find area for ROI

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Max Bernstein
Max Bernstein 2011-10-20
Hello,
I dont have the image processing toolbox and I'm trying to find an area of an image. I have been able to reduced it to bw double array but I'm having trouble trying to calculate the area. Should I calculate the area for 1 pixel and multiply it by the number of black pixels?

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回答（2 个）

Walter Roberson
Walter Roberson 2011-10-20
ROI usually use white (1) for a selected pixel, and black (0) for one that is not selected.
ROIarea = sum(sum(YouImage.*ROI))
Or simply regionprops(ROI) and look at the 'Area' field.
Yes you would want to multiply by area of a pixel. But unless you have some kind of absolute measurement, or you know your pixels are non-square, a pixel would normally be assumed to have an area of 1.
Image Analyst
Image Analyst 2011-10-21
Hmm... Usually BW in the MATLAB examples is a logical image (true or false), not a double. Anyway I'd do it like Walter said because I prefer pixel-based measurements. However just to let you know there is an alternate definition of area that bwarea uses. Run this code to see:
m = [0 1; 1 1]
numberOfPixels = sum(m(:))
bwAreaMeasurement = bwarea(m)
x = [2 2 1];
y = [1 2 2];
poly_area = polyarea(x, y)
m =
0 1
1 1
numberOfPixels =
3
bwAreaMeasurement =
3.1250
poly_area =
0.5000
Tell me, what do you think the area is? is it 3 pixels? Or is it 0.5, which it is if you go from center of pixel to center of pixel and along the diagonal? Or is it 3.1250? Is there a right answer, or is there no right answer, or multiple answers?
regionprops() returns the number of pixels, 3 in this example.
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Image Analyst
Image Analyst 2011-10-21
By the way, by your definition this array
0 0 0 0 0
0 1 1 1 0
0 0 0 0 0
would give the object an area of zero.
Walter Roberson
Walter Roberson 2011-10-21
@IA:
I think you missed the "and connect to the corner for ending pixels". It sounds like Max would want it to be measured as if
|>----<|
where the > and > should have 45 degree angles and each "-" is a half-width. That would, I figure, give a total area of sqrt(2)
"Connect to the corners" is, though, ambiguous since it doesn't really say which point to start the connection from.
@Max:
Your algorithm confuses distance with area. For example it leaves confused a case such as looking near the diagonal on
0 1
1 ?
as it does not take in to account whether the ? is occupied or not in determining an area contribution.

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