I'm not sure exactly what you are trying to do.

What exactly do you mean by "moves"? By "object" do you mean a raster color run (a horizontal line of the same color)?

What is the purpose of doing this? The ppm format is as horribly inefficient as pbm, the only real difference is that it stores RGB colors instead of on/off. It has no compression or organization other than a simple list of pixels in raster order.

If you can explain it better with pictures, I don't mind.