I am trying to create a filled contour plot from an image in MATLAB. However, the command imcontour does not seem to have an option for filled contours. If I use contourf, it draws all the contour lines in black. Since the image has a lot of contour levels, it is shown almost completely in black.
Does anybody know how to make a filled imcontour or how to meaningfully apply contourf on an image?
There is no filled version of imcontour because in theory, the image itself is the filled version.
data = load('clown');
img = ind2rgb(data.X, data.map);
imshow(img);
hold on
imcontour(img(:,:,1), 3);
You could use contourf though, and specify the line color. By specifying a value of 'none' no lines will be shown.
c = contourf(data, 2, 'LineColor', 'none')
Related
The following code shows an image that is combination of blue and red colors. But if I remove the close statement it yields a grayscale image (which is what I want).
Why does it happen, so that I can avoid it in the future?
I am following this tutorial on youtube.
clear;
animal1=imread('animal1.jpg');
%% GrayScale
animal2=rgb2gray(animal1);
%% scale
bright=animal2*1.5;
imshow(bright);
close;
%% threshold
binary= bright>220;
imagesc(binary);
When you call the imshow function, a new figure is created, and default colormap is set to grayscale. If you do not call close, the imagesc uses the same colormap, and uses gray levels to show the binary image.
Otherwise, the current figure is destroyed, a new one is created, and the imagesc function defines a new colormap. The default colormap in this case is parula, which shades from blue to yellow.
Note that you can display the binary image by using the imshow function directly.
As #dlegland has pointed out, it's an issue with colormaps.
In MATLAB a colormap defines the way that data (in your image, plot, whatever) is mapped to a color on the screen. This is done via a linear mapping which can be different for each axes.
When you call imshow, it is a relatively high-level function which alters a number of properties of the axes on which is it displayed. This includes the colormap, color limits, and other things like the tick marks. In your case, since you fed it a grayscale image (you created using rgb2gray), it set the figure/axes to use the gray colormap.
imagesc, however, is a lower-level function that doesn't make any changes to the current axes with the exception that it alters the color limits to span the entire dynamic range of the image. Because of this, when you use imagesc to plot an image on an axes that was previously used by imshow. It simply uses the colormap that imshow was using (gray).
If imshow hadn't been called, then the figure would be using the default colormap (typically parula) and your image would be displayed using this colormap.
Now the nice thing is that you can change the colormap that is being used with the colormap command. For example to use grayscale, you would do
colormap gray
Or if you wanted to specify that colormap for only a specific axes you could do the following
ax = axes();
colormap(ax, gray)
Your only options aren't gray or parula. MATLAB has a number of built-in colormaps or you can even specify your own custom colormap.
I have written a program to process and print a 81x81 2D surface plot, which needs to be used as an input. Saving the plot made by Matlab includes side axes as well as white margins on the sides.
How do I crop this image to get just the (81pixels)x(81pixels) output as an image?
Try placing this after your figure code, it will remove the margins around your figure.
set(gca,'units','pixels') % set the axes units to pixels
xx = get(gca,'position'); % get the position of the axes;
set(gcf,'units','pixels') % set the figure units to pixels
yy = get(gcf,'position'); % get the figure position;
set(gcf,'position',[yy(1) yy(2) xx(3) xx(4)]) % set the position of the figure to the length and width of the axes
set(gca,'units','normalized','position',[0 0 1 1]) % set the axes units to pixels
You can avoid using surf plot and just save your array as image. So, you have 81x81 array. I'll use this one for the example:
a = magic(81);
Now you need to normalize image in order to have values from 0 to 255:
a = (a - min(min(a)))/(max(max(a))-min(min(a)))*255;
Finally you use imwrite to save your array as image.
imwrite(a,jet(256),'SO_4.png')
The first argument is your 81x81 array. The second argument is colormap. You can specify any colormap you want, but I like the heatmap jet(256). You can skip this argument, in this case the image would be grayscale. From here you can find what colormaps are available and what they look like. The last argument is image name. The result is shown below:
Hope that helps, good luck.
I have some data I would like to display in a picture form. In one case I want to rescale the x and y axis, which leads me to using imagesc. The issue is that the same colormap (jet) looks different in imagesc as compared to image.
Is there a way to make them the same?
I am using MATLAB R2014a.
Demonstration:
Here is how I show them:
figure; image(cancelledmap); colormap(jet); %image
figure; imagesc(y,x,cancelledmap); colormap(jet); %imagesc
And the output:
The colormap settings for both figures are somehow the same, however:
imagesc scales the color axis, whereas image does not. That's why the colors look different. If you click the colorbar button you'll see that they are on different color scales.
You can change the colorscale with caxis.
By the way, if you only want to scale the X- and Y-axes, you can use either function. Just supply your own scaled x and y vectors.
I am running Isomap Dimensionality reduction in MATLAB on a series of images. I want to plot the image's thumbnail beside the point on the manifold corresponding to it.
I am currently using 2 differnt isomaps http://isomap.stanford.edu/ and http://robotics.cs.brown.edu/projects/stisomap/ .
The imagesc function can take arguments which dictate where the image is drawn, so I would use this. Here's an example of imagesc being drawn on top of a plot:
% Draw plot
vals=rand(2,100);
plot(vals(1,:),vals(2,:),'x');
hold on;
% Draw image
im=imread('moon.tif');
xs=linspace(0.1, 0.2, size(im, 2) );
ys=linspace(0.1, 0.2, size(im, 1) );
colormap gray;
imagesc(xs,ys,im)
Which looks like this:
Note the first two arguments to imagesc which define the range over which the image is drawn. Obviously you'll want to change the arguments to linspace, which will define the position and size of the image, and you'll need to take account for the aspect ratio if the image isn't square, but hopefully this will get you along the right lines.
I have a similar question than the one in this post.
I have a grayscale image and I plot points on it. Fro plotting the points I use colormap('jet') but as I want the image to be grayscale, after plotting the points I reset the colormap, colormap('gray').
But I want to show a colorbar! And the colorbar is plotted in grayscale, not 'jet'.
How can I do that?
EDIT:
I want a Colorbar showing the color of the points!
You should convert your image to RGB by putting the same data into R-, G-, and B-channels (this will be grayscale RGB image). Colormap in MatLab is not applied to RGB images, only to indexed ones. Then plot your points over the image with colormap you like.
As discussed here, there's a few ways:
If you have the image processing toolbox, use subimage to create an independent image with a separate colormap. Then plot the image, your points, and join them into one using linkaxes.
Use freezeColors from the file exchange (or multiple colormaps, which I haven't ever tested personally). This is a very easy way to create a larger colormap, and automatically selecting the right portion of the colormap for display of images and colorbars.
As answered by anandr, convert your greyscale image to RGB; Matlab doesn't use colormaps on RGB images, which leaves you freedom to plot your points and show their colorbar independent of the image.
Example code for (3):
I = imread('cameraman.tif');
imshow(cat(3,I,I,I))
hold on
x = #() round(size(I,1) * rand(50,1));
y = #() round(size(I,2) * rand(50,1));
plot(x(), y(), 'r.')
plot(x(), y(), 'g.')
plot(x(), y(), 'b.')
colormap('jet')
colorbar
result: