Progressive coding of palette images and digital maps
A 2D version of PPM (Prediction by Partial Matching) coding is introduced simply by combining a 2D template with the standard PPM coding scheme. A simple scheme for resolution reduction is given and the 2D PPM scheme extended to resolution progressive coding by placing pixels in a lower resolution image layer. The resolution is increased by a factor of 2 in each step. The 2D PPM coding is applied to palette images and street maps. The sequential results are comparable to PWC. The PPM results are a little better for the palette images with few colors (up to 4-5 bpp) and a little worse for the images with more colors. For street maps the 2D PPM is slightly better. The PPM based resolution progressive coding provides a better result than coding the resolution layers as individual images. Compared to GIF the resolution progressive 2D PPM's coding efficiency is significantly better. An example of combined content-layer/spatial progressive coding is also given.