XMPNG (Differential Motion PNG) is a simple lossless video codec, particularly well-suited for high quality screencasts and screen sharing applications. This is a quick, proof of concept C#/.NET implementation from an old academic project.
The concept is pretty straightforward: the encoder writes the initial I frame as a full PNG image, then it writes each subsequent P frame as the PNG encoded XOR "difference" with respect of its preceding frame (i.e. P2 = Frame 2 XOR Frame 1).
This method allows to greatly reduce both the inter-frame temporal redundancy, by means of the fast and effective XOR "difference", and the intra-frame spatial redundancy, thanks to PNG's own 2D prediction filter.
Then, the decoder can reconstruct the exact, original frames by making a XOR operation between each P frame and the previous reconstructed frame (e.g. Frame 2 = P2 XOR Frame 1)
