squares" by turning anti-aliasing off). These points act the way I like - i.e. they don't change size as I zoom in, although their x/y/z locations move appropriately as I zoom, pan, etc.

What I'd like to be able to do is plot points using a myriad of shapes (^,<,>,*,., etc.). From what I understand of OpenGL (which isn't very much) this is not trivial to accomplish because OpenGL treats everything as a "real" 3-D object, so zooming in on any openGL shape but a "point" changes the object's projected size.

After doing some reading, I think there are (at least) 2 possible solutions to this problem:

1. Use OpenGL textures. This doesn't seem to difficult, but I believe that the texture images will get larger and smaller as I zoom in - is that correct?

2. Use OpenGL polygons, lines, etc. and draw *'s, triangles, or whatever. But here again I run into the same problem - how do I prevent OpenGL from re-sizing the "points" as I zoom?

Is the solution to simply bite the bullet and re-draw the whole data set each time the user zooms or pans to make sure that the points stay the same size? Is there some way to just tell openGL to not re-calculate an object's size?

Sorry if this is in the OpenGL doc somewhere - I could not find it.

Answer 1

There's no easy way to do what you want to do. You'll have to dynamically resize the primitives you're drawing depending on the camera's current zoom. You can use a technique known as billboarding to make sure that your objects always face the camera.

Answer 2

What you want is called a "point sprite." OpenGL1.4 supports these through the ARB_point_sprite extension.

Try this tutorial

http://www.ploksoftware.org/ExNihilo/pages/Tutorialpointsprite.htm

and see if it's what you're looking for.

Answer 3

The scene is re-drawn every time the user zooms or pans, anyway, so you might as well re-calculate the size.

You suggested using a textured poly, or using polygons directly, which sound like good ideas to me. It sounds like you want the plot points to remain in the correct position in the graph as the camera moves, but you want to prevent them from changing size when the user zooms. To do this, just resize the plot point polygons so the ratio between the polygon's size and the distance to the camera remains constant. If you've got a lot of plot points, computing the distance to the camera might get expensive because of the square-root involved, but a lookup table would probably solve that.

In addition to resizing, you'll want to keep the plot points facing the camera, so billboarding is your solution, there.

An alternative is to project each of the 3D plot point locations to find out their 2D screen coordinates. Then simply render the polygons at those screen coordinates. No re-scaling necessary. However, gluProject is quite slow, so I'd be very surprised if this wasn't orders of magnitude slower than simply rescaling the plot point polygons like I first suggested.

Good luck!