Changing the colour intensity of a 2D plot

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John 2012-12-25

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Hi,

I'm plotting lat long data on a map like this.

I'm using the plotm() function:

plotm(x,y,'LineStyle','none','Marker','o','MarkerSize',20);

I'm trying to change the colours so that areas with more data are a different colour. I would like the intensity of the colour to change with the amount of data points.

Would anybody know how I could achieve this?

Thanks

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Walter Roberson 2012-12-26

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plotm() cannot have different colors within any one line (column of data).

There are tricks for using patch() for drawing very narrow lines.

It appears to me that your data is sufficiently dense that you could use scatterm() and have the output look relatively continuous. scatterm() accepts a matrix of colors, one RGB row per point.

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Walter Roberson 2013-1-14

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What I think I would suggest at this point is to use interp1() on relative_counts, using a look-up table that emphasized the low values but still had high values. For example,

mapval = interp1( 0:.1:1, [0 .3 .5 .7 .725 .75 .775 .8 .825 .9 1], relative_counts);
rgbarray = [zeros(length(x), 2), mapval(:)];

You would adjust the first parameter (possibly unevenly) according to where you wanted the intensity boundaries to be, and would adjust the second parameter according to the percentage blue you wanted each range to show up with.

John 2013-1-17

编辑：John 2013-1-17

Hello Walter,

I tried your new suggestion. Unfortunately the points were all black, similar to the image posted below on Image Analyst's post. When you say 'adjust the first parameter', do mean, 0:.1:1? What would you suggest I change it to? Apologies for all the questions but I don't really understand the code and I'm not sure on how to proceed.

Thank you

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Image Analyst 2012-12-26

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编辑：Image Analyst 2013-1-13

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If you treat it as an image you could do that, with some work. You could run the thing through a convolution, conv2(). This would give you a higher signal where the roads are more dense. Essentially that is an indexed image. Then you apply a colormap to it, and call ind2rgb to turn it into a color image. Then you use the roads as a mask over the color image so that roads in concentrated areas show up as a different color than roads in sparse areas. Hopefully that described it enough for you to carry out the code. Though I think this representation would be very confusing and distracting to look at.

for k = 1 : length(x)
  yourImage(round(y(k)), round(x(k)) = 1;
end
densityImage = conv2(yourImage, ones(5)/25, 'same');
imshow(densityImage, []);
colormap(jet(256));

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John 2013-1-16

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Hello Image Analyst,

I am not using the mapping toolbox to source the image. I am using this file on the file exchange to retrieve the map and then I just overlay the data on it. I code I use is below:

http://www.mathworks.com/matlabcentral/fileexchange/27627

 latlim = [53.0 53.6];
 lonlim = [-6.5 -6];
  figure
  axis([lonlim, latlim])
  axis image
  plot_google_map
  hold on
  plot(x,y,'LineStyle','none','Marker','.','MarkerSize',3);