3x3 Sobel operator and gradient features

Question

Reading a paper, I'm having difficulty understanding the algorithm described:

Given a black and white digital image of a handwriting sample, cut out a single character to analyze. Since this can be any size, the algorithm needs to take this into account (if it will be easier, we can assume the size is 2^n x 2^m).

Now, the description states given this image we will convert it to a 512-bit feature (a 512-bit hash) as follows:

1. (192 bits) computes the gradient of the image by convolving it with a 3x3 Sobel operator. The direction of the gradient at every edge is quantized to 12 directions.

2. (192 bits) The structural feature generator takes the gradient map and looks in a neighborhood for certain combinations of gradient values. (used to compute 8 distinct features that represent lines and corners in the image)

3. (128 bits) Concavity generator uses an 8-point star operator to find coarse concavities in 4 directions, holes, and lagrge-scale strokes.

The image feature maps are normalized with a 4x4 grid.

I'm for now struggling with how to take an arbitrary image, split into 16 sections, and using a 3x3 Sobel operator to come up with 12 bits for each section. (But if you have some insight into the other parts, feel free to comment :)

Answer 1

I'm struggling with the same paper by Srihari et al. (2002) for my Ph.D. thesis. I would say this text is not very specific, but the authors refer to a technical report (CEDAR-TR-01-1) for more details. This report seems not to be accessible on internet, so my suggestion is to contact the authors by e-mail and ask for this report. If something is not clear, you could ask for clarification as well.

Answer 2

Hi, i am also looking into these features and didn't understand the underlying details. i am stuck in how to "quantized to 12 directions." for gradient feature?

1.(192 bits) computes the gradient of the image by convolving it with a 3x3 Sobel operator. The direction of the gradient at every edge is quantized to 12 directions.

2.(192 bits) The structural feature generator takes the gradient map and looks in a neighborhood for certain combinations of gradient values. (used to compute 8 distinct features that represent lines and corners in the image)

3.(128 bits) Concavity generator uses an 8-point star operator to find coarse concavities in 4 directions, holes, and lagrge-scale strokes.

Can anybody help me please ?