CorePNG is a lossless codec based on PNG. Essentially, each frame is compressed as a PNG, so if PNG does it, this codec does too.
The P frame takes the difference of the previous frame and the current frame and encodes that as a PNG.
CorePNG was orignally developed for use with the subtitles, it compresses cartoons and CGI very well and in many cases better than HuffYUV
This codec also has the ability to write P frames and to autodetect when it should.