How do I get a colormap from JPEG file?

Question

Julián Francisco 2011-10-24

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https://ww2.mathworks.cn/matlabcentral/answers/19117-how-do-i-get-a-colormap-from-jpeg-file

评论： Walter Roberson 2016-6-7

Hi. I have a jpg image file of the surface of Neptune. My intention is to build a texture mapping ( see Matlab's help about this topic ). I have used the command imread with the file but jpg files have not a colormap (in general, the command imread produces an MxNx3 matriz and a colormap is a Mx3 matrix). I would like to know how I could do it.

Like a image is more valuable than 1000 words (sometimes), my purpose is doing something like that (see example) but for Neptune.

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Image Analyst 2011-10-24

The jpg image is a true color image. What do you mean by a topographic image? You mean like a geological survey topographic map with contour lines on it and you want that somehow transferred to the surface of a spherical object? What proportion of the sphere should this 2D rectangle cover? A tiny spot? A whole hemisphere? Something in between? Or do you want some kind of 2.5D terrain image with mountains and things sticking above the surface of the sphere?

Julián Francisco 2011-10-24

@Image Analyst: Thank you for your interest. The image is a 2d representation of the whole surface (longitude [0,360] degrees and latitude [-90,90] degrees). I want to transform the colordata into altitude z about the sphere.

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Answer 1

Walter Roberson 2011-10-24

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You can use the two-output form of rgb2ind() to extract a colormap from a true-color image.

I am not certain how you plan to map colormap indices to altitudes, though.

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Julián Francisco 2011-10-24

@Walter Roberson: Ok, you are right. I was wrong (I thought the colour of the image was based on the altitude about Neptune's surface through some type of transformation). Thank you so much for your answer.

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Answer 2

Julián Francisco 2011-10-24

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https://ww2.mathworks.cn/matlabcentral/answers/19117-how-do-i-get-a-colormap-from-jpeg-file#answer_25583

在 MATLAB Online 中打开

Here is the solution for my question based on the answer of Walter Robertson:

    [X, map] = rgb2ind(imread('neptune.jpg'),128);
    [x,y,z] = sphere(50);
    x = 24764*x;
    y = 24764*y;
    z = 24764*z;
    props.FaceColor= 'texture';
    props.EdgeColor = 'none';
    props.Cdata = flipud(X); % it is necessary to do this for getting the 
    % appropiate image on the sphere
    surface(x,y,z,props);
    colormap(map);
    axis equal;
    view([71 14]);