Map and Model Resolutions

From ZBrush Info

In spite of the nondescript title, the information in this section is quite important to achieving the best normal map possible. When working with ZMapper, there are up to four different resolutions you must consider:

• The resolution of the 3D object being mapped, in terms of number of polygons and uniformity of distribution.
• The resolution of the final normal map.
• If creating a bump map in ZBrush, the resolution of the bump map.
• If using Projection Master as part of the process, the resolution of the canvas.

For optimal results, you need to make sure that all of these resolutions are set to be compatible with one another. Here's how to do it.

• Assuming the polygons on your high-resolution model are distributed fairly equally over the UV unwrapping of the model, the number of pixels in the normal map (i.e., map pixel height times map pixel width) should be about equal to the number of polygons in the high-resolution model. If you have many more pixels than polygons, many pixels will end up mapping to the same polygons, which means your map will be carrying redundant information. Conversely, if you have many fewer pixels than polygons, some of your polygons will not be reflected in the normal map, and detail will be lost. Note that whenever you increase the size of a normal map, the number of pixels in it goes up by a factor of four, which might make it hard to get a good match for the number of model polygons. If your end use application can handle it, too many pixels will give better results than too few in this case.
• If you will be incorporating a bump map, the bump map dimensions (width and height) should generally be between one and two times the width and height dimensions of the normal map. Bump maps smaller than this will not be taking full advantage of the detail they can contribute to the normal map, and bump maps larger than this may lose fine detail when rolled into the normal map. Note that bump map dimensions are not constrained to have the same width and height, or to be powers of two.
• If using Projection Master to create a bump (or similar) map, we need to consider the resolution of the ZBrush canvas. The rule is that there should be an approximate one-to-one correspondence between pixels on the canvas and mapped pixels on the viewed portion of the model after it has been dropped, assuming it is sized so as to fill most of the canvas. This ensures that any change to a pixel on the canvas will result in a more-or-less equivalent change to a pixel (or a couple of pixels) in the bump map. When working with a flat one-sided model such as a plane representing a wall, this means that the resolution (and for that matter, the pixel dimensions) of the canvas should be about the same as the model. If the model is two sided, then on average only one-half of its mapped pixels will be exposed at any time, so the number of pixels on the canvas should be about half of the number of pixels in the bump map.