regionprops calculation

hi sir,this is my result of measuring bounding box,area and perimeter Bounding Box are: 30.50 30.50 422.00 412.00 Centroid are: 240.47 236.34 Area: 133907.00 Perimeter: 1567.80

As far as Im concerned, 422 is width,412 is height of bounding.then if I times for example 422*412 to find the area. It shud be 173864. then why the area above just only 133907? same to perimeter. actually to find perimeter, 2*width+2*height=1668. then why it's 1567 only!!!

Answer 1

[Walter Roberson]

Bounding Box gives the rectangle that includes all pixels. Area does not give the area of the rectangle, it gives the area of the item within the rectangle.

[Tulips]

actually in regionprops, the area/perimeter value will be cover the area/perimeter of bounding box or not?pls advie.thx

[Walter Roberson]

Not.

[Tulips]

it means that my answer was correct!!thanks sir!

[Walter Roberson]

I am not sure which part was your answer. Your point starting "As far as Im concerned" about what the area "should be", disagrees with the definition of "Area" used in MATLAB.

Answer 2

[Image Analyst]

40 thousand pixels - that's a big difference. Sounds like your area doesn't take up the full area of the box, like a circle wouldn't take up the full area of its bounding box.

[Tulips]

that's why I asked . my result above is correct or not..?Is it true that the area does not give the area of rectangle,?just like mr walter roberson said?

[Walter Roberson]

You can read the documentation:

http://www.mathworks.com/help/toolbox/images/ref/regionprops.html#bqkf8hc

'Area' — Scalar; the actual number of pixels in the region.

'BoundingBox' — The smallest rectangle containing the region

You might also be interested in FilledArea

[Tulips]

so my result is correct.becoz it's clearly said that area just cover the number of pixel in region not the bounding box area. thanks guys!

[Image Analyst]

Your result for the bounding box was correct. Your and regionprops()'s answers for the area are also correct because the blob you are getting the area of is smaller than the bounding box of the blob. Like I said, the area of a circle is smaller than the area of its bounding box. So everything agrees, as long as you understand what's going on.