Converting a rectangle position on an image to image matrix index

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Itzik Ben Shabat
Itzik Ben Shabat 2012-10-31
Hi, I have drawn an greyscale image from a matrix using imshow. I used Imrect to draw a rectangle on a region of the image. from Imrect i get the xmin, ymin hight and width of the rectangle. but what i really need is to know which pixels it contains e.g the image matrix indexes. how do i do this conversion? thanks

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Matt J
Matt J 2012-10-31
编辑：Matt J 2012-10-31
If H is the handle returned by IMRECT, then you can use its createMask method to get a logical index mask of the region,
Indices=H.createMask;
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Itzik Ben Shabat
Itzik Ben Shabat 2012-10-31
great! did this and its almost what i wanted. i wanted the indexes (rows and columns) if the selected area. is there a simple way to convert the index mask to the (row,column) format? thanks a lot, this helped!
Matt J
Matt J 2012-10-31
编辑：Matt J 2012-10-31
Yes. You use the FIND function
[I,J]=find(mask);
I hope you realize, however, that this conversion is not necessary for indexing the image. You could just use logical indexing:
values = Image(mask);

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Image Analyst
Image Analyst 2012-10-31
Here's a snippet from my code:
hBox = imrect;
roiPosition = wait(hBox);
roiPosition
% Erase all previous lines.
ClearLinesFromAxes(handles);
xCoords = [roiPosition(1), roiPosition(1)+roiPosition(3), roiPosition(1)+roiPosition(3), roiPosition(1), roiPosition(1)];
yCoords = [roiPosition(2), roiPosition(2), roiPosition(2)+roiPosition(4), roiPosition(2)+roiPosition(4), roiPosition(2)];
% Plot the mask as an outline over the image.
hold on;
plot(xCoords, yCoords, 'linewidth', 2);
Alternatively, if you want, you can use rbbox instead of imrect. Here's a snippet showing how to do that:
k = waitforbuttonpress;
point1 = get(gca,'CurrentPoint'); % button down detected
finalRect = rbbox; % return figure units
point2 = get(gca,'CurrentPoint'); % button up detected
point1 = point1(1,1:2); % extract x and y
point2 = point2(1,1:2);
p1 = min(point1,point2); % calculate locations
offset = abs(point1-point2); % and dimensions
xCoords = [p1(1) p1(1)+offset(1) p1(1)+offset(1) p1(1) p1(1)];
yCoords = [p1(2) p1(2) p1(2)+offset(2) p1(2)+offset(2) p1(2)];
x1 = round(xCoords(1));
x2 = round(xCoords(2));
y1 = round(yCoords(5));
y2 = round(yCoords(3));
hold on
axis manual
plot(xCoords, yCoords); % redraw in dataspace units
croppedImage = imgOriginal(y1:y2,x1:x2,:);
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Matt J
Matt J 2012-11-1
So I don't really understand what you were wondering about - I guess it never applies to me.
I just meant that, for example, when you use get(gca,'CurrentPoint') to obtain coordinates of the box, the output will depend on whether the axes is in matrix mode (axis ij) or xy mode (axis xy). Since the task of the OP is to obtain the pixel coordinates inside the box, you have to make sure the former is true. Conversely, imrect.createMask seems to take care of that for you innately.
Image Analyst
Image Analyst 2012-11-1
Well since I always have loaded an image in first, before calling plot() to plot the box, I guess it's always in pixel mode for me.

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