Use of 2D colormaps possible in Matlab?

Is there a way to use colormaps in Matlab which change in two dimensions?

If yes, how? And how do you create them?

One example could be to have magitude and orientation of a 2D vector field in one plot.

I have found some examples on other platforms:

Source: https://dominikjaeckle.com/projects/color2d/

Or this discussion here about circular 2D colormaps:

Source: https://github.com/BIDS/colormap/issues/6

Best regards

Answer 1

[Dave B]

There's nothing (that I know of) that provides this as a built-in utility in MATLAB, but it's pretty easy to do this kind of thing by interpolating indices into a colormap. For example, to recreate the scatter at the top of the link:

%% Load a colormap and store in a matrix of RGB values
im = imread('https://dominikjaeckle.com/projects/color2d/data/bremm.png');
rgb = double(reshape(im, [], 3))./255;

%% Example mapping
x=rand(100,1);
y=rand(100,1);

% interpolate to find a color for each x and y, mapping the range of the
% colormap onto [0 1]
c_column=round(interp1(linspace(0,1,size(im,2)), 1:size(im,2), x));
c_row=round(interp1(linspace(0,1,size(im,1)), 1:size(im,1), y));
rgb_row=sub2ind(size(im),c_row,c_column,ones(size(c_row))); % this is the same as the index into im of red values
c=rgb(rgb_row,:);

%% Make some plots
t=tiledlayout(1,3);
nexttile;image(im)
nexttile;scatter(x,y,'filled')
nexttile;scatter(x,y,[],c,'filled')
set(t.Children,'XTick',[],'YTick',[],'box','on','DataAspectRatio',[1 1 1],'YDir','reverse')

[Marcus Becker]

Thank you! That certainly helps.

I adapted your plotting to allow for surface plots:

% [...] equal code, other colormap
%% Find faces
DT = delaunay(x,y);
%% Make some plots
t=tiledlayout(2,2);
nexttile;image(im)
nexttile;scatter(x,y,'filled')
nexttile;scatter(x,y,[],c,'filled')
nexttile;patch('Faces',DT,'Vertices',[x,y],'FaceVertexCData',c,... % Surface plot
    'EdgeColor','none','FaceColor','interp')
set(t.Children,'XTick',[],'YTick',[],'box','on','DataAspectRatio',[1 1 1],'YDir','reverse')

Then you get this:

[Dave B]

It's an interesting idea, I wonder if there's potential for something to put on file exchange here (it's hard to imagine what this would look like as a MATLAB feature as it seems like you might think about it a bit differently depending on the plot type?)

[Marcus Becker]

For me, ideally there would be function calls like

mesh4d(x,y,u,v,ulabel,vlabel,'Cmap2d','bremm')

or

scatter4d(x,y,u,v,'Cmap2d','bremm')

which provide a selection of useful colormaps and maybe even an interface for custom ones.

Issue might be that the colorbar becomes a colorplane and needs x and y labels.

But I do think that it would be helpful to put it on file exchange!

[Marcus Becker]

I have created a Git with a working version of the function. It's not amazing code but it works for my needs:

https://github.com/MarcusBecker-GitHub/plot2D2D

Feel free to use, share, adapt.