TIL signed distance field. First I need to explain the concept of alpha testing. Alpha testing is a rendering technique for textures. It dictates that when rendering a texture, perform a test on the alpha value of that pixel against a threshold. Completely throw out or keep the data depending on the comparison result. Typically the alpha value for each pixel is set to the actual transparency of the pixel. This signed distance field method calculates the signed distance of each pixel to the nearest color pixels. The signed instance field is then saved in alpha channel in the texture. All the remaining alpha-testing mechanism is the same. It's claimed that this technique improves the rendering quality of low-resolution textures drastically.

Read the original paper: https://steamcdn-a.akamaihd.net/apps/valve/2007/SIGGRAPH2007_AlphaTestedMagnification.pdf