Shaders and Texturing - An Introduction to Computer Graphics for Artists - page 200

Fig. 11.21 Photographs do not convert well to bump maps, as shown in this image

11.5.4.3 Bump

If you use shadow impressions from a photo as reference for a bump map, as is
often done, remember that a bump map uses the intensity of pixels in a map to
determine their height. If you use the shadow information from your photograph
exactly as-is, you will not get the result you want. You will have enough informa-
tion to paint it correctly, but the values in the image are not intensity values based
on height information, as your fi nished map should be, but intensity values based
on shadows in a 3D scene (Fig. 11.21 ). Because it is not the same information, it
will not yield the same result.
The outlines of each bump area can give you an idea of shape, just as intensity
values can give you an idea of height, but the correct values and shape are some-
thing you will have to extrapolate from the data in the photograph.
If you use a single cylindrical bump on a wall as an example, with a strong light
hitting it from one side, then the opposite side of the cylinder will have a crescent
shaped shadow, and the side the light hits will have an arc shaped highlight. In
between will be a tonal gradient, unless the bump is too small for a gradient to be
represented. When rendered as a bump, you will not get a raised cylinder, but a
crescent-shaped crevice and an arc-shaped wall, with a short ramp in between. This
is true for any bump or displacement map.
If you do allow shadows to remain in your maps, keep in mind that they will not
change based on lighting in your scene, and if they are prominent, it will be clear
that these textures are double-lit, by the lighting present when the photo was taken,
and by the CG lighting in the scene. Shadows can be the most easily spotted texture
error, so great care should be taken with them if they exist in your source images.
To paint a height map properly, the full range of desired height difference must
be scaled to the full 0-255 grayscale values possible in a bump or displacement map
(Fig. 11.22 ).