Defining value range in colormap manually

2016 1 11
2 个回答
回答已采纳
更新时间：2021 4 12
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0 个投票

My data range is 0-15
I plotted
scatter(X,Y,Data);
colormap([0 0 0; 1 0 0;0 0 1]);
it takes color 0 0 0 as 0-5 and so on
I need
0 0 0 as <4
1 0 0 as>4 and <9
0 0 1 as >9
How do I define value range in colormap

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Stephen23
Stephen23 2016-1-11
编辑：Stephen23 2016-1-11

1 个投票

You just need to supply the fourth option to scatter, which maps the values onto the colormap. One very easy way to do this is to use histc / histcounts:
X = rand(50,1);
Y = rand(50,1);
data = randi(16,50,1)-1;
[~,C] = histc(data,[0,4,9,Inf]);
scatter(X,Y,10,C)
colormap([0,0,0;1,0,0;0,0,1]);

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Utsav
Utsav 2016-1-11
Hello Stephen. Thanks for your reply, But I have X and Y and a corresponding Z, Z value 0 to infinity (say). I wish 0-4 as black, 4-9 as red and above 9 as blue in scatter format so as to say.
I am not so well versed in this art of Matlab, so I may be excused of this ignorance.
Stephen23
Stephen23 2016-1-11
编辑：Stephen23 2016-1-11
This is exactly what my code does: it colors the scatter points according to the data values, exactly as you have told us, using the three colors that you defined. Why do you think that it does not do this?
Utsav
Utsav 2016-1-11
Hi Stephen, Thanks a ton for your answer
But histc makes the datasets as 1,2,and 3 corresponding to three ranges
When I use colorbar I get 1-3 instead of the real data values 0-infinity
Any solution friend?
Stephen23
Stephen23 2016-1-11
编辑：Stephen23 2016-1-12
Try specifying the colorbar tickmark locations and labels:
>> colorbar('YTicklabel',[0,4,9,Inf],'YTick',linspace(1,3,4))
Utsav
Utsav 2016-1-12
Hello Stephen,
I am really grateful to you for your continual help. Thanks. I could get around it with your philanthropy.
Stephen23
Stephen23 2016-1-12
My pleasure. You can also Accept answer that best resolve your question. This gives points to those who volunteer to help you.
Hasan Jamil Apon
Hasan Jamil Apon 2021-4-12
If I want to set color limit for 3D plot,then what sort of changes I have to make? Like I want the red color for the Z axis value between 0.8 to 0.93, cyan color for Z axis value between 0.93 to 0.95, yellow color for Z axis value between 0.95 to 1 for given X and Y values. Please give me some guidance in this issue. Will be really be grateful!

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更多回答（1 个）

Image Analyst
Image Analyst 2016-1-11

1 个投票

I'm not sure what you mean. All of data (if it's a 1-D array) will show up in the same color, and as circles with whatever markersize you specified as Data (which, by the way, I think is a bad name for you to choose as the marker size).
If you want multiple colors, you have to call scatter() twice.
Data = 20; % Circle diameter
% Define sample data.
X = 12 * rand(1,20);
Y = 12 * rand(1,20);
scatter(X,Y,Data);
% Define second sample data.
X = 12 * rand(1,20);
Y = 12 * rand(1,20);
hold on;
scatter(X,Y,Data);
legend('Set #1', 'Set #2');
grid on;
% colormap([0 0 0; 1 0 0;0 0 1]);
If you want, you can pass in an array that is the colormap for markers based on the index.
markerSize = 20; % Circle diameter
% Define sample data.
numPoints = 500;
X = 12 * rand(1, numPoints);
Y = 12 * rand(1, numPoints);
cMap = zeros(length(X), 3);
% Define markers to be red if both X and Y >4 and < 9
red = X > 4 & X < 9 & Y > 4 & Y < 9;
cMap(red, 1) = 1;
% Define markers to be blue if both X and Y >= 9
blue = X >= 9 & Y >= 9;
cMap(blue, 3) = 1;
scatter(X,Y, markerSize, cMap);
grid on;

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Utsav
Utsav 2016-1-11
Hi Thanks
Actually, for given X and Y pair I have a Z value (used Data for brevity) Z ranges from 0 to infinity(say) I wished scatter plots with just three colors 0-4 as black 4-9 as red and above 9 as blue.
Please Help.
Image Analyst
Image Analyst 2016-1-11
Like I and Stephen area saying, you can't have Data as the third argument because if there's only 3 arguments the third argument must be the marker size. You have to supply a fourth argument if you want to make the marker a certain color. A slight variation from my second code example (to look at the magnitude of Z instead of X and Y) is:
markerSize = 20; % Circle diameter
% Define sample data.
numPoints = 100;
% Make up location data X and Y
X = 100 * rand(1, numPoints);
Y = 100 * rand(1, numPoints);
% Generate magnitude data Z
Z = 15 * rand(1, numPoints);
% Allocate colormap
cMap = zeros(length(X), 3);
% Define markers to be red if Z >4 and < 9
red = Z > 4 & Z < 9;
cMap(red, 1) = 1;
% Define markers to be red if Z >4 and < 9
blue = Z >= 9;
cMap(blue, 3) = 1;
% Now scatter the data with colors depending on Z magnitude.
scatter(X,Y, markerSize, cMap);
grid on;
Utsav
Utsav 2016-1-12
Thanks Sir,
I was able to get around the issue with all support from you and Stephen. Take care.
Hasan Jamil Apon
Hasan Jamil Apon 2021-4-12
If I want to set color limit for 3D plot,then what sort of changes I have to make? Like I want the red color for the Z axis value between 0.8 to 0.93, cyan color for Z axis value between 0.93 to 0.95, yellow color for Z axis value between 0.95 to 1 for given X and Y values. Please give me some guidance in this issue. Will be really be grateful!
Image Analyst
Image Analyst 2021-4-12
I'm sure you made the obvious changes (something like)
% Define markers to be red if Z >= 0.8 and Z <= 0.93
redRows = Z >= 0.8 & Z <= 0.93;
cMap(redRows, :) = 1;
% Define markers to be cyan if Z >= 0.93 & Z <= 0.95
cyanRows = Z >= 0.93 & Z <= 0.95;
cMap(cyanRows, :) = [0, 1, 1];
but it didn't work, so you're asking me. I don't know what other changes you made in the code, which you forgot to post. Please post your non-working code along with your XYZ data in a .mat file with the paperclip icon.

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