Talk:Sobel operator

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[edit] "Solving"

I'm a university AI student who's done a module in Computational Vision, and still I don't understand the first line: "a discrete differentiation operator solving for the 1st derivatives". What does the "solving" mean?

Well, I think the "solving" is in the algebraic sense, as in "Solve the following equation for x" which basically means "Find the value of x", but I agree that that was a horrible way to start the article, useful only to seasoned mathematicians. I've tried to add a more "general audience" explanation of what the operator actually does. - IMSoP 22:30, 16 December 2005 (UTC)

[edit] Technical Details - Matrix maths wrong way round

Surely the matix maths in the Technical Details section is the wrong way round. The result shown would make Gx and Gy = 0. It should be a 3x1 column * 1x3 row = 3x3 matix.

[edit] Technical Details - Convolution operator get confuse with the times operator

First we get a squared matrix with a 3x1 matrix multiplied by a 1x3 row (so the times operator \times)

\quad \quad \begin{bmatrix} +1 & +2 & +1 \\ 0 & 0 & 0 \\ -1 & -2 & -1 \end{bmatrix} = \begin{bmatrix} +1 \\ 0 \\ -1 \end{bmatrix} \times \begin{bmatrix} 1 & 2 & 1 \end{bmatrix}

Then we convolve the squared matrix with the input image to get another image, in this case the Gy derivative (and the convolution operator * is used):

\mathbf{G}_y =\quad \begin{bmatrix} +1 & +2 & +1 \\ 0 & 0 & 0 \\ -1 & -2 & -1 \end{bmatrix} * \mathbf{A} = \begin{bmatrix} +1 \\ 0 \\ -1 \end{bmatrix} \times \begin{bmatrix} 1 & 2 & 1 \end{bmatrix} * \mathbf{A}

And, because they are separable in both x and y directions, we can write it as:

\mathbf{G}_y =\left(\begin{bmatrix} +1 \\ 0 \\ -1 \end{bmatrix} * \mathbf{A} \right) * \begin{bmatrix} 1 & 2 & 1 \end{bmatrix}


[edit] Simple Description

"giving the direction of the largest possible increase from light to dark"

I think "light to dark" is backwards, since usually dark is 0 and light is 1. And in fact, isn't it redundant since you say the largest possible "increase" ?

Ian Stewart