How to plot some points on an image?

Hi all,

I have an image, and I have the coordinates of a few points which I'd like to draw on the image. When using the hold on & plot() functions, the coordinates of the image is from the bottom left corner of the window! not the corner of the figure! Someone help me please!

Thanks,

[Geoff Hayes]

Hamed - can't you just apply an offset (to the y-coordinate) which is the number of rows of the image? For example, if your y-coordinate is 50 and the number of rows in your image is 500, then the transformed y-coordinate would be 500-50=450.

Answer 1

[Image Analyst]

You'd have to show us your code. The origin for both the image array and the plotting coordinates is at the upper left, not the bottom left. Run this code:

imshow('cameraman.tif');
axis on
hold on;
% Plot cross at row 100, column 50
plot(50,100, 'r+', 'MarkerSize', 30, 'LineWidth', 2);

Are you sure you're not making the common beginner mistake of mixing up the order of row,column and x,y - remember (x,y) is (column,row) not (row,column).

Answer 2

[KSSV]

First you have to read the image using imread. Then you have to fix the axes of the image. Later you can plot your points using hold on and plot.

data = imread('yourimage.ext') ;

image([xmin xmax], [ymin ymax],data);

Answer 3

[MathWorks Computer Vision Toolbox Team]

As demonstrated in the two previous answers, fixing the axes of the image first and then using hold on and plot function should work correctly. Simply calling axis on before hold on, as in the first answer, should fix the axes.