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How to have a discrete color bar for scatter3?

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Fajri
Fajri on 23 Mar 2016
Answered: Makarand on 27 Apr 2018
Hi all,
I have here residual errors as function of variable X and Y. The error ranges from 0 to (I think maximum) 3.7 something...
How can I have a discrete color for the marker and the color bar? Let's say that for error values from:
r < 0.5 --> color dark blue
0.5 < r < 1.0 --> color green
1.0 < r < 2.0 --> color yellow
2.0 < r --> color red
I have used scatter3 for the plot, the code looks like this:
scatter3(variable_X,variable_Y,residual_errors,[],residual_errors,'filled')
Thanks for your help!

Accepted Answer

Mike Garrity
Mike Garrity on 23 Mar 2016
Edited: Mike Garrity on 23 Mar 2016
You want to use the colormap function to tell the axes what colors you want to use. That's pretty easy:
colormap([0 0 .5; 0 1 0; 1 1 0; 1 0 0])
But the problem is that MATLAB's colormap lookup is always linear. If your bins aren't all the same width, then it isn't that simple. You'll probably need something like this:
npts = 1e5;
x = randn(npts,1);
y = randn(npts,1);
z = randn(npts,1);
scatter3(x,y,z,32,z,'.')
crange = caxis;
t = linspace(crange(1),crange(2),64);
cmap = zeros(64,3);
mask = t<=.5;
cmap(mask,:) = repmat([.5 0 0],[sum(mask), 1]);
mask = t>.5 & t<=1;
cmap(mask,:) = repmat([0 1 0],[sum(mask), 1]);
mask = t>1 & t<=2
cmap(mask,:) = repmat([1 1 0],[sum(mask), 1]);
mask = t>2;
cmap(mask,:) = repmat([1 0 0],[sum(mask), 1]);
colormap(cmap);
You actually could simplify that a bit because the colormap will clamp to the last color for values that are off the end. That means that you could just have the linspace go between any two fixed values that are convenient. If you do that, you need to pass the values you use into the caxis function so the axes knows the range you're defining the colormap for.
  1 Comment
Fajri
Fajri on 24 Mar 2016
Thank you Mike, I have tried your code and it works :)
I don't get the part with the simplification yet...do you mean I could let this part go?:
mask = t>2;
cmap(mask,:) = repmat([1 0 0],[sum(mask), 1]);
and the rest of error values greater than 2 would be automatically assigned a specific color?
Thanks again!

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