I am trying to make a contour plot using the following matlab code:
x=linspace(-10,10);
y=linspace(-10,10);
[X,Y]=meshgrid(x,y);
Z=X.^3-Y.^3;
figure
[c,h]=contour(X,Y,Z,[3]);
clabel(c,h)
This gives me the wrong picture actually:
I really don't understand what goes wrong here, because when I do [c,h]=contour(X,Y,Z,[3 0]) for example, it does give me the correct contour plots for the levels 3 and 0, I need help.
If you only give a single number to contour there, it interprets it as the number of contour lines you want and picks the levels automatically. From the docs:
contour(Z,v) draws a contour plot of matrix Z with contour lines at the data values specified in the monotonically increasing vector v. To display a single contour line at a particular value, define v as a two-element vector with both elements equal to the desired contour level. For example, to draw contour lines at level k, use contour(Z,[k k]). Specifying the vector v sets the LevelListMode property to manual.
e.g. to get a single contour at "3", you need to do it this way instead:
figure
[c,h]=contour(X,Y,Z,[3,3]);
clabel(c,h)
The fourth argument of contour can be two things.
If it is an array of numbers (more than 1) then its the contour value you want to show. Else, if its a single value, its the amount of contour lines you want to show.
Example:
x=linspace(-10,10);
y=linspace(-10,10);
[X,Y]=meshgrid(x,y);
Z=X.^3-Y.^3;
figure
subplot(121)
[c,h]=contour(X,Y,Z,[10]);
clabel(c,h)
subplot(122)
[c,h]=contour(X,Y,Z,[1000 -1000 50 -70 3 0]);
clabel(c,h)
Related
I am trying to make a simple plot (for this example doing a plot of y=x^2 will suffice) where I want to set the colors of the points based on their magnitude given some colormap.
Following along my simple example say I had:
x = 1:10;
y = x.^2;
Use gscatter(x,y,jet(10)); legend hide; colorbar which produces a plot with the points colored but the colorbar does not agree with the colored values. (Can't post picture as this is my first post). Using a caxis([1,100]) command gives the right range but the colors are still off.
So I have two questions:
(1) How can I fix the colors to fit to a colorbar given a range? In my real data, I am looking at values that range from -50 to 50 in some instances and have many more data points.
(2) I want to create a different plot with the same points (but on different axes) and I want the colors of each point on this new plot to have the same colors as their counterparts in the previous plot. How can I, programmatically, extract the color from each point so I can plot it on two different sets of axes?
I would just move the points into a matrix and do an imagesc() command but they aren't spaced as integers or equally so simple scaling wouldn't work either.
Thanks for any help!
Regarding you first question, you need to interpolate the y values into a linear index to the colormap. Something like:
x = 1:10;
y = x.^4;
csize = 128;
cmap = jet(csize);
ind = interp1(linspace(min(y),max(y),csize),1:csize,y,'nearest');
scatter(x,y,14,cmap(ind,:),'filled')
colorbar
caxis([min(y) max(y)])
Using interp1 in this case is an overkill; you could calculate it directly. However, I think in this way it is clearer.
I think it also answers your 2nd question, since you have the index of the color of each data point, so you can use it again in the same way.
I'm currently producing a contour plot using
contour(x,y,z)
However, I would like to specify some additional contour lines to the ones provided.
I understand that I can use contour(x,y,z,v) where v is some vector containing values of the contour levels I would like but I don't really want to use this since I don't know exactly the levels.
Instead is it possible to plot the contour that goes through a specific point (x,y)?
Thanks.
You can overplot a second contour with a single, specific value for the contour, optionally specifying parameters like line width to make it obvious:
contour(x,y,z)
hold on
lev = z(n,m); % find the value you want in z
contour(x,y,z,lev,'Linewidth',2);
I am looking for help for my particular problem.
I have a contour plot created from XYZ data. This plot contains 2 broad peaks with one more intense than the other.
When the most intense peak is aligned with the Y axis, I can perform a fitting of every YZ curve at each X values. I usually do a gaussian fit to plot the peak center on the same graph.
In some cases I need to perform the same fitting but no along the Y axis direction (in this case I just plot YZ scan at every different X values) but along another arbitrary direction.
For the moment the only way I found is the following:
-plot the contour plot and find for the position of the most intense peak
-if the position is not aligned with the Y axis, then rotate all the datas and plot again the contour
-perform the YZ gaussian fit for every X value
- Rotate the resulting XY position to go back to the original plot
-plot the XY position as a line on the original contour plot
this is quite long and requires a lot of memory. i would like ot know if there is a more elegant/faster way.
Thanks for your help
David
I take it you want to extract data from the (x,y,z) data along some arbitrary line in order to make a fit. A contour plot will show only part of the data, the full z(x,y) data can be shown with imagesc etc. Say you want the data along line defined by two points (x1,y1) -> (x2,y2). According to the eq of the line, the line y=a*x+b the slope a is (y2-y1)/(x2-x1) and b=y1-a*x1. For example, I'll select (x,y) coordinates in the following contour:
Create data and end points:
m=peaks(100);
x1=11 ; x2=97;
y1=66; y2=40;
Thus the line parameters are:
a=(y2-y1)/(x2-x1);
b=y1-a*x1;
and the line is:
x=x1:x2;
y=round(a*x+b);
select the proper (x,y) elements using linear indexing:
ind=sub2ind(size(m),y,x)
plot:
subplot(2,1,1)
contour(m,10); hold on
line([x1 x2],[y1 y2],'Color',[1 0 0]);
subplot(2,1,2)
plot(m(ind))
You can now use vec=m(ind) to fit your function.
I have made a contour plot in matlab using the inbuilt contour function. It plots a group of lines in a figure, each of which represents a contour. I would like to obtain the data points that comprise each of these contours. How can I do this?
So given a contour plot how would I get the actual underlying data points that make up the equation for each contour line. For example, if the contours ended up being straight lines and one of the contour lines went through the origin, I would like to be able to obtain data points that describe this line. e.g. [0 0.1 0.2 0.3 0.4 ; 0 0.25 0.5 0.75 1].
Thanks.
[C,h] = contour(...) returns a contour matrix, C, that contains the x, y coordinates and contour levels for contour lines derived by the low-level contourc function, and a handle, h, to a contourgroup object. The clabel function uses contour matrix C to label the contour lines. ContourMatrix is also a read-only contourgroup property that you can obtain from the returned handle.
If X or Y is irregularly spaced, contour calculates contours using a regularly spaced contour grid, and then transforms the data to X or Y.
By the way, this text was taken from Matlab documentation...
I have a matrix M, 135*191*121 double and want to plot it in 3D volume by using those 121 slices. How can I do this? (I need a grayscale image)
Regards
Check out vol3d v2 , it an update to Joe Conti's vol3d function, allowing voxel colors and alpha values to be defined explicitly. In cases where voxels can be any RGB color, use:
vol3d('CData', cdata);
where cdata is an MxNxPx3 array, with RGB color along the 4th dimension. In cases where color and alpha values are highly independent, specify an MxNxP alphamatte as follows:
vol3d('CData', cdata, 'Alpha', alpha);
if you have 3 arrays, storing (x,y,z) coordinates of every point that you need to plot, then you can use function plot3
From matlab help
PLOT3 Plot lines and points in 3-D space.
PLOT3() is a three-dimensional analogue of PLOT().
PLOT3(x,y,z), where x, y and z are three vectors of the same length,
plots a line in 3-space through the points whose coordinates are the
elements of x, y and z.
PLOT3(X,Y,Z), where X, Y and Z are three matrices of the same size,
plots several lines obtained from the columns of X, Y and Z.
Various line types, plot symbols and colors may be obtained with
PLOT3(X,Y,Z,s) where s is a 1, 2 or 3 character string made from
the characters listed under the PLOT command.
PLOT3(x1,y1,z1,s1,x2,y2,z2,s2,x3,y3,z3,s3,...) combines the plots
defined by the (x,y,z,s) fourtuples, where the x's, y's and z's are
vectors or matrices and the s's are strings.
Example: A helix:
t = 0:pi/50:10*pi;
plot3(sin(t),cos(t),t);
PLOT3 returns a column vector of handles to lineseries objects, one
handle per line. The X,Y,Z triples, or X,Y,Z,S quads, can be
followed by parameter/value pairs to specify additional
properties of the lines.
for 3d plots you may also want to look into surf function