initialising a color map

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Harry 2013-7-16

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How do I create a colormap with n*m pixels which is all white and then display it?

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Answer 1

Image Analyst 2013-7-16

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https://ww2.mathworks.cn/matlabcentral/answers/82188-initialising-a-color-map#answer_91889

在 MATLAB Online 中打开

whiteColorMap = ones(256,3);
colormap(whiteColorMap);
colorbar;

Not sure what good that would be. What's the point?

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Harry 2013-7-16

I'm just trying to learn how they work as I find the documentation about colormaps confusing.

Is there a way of specifying the dimensions of the image as at the moment it will only give a single pixel?

Essentially I want to set up an image so that I can manipulate it later.

Image Analyst 2013-7-16

在 MATLAB Online 中打开

No, that doesn't make sense. Colormaps are used as a pseudocolor look up table. For a typical uint8, 256 gray level image, use a 256 row by 3 column look up table. The row is the gray level, and the values in the row are the fraction of Red, Green, and Blue that that gray level will be displayed as. So it maps a gray level into a color. For example if you make a colormap

myColorMap = zeros(256,3); % All black

That will map all gray levels into black. Now if you set row 42 equal to a color

myColorMap(42, :) = [1, 0, 0]; % GL 42 maps to red

Then when you apply the colormap with the colormap() function, the look up table (myColorMap) says to make any/all pixels with gray level 42 show up as pure red instead of a gray with intensity 42. Does that explain it better? There are predefined colormaps you can use where you specify the number of levels, for example jet(256) or cool(256). Use fewer gray levels if you want a more "posterized" look.

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Answer 2

Iain 2013-7-16

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https://ww2.mathworks.cn/matlabcentral/answers/82188-initialising-a-color-map#answer_91893

在 MATLAB Online 中打开

 image_dat = uint16(randn(1024,1280)*5+5000); % random image (single colour channel)
imagesc(image_dat) % display it
colorbar % put the scale on the screen
colormap white % use the "white colormap"
 custom_colormap = [r1 g1 b1;
                    r2 g2 b2
                    r3 g2 ...
                    ...
                    rn gn bn];

r1, is the amount of red in the first colormap bin, g2 is the green in the second colormap bin, b3 is the blue in the 3rd colormap bin, (and so on) they are scaled from 0 (none) to 1 (max)

colormap(custom_colormap) % changes the display to the colormap you defined.

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Harry 2013-7-16

I am still having trouble understanding how theses colormaps work as I don't see how you can give the image x and y dimensions. Because that custom colormap looks like series of bins with three different colours, but no indication of which row and column its on.