plscmap1l: Set color map1 colors using a piece-wise linear relationship

plscmap1l (itype,
 npts,
 pos,
 coord1,
 coord2,
 coord3,
 alt_hue_path);
 

Set color map1 colors using a piece-wise linear relationship between position in the color map (from 0 to 1) and position in HLS or RGB color space (see the section called “Color Map1”). May be called at any time.

The idea here is to specify a number of control points that define the mapping between palette 1 input positions (intensities) and HLS (or RGB). Between these points, linear interpolation is used which gives a smooth variation of color with input position. Any number of control points may be specified, located at arbitrary positions, although typically 2 - 4 are enough. Another way of stating this is that we are traversing a given number of lines through HLS (or RGB) space as we move through color map1 entries. The control points at the minimum and maximum position (0 and 1) must always be specified. By adding more control points you can get more variation. One good technique for plotting functions that vary about some expected average is to use an additional 2 control points in the center (position ~= 0.5) that are the same lightness as the background (typically white for paper output, black for crt), and same hue as the boundary control points. This allows the highs and lows to be very easily distinguished.

Each control point must specify the position in color map1 as well as three coordinates in HLS or RGB space. The first point must correspond to position = 0, and the last to position = 1.

The default behaviour is for the hue to be linearly interpolated between the control points. Since the hue lies in the range [0, 360] this corresponds to interpolation around the "front" of the color wheel (red<->green<->blue<->red). If alt_hue_path[i] is true, then an alternative interpolation is used between control points i and i+1. If hue[i+1]-hue[i] > 0 then interpolation is between hue[i] and hue[i+1] - 360, otherwise between hue[i] and hue[i+1] + 360. You can consider this as interpolation around the "back" or "reverse" of the color wheel. Specifying alt_hue_path=NULL is equivalent to setting alt_hue_path[] = false for every control point.

Table 19.1.  Examples of interpolation

Huealt_hue_pathcolor scheme
[120 240]falsegreen-cyan-blue
[240 120]falseblue-cyan-green
[120 240]truegreen-yellow-red-magenta-blue
[240 120]trueblue-magenta-red-yellow-green


Table 19.2.  Bounds on coordinates

RGBR[0, 1]magnitude
RGBG[0, 1]magnitude
RGBB[0, 1]magnitude
HLShue[0, 360]degrees
HLSlightness[0, 1]magnitude
HLSsaturation[0, 1]magnitude


itype (PLBOOL, input)

true: RGB, false: HLS.

npts (PLINT, input)

number of control points

pos (PLFLT *, input)

position for each control point (between 0.0 and 1.0, in ascending order)

coord1 (PLFLT *, input)

first coordinate (H or R) for each control point

coord2 (PLFLT *, input)

second coordinate (L or G) for each control point

coord3 (PLFLT *, input)

third coordinate (S or B) for each control point

alt_hue_path (PLBOOL: *, input)

alternative interpolation method flag for each control point. (alt_hue_path[i] refers to the interpolation interval between the i and i + 1 control points).

Redacted form: plscmap1l(itype, pos, coord1, coord2, coord3, alt_hue_path)

This function is used in examples 8,11,12,15,20,21.