Question

I have image which size was 600 * 600 and it was displayed on 800 * 800 pixel screen.
The x,y coordinate in which the user look on screen was recorded in an array:

x =[250,300,390,750,760];
y =[120,550,250,130,420]; 

In other program, I want to plot the x,y coordinate on the 600 * 600 image. The problem is that some of the x,y plot were out of the image (as shown on the picture below) since the coordinate was more that the maximum size of the image (600 * 600).

EDITED:
How to transform/adjust the coordinate of the bigger image (800*800) into the smaller image (600*600) so all x,y coordinate are inside the smaller image (600*600)?

Lets say for example, the coordinate of top left image of the 600*600 inside the image of the 800*800 image is e.g. x = -10, y = 3.

Thanks.

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alt text http://img9.imageshack.us/img9/8836/e47184420f.jpg

Answer 1

To get the pixels in image coordinates, you need to know where the bottom left and top right corners of your image were placed on the screen. From that you can both calculate offset and zoom of the image.

%# define some parameters
imageSize = [600 600];
topLeftPixScreen = [200,100]; %# position of the top left image corner in screen pixels
bottomRightPixScreen = [800,750]; %# position of the bottom right image corner in screen pixels

%# transform coordinates
oldX =[250,300,390,750];
oldY =[120,550,250,130,420];

newX = (oldX - topLeftPixScreen(1))/(bottomRightPixScreen(1) - topLeftPixScreen(1) + 1);
newY = (oldY - topLeftPixScreen(2))/(bottomRightPixScreen(2) - topLeftPixScreen(2) + 1);

Having said that, I’d suggest using ginput to select the points with Matlab, since this function directly returns image pixels.

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EDIT

If you only have the top left corner, you have to hope that there has not been any scaling – otherwise, there is no way you can transform the points.

With offset only, the above simplifies to

%# define some parameters
imageSize = [600 600];
topLeftPixScreen = [200,100]; %# position of the top left image corner in screen pixels

%# transform coordinates
oldX =[250,300,390,750];
oldY =[120,550,250,130,420];

newX = oldX - topLeftPixScreen(1);
newY = oldY - topLeftPixScreen(2);