Plot points (x,y) and use a third (z) as the color code in Matlab

Question

Possible Duplicate:
matlab: scatter plots with high number of datapoints

I have 3 vectors of 315,000 elements each. X,Y, and Z. X & Y are coordinates and Z is a value. I must plot the coordinates as points in a 2D graph, the Z is a color indicator at each coordinate of X and Y. I've tried the "scatter" command, but it extremely slow. Would anybody suggest a better way?

thanks!

Do you really need to plot all 315000 elements? typical screens has 2K pixels anyway, can't just down-sample you vectors, using interp1 or similar, and use scatter? — bla, Jan 14 '13 at 19:34

score 1 · Answer 1 · answered Jan 14 '13 at 19:20

Depending on what kind of color map you are looking for, you can try something like

zmin=min(Z);
zmax=max(Z);
map=colormap;
color_steps=size(map,1);

hold on
for i=1:color_steps
    ind=find(Z<zmin+i*(zmax-zmin)/color_steps & Z>=zmin+(i-1)*(zmax-zmin)/color_steps);
    plot(X(ind),Y(ind),'o','Color',map(i,:));
end

The finding is a little expensive but it seems to be quicker than scatter. I'm sure you could optimize this further.

score 0 · Answer 2 · answered Jan 14 '13 at 18:43

0

Try cline from MATLAB file exchange here. It looks like it does exactly what you want.

answered Jan 14 '13 at 18:43

dinkelk

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I'm trying to plot the elements as points.... but thanks for the comment. I'll update the question – Oliver Amundsen Jan 14 '13 at 18:54

score 0 · Answer 3 · answered Jan 14 '13 at 19:02

Your code is slow because of the large size of the vectors, not because of the SCATTER function. Try breaking them down into vectors of smaller size (say, 10 elements each) and putting each vector into a cell of a cell array. Then loop through the cell array and scatter each smaller vector individually to avoid loading too much into the memory.

hold on
for i=1:numel(XcoordCellArray):
  scatter(XcoordCellArray{i},YcoordCellArray{i},S,ZcoordCellArray{i})
end

Plot points (x,y) and use a third (z) as the color code in Matlab

3 Answers3