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Gradient vector flow (GVF), a computer vision framework introduced by Chenyang Xu and Jerry L. Prince, [1] [2] is the vector field that is produced by a process that smooths and diffuses an input vector field. It is usually used to create a vector field from images that points to object edges from a distance.
Normalized y-gradient from Sobel–Feldman operator The images below illustrate the change in the direction of the gradient on a grayscale circle. When the sign of G x {\displaystyle \mathbf {G_{x}} } and G y {\displaystyle \mathbf {G_{y}} } are the same the gradient's angle is positive, and negative when different.
A line is then sampled at unit intervals in one coordinate and corresponding integer values nearest the line path are determined for the other coordinate. Considering a line with positive slope, if the slope is less than or equal to 1, we sample at unit x intervals (dx=1) and compute successive y values as + = +
Here is an example gradient method that uses a line search in step 5: Set iteration counter k = 0 {\displaystyle k=0} and make an initial guess x 0 {\displaystyle \mathbf {x} _{0}} for the minimum.
For a given iterated function :, the plot consists of a diagonal (=) line and a curve representing = (). To plot the behaviour of a value x 0 {\displaystyle x_{0}} , apply the following steps. Find the point on the function curve with an x-coordinate of x 0 {\displaystyle x_{0}} .
is the derivative with respect to y (gradient in the y direction). The derivative of an image can be approximated by finite differences . If central difference is used, to calculate ∂ f ∂ y {\displaystyle \textstyle {\frac {\partial f}{\partial y}}} we can apply a 1-dimensional filter to the image A {\displaystyle \mathbf {A} } by convolution :
Bresenham's algorithm chooses the integer y corresponding to the pixel center that is closest to the ideal (fractional) y for the same x; on successive columns y can remain the same or increase by 1. The general equation of the line through the endpoints is given by:
Slope illustrated for y = (3/2)x − 1.Click on to enlarge Slope of a line in coordinates system, from f(x) = −12x + 2 to f(x) = 12x + 2. The slope of a line in the plane containing the x and y axes is generally represented by the letter m, [5] and is defined as the change in the y coordinate divided by the corresponding change in the x coordinate, between two distinct points on the line.