I'm trying to understand what causes the blobby edges when using alpha testing to create transparency with 1-bit alpha channels. The edges appear pixelated, but with rounded corners and diagonal lines. [Image from http://wiki.polycount.com/wiki/File:Alphatest_8bit_vs_1bit.jpg]
My theory is that the fragment shader is sampling from the alpha channel using bicubic interpolation then comparing with the alpha threshold. (I would expect straighter edges with bilinear interpolation). Is this correct? Is something special done to prevent overshoot artifacts?