Question

I am trying to implement a colorbar in Matlab that looks linear when printed in grey. The most straight forward approach I though would be to implement the gnuplot pm3d 30,31,32 RGB colour space. On this website I found a good introduction, which works OK for some of the easier gnuplot schemes. However, when trying to implement the 30,31,32 scheme I run into trouble.

The gnuplot instructions are

30: x/0.32-0.78125 31: 2*x-0.84       32: 4x;1;-2x+1.84;x/0.08-11.5

And I interpreted this such that for the blue channel I have to apply four different equations. One for each quarter of intensity values. This is what I have tried so far

x       = linspace(0,1,128);
r                               = x/0.32-0.78125;
g                               = 2*x-0.84;
b(1:length(x)/4)                = 4*x(1:length(x)/4);
b(length(x)/4:length(x)/2)      = 1;
b(length(x)/2:length(x)*.75)    = -2*x(length(x)/2:length(x)*.75)+1.84;
b(length(x)*.75:end)            = x(length(x)*.75:end)/0.08-11.5;
pm3d303132=[r;g;b]';

but unfortunately it doesn’t work. I end up with negative values and values that exceed 1, which Matlab cannot interpret.

I did read in the show palette rgbformulae help that

* negative numbers mean inverted=negative colour component

But I don’t know how to implement this in Matlab; and I also don’t understand how to deal with values exceeding 1. Can anyone help?

Answer 1

Ignoring the out of bound values seems to work. (http://juluribk.com/2011/05/18/843/)

pm3d303132(pm3d303132<0)=0;

pm3d303132(pm3d303132>1)=1;