If it is not a power of two, the nearest power of two less than n is substituted. This sets the size of the squares, measured in pixels. If it is not a power of two, the nearest power of two less than n is substituted. The default ending value is 1. Future versions of POV-Ray will include some system of temporary files or buffers which will eliminate these inefficiencies and limitations.
Mosaic preview is still a very useful feature for test renderings.
When you are developing a scene and doing test renders, the graphic preview may be sufficient. See your platform-specific documentation to see what your default file type is. PPM, PNG, and system specific formats have been added. PPM format images are uncompressed, and have a simple text header, which makes it a widely portable image format. PNG is a new image format designed not only to replace GIF, but to improve on its shortcomings.