The color of each quadrangle in a pm3d surface is assigned based on the color
values of its four bounding vertices.
The options 'mean' (default), 'geomean', 'harmean, 'rms', and 'median' produce
various kinds of surface color smoothing, while options 'min' and 'max' choose
minimal or maximal value, respectively. This may not be desired for pixel
images or for maps with sharp and intense peaks, in which case the options
'c1', 'c2', 'c3' or 'c4' can be used instead to assign the quadrangle color
based on the z-coordinate of only one corner. Some experimentation may be
needed to determine which corner corresponds to 'c1', as the orientation
depends on the drawing direction. Because the pm3d algorithm does not extend
the colored surface outside the range of the input data points, the 'c4#4j5#5'
coloring options will result in pixels along two edges of the grid not
contributing to the color of any quadrangle. For example, applying the pm3d
algorithm to the 4x4 grid of data points in script demo/pm3d.dem (please have
a look) produces only (4-1)x(4-1)=9 colored rectangles.
