Standard: Multiply your resolution by two constants. If your resolution was 100x100 and your constants are [8, 8] then you would render an 800x800 image. Next, average blocks of pixels together. The size of each block depends on your constants. If you chose the constants [8, 8] then you would average the 8x8 blocks. I you chose [4, 4] then you would average every 4x4 block.

The resulting quality is directly related to your constants. The higher, the better. Note that all of the extra time is spent is in your rasterizers.

Alternate: Render the image a number of times (for this example, we'll use a constant of 4). Before each render, offset the vertices in the image by a fractional amount in each direction. If you were to render the image 4 times, you would render the image with these offsets to each screen-space vertex: [-0.5, -0.5] [+0.5 -0.5] [-0.5, +0.5] and [+0.5, +0.5]. Of course, these offsets need to change depending on the number of times you render your image. Once these images have been rendered, simply average them together.