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I.5 Artificial illumination
GMT uses the HSV system to
achieve artificial illumination of colored images (e.g. -I
option in grdimage) by changing the saturation s and value v
coordinates of the color. When the intensity is zero (flat illumination), the data
are colored according to the cpt file. If the intensity is
non-zero, the color is either lightened or darkened depending on the illumination.
The color is first converted to HSV (if necessary) and then
darkened by moving (
,
) toward (HSV_MIN_SATURATION,
HSV_MIN_VALUE) if the intensity is negative, or lightened by sliding (
,
) toward
(HSV_MAX_SATURATION, HSV_MAX_VALUE) if the illumination is positive.
The extremes of the
and
are defined in the .gmtdefaults4 file and are usually
chosen so the corresponding points are nearly black (
= 1,
= 0) and white (
= 0,
= 1).
The reason this works is that the HSV system allows movements in
color space which correspond more closely to what we mean by
``tint'' and ``shade''; an instruction like ``add white'' is
easy in HSV and not so obvious in RGB.
Next: I.6 Thinking in RGB
Up: I. Color Space: The
Previous: I.4 Color interpolation
Contents
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Paul Wessel
2010-01-14