im_lab_morph(3) calculate colour differences


#include <vips/vips.h>

int im_lab_morph( IMAGE *in, IMAGE *out,


  double L_offset, double L_scale,

  double a_scale, double b_scale )


This function tweaks the colour in an LAB image. It's useful for making a 'tweaked' image for sending to a colour printer.

It performs three corrections: first, it straightens the neutral axis (this is useful if your printer tends to tint shadows slightly red, for example); it moves L* by adding an offset and then multiplying by a scale (useful if your printer thinks in relative colorimetry, for example), and finally scales a* and b* by a factor (useful if your printer desaturates to avoid gamut clipping).

The neutral axis straightening is specified as a DOUBLEMASK containing L*, a* and b* readings taken from a print of a neutral greyscale (one with a* and b* zero). For example:

                3 4

                14.23   4.8     -3.95

                18.74   2.76    -2.62

                23.46   1.4     -1.95

                27.53   1.76    -2.01

This is interpolated to make an a/b offset for each input value of L*. The top and tail are interpolated towards [100,0,0] and [0,0,0]. Entries in the mask can be in any order.


All functions return 0 on success and -1 on error.


National Gallery, 2001