Blur with OpenGL?

Question

I'm using OpenGL and drawing polygons in a 2D view. How could I blur a polygon, without using glsl, and only things like stencil buffer and stuff. Thanks

Answer 1

There are two quick and dirty ways. GLSL or Cg is by far your best solution, especially if you need any decent blur (Gaussian, box, motion, etc). However, you can:

• Render the image at a lower resolution, usually to a texture, then render that texture to the screen at high-res. It will blur the image, but you need to use trilinear or anistropic filtering for it to look good. Usually it still won't, but those help.
• Render the image to a texture, render once to screen with full opacity, then turn on blending, turn down alpha, and render the image shifted left a bit, right a bit, up a bit, down a bit... etc. You need low opacity for the in-front renders, but they will effectively blur the scene. You may also want to play with blending modes, SrcColor/InvSrcColor or DstColor/InvDstColor may be helpful.

There are a few ways to do this without shaders, but none of them are optimal.

Answer 2

The normal method uses the accumulation buffer instead of the stencil buffer. You basically re-draw your polygon(s) a number of times, but change the viewing perspective slightly each time. Exactly what you change determines the style of blur you get. For example, if you want an effect like zooming a camera lens, you can change the view frustum slightly between frames. If you want motion blur, you change the camera view angle instead. With some extra work, you can do some slightly odd-looking effects, such as moving your viewpoint forward, and zooming back at the same time, so (most of) the scene remains roughly the same size, but the perspective you're viewing it from constantly changes.