Blobs

Question 1

What is a blob?

Answer 1

A set of pixels that

• share some property
• are connected

Question 2

What is 4 connectedness?

Answer 2

When we consider the 4 pixels directly above, below, to the left and right of a pixel to be connected.

Question 3

What is 8 connectedness?

Answer 3

When we consider the 8 pixels surrounding a single pixel to be connected.

Question 4

How do we work out the center of gravity of a blob?

Answer 4

Take the average of all the positions of the pixels in the blob, weighted by their intensity.

Question 5

Are two chain codes that are produced from starting at two different pixels equal?

Answer 5

Yes the two chain codes are the same, as one can be rotated to match the other.

Question 6

How can we make sure we can generate the same chain code regardless of rotation?

Answer 6

We can use differential chain codes, where we set the last direction taken as being the zero direction, and compute the next direction in the chain code relative to this.

Question 7

How can we calculate a differential chain code from an ordinary chain code?

Answer 7

Let the ordinary chain code be:

c1, c2, c3 … cm

dn = cn cn-1 (mod 8)

Question 8

How can we calculate the perimeter for an ordinary chain code?

Answer 8

Add 1 for each even code, and add sqrt(2) for every odd code

e.g. 1,1,1,2,3,4,5,6,2

= 3(4) + 3(sqrt(5))