Very much this -- it will unfortunately have a horizontal bias if done naively, but I've been exploring the space of doing lossy preprocessing to improve image

Very much this -- it will unfortunately have a horizontal bias if done naively, but I've been exploring the space of doing lossy preprocessing to improve image compression, but the layered heuristics in png make this difficult. This format is simple enough that we have a number of approximation heuristics -- find the closest color in the last 64, use a smaller delta, or preserve one or two channel colors in the next pixel.