Jet function output error

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WizKing 2018-7-29

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评论： Walter Roberson 2018-8-4

I got a script that is to create a flux map using two images; image of reflected rays and image of the sun. The inital lines make sense and produce an output but the total result is null (the is no flux map) after running it. I realized the

jet (ceil (max(fmap_img(:)))

did not produce anything. Any help?

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Walter Roberson 2018-7-29

You can attach the script.

WizKing 2018-7-30

phlux_v4.m

@Walter Robertson. Okay

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Answer 1

Image Analyst 2018-7-30

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编辑：Image Analyst 2018-7-30

在 MATLAB Online 中打开

You must pass Jet an integer that is the number of colors it is supposed to create.

Try this:

numberOfColors = ceil (max(fmap_img(:)))  % No semicolon
cmap = jet(numberOfColors) % Create a color map.
colormap(cmap);  % Apply the colormap to the indexed/gray scale image.

What do you see in the command window? Does the image look pseudocolored now?

You might also want to look into caxis().

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Walter Roberson 2018-7-30

在 MATLAB Online 中打开

You need to take into account that for uint8 or uint16 data type, the data value 0 is valid and means "intensity all the way at minimum".

You use the largest value in the green plane as the number of colors to use from the jet color map, but suppose the largest value is 0, then you say that the number of colors to use is 0. That is incorrect: if the largest value is 0, that means you should use 1 color. If the largest value is N then you need to use N+1 colors.

Note too that using

rcv_colormap = colormap(jet(ceil(rcv_img_max)));
colormap(rcv_colormap);

is a bit redundant. jet(ceil(rcv_img_max)) is building the colormap array, and then the colormap() call around that already puts it into effect and returns it; you do not need to colormap() it again. I would suggest you should instead

rcv_colormap = jet(ceil(rcv_img_max));

without the colormap() call at that point.

I have to wonder if using a number of colors based upon the maximum value is what you really want to do. For example when the number of colors is odd, then the middle of the jet map is always color [1/2 1 1/2] . So if you see that color on an image with (say) 101 colors then it would represent one numeric value, but if you see it on a different image with (say) 43017 values, it would represent a completely different value, making it impossible to determine absolute values by visual comparison. Is that what you want, that the colors represent relative values? Or do you want the colors to represent absolute values?

WizKing 2018-8-3

编辑：Walter Roberson 2018-8-4

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Okay please I have done that.

Also, after the colormap() call. I want to create a surface that looks like:

[X,Y] = meshgrid(-8:.5:8);
R = sqrt(X.^2 + Y.^2) + eps;
Z = sin(R)./R;
% surface in 3D
figure;
surf(Z,'EdgeColor','None'); view(2)% surface in 2D

Using the colormap() output in the script. So that the red will be the centre color (peak), followed by yellow up to blue in a concentric circular transition.