Calculate weights for image pixels based on image gradient
calculates the pixel weight for each pixel in image
W = gradientweight(
I based on the gradient magnitude at
that pixel, and returns the weight array
weight of a pixel is inversely related to the gradient values at the
pixel location. Pixels with small gradient magnitude (smooth
regions) have a large weight and pixels with large gradient
magnitude (such as on the edges) have a small weight.
This example segments an image using the Fast Marching Method based on the weights derived from the image gradient.
Read image and display it.
I = imread('coins.png'); imshow(I) title('Original Image')
Compute weights based on image gradient.
sigma = 1.5; W = gradientweight(I, sigma, 'RolloffFactor', 3, 'WeightCutoff', 0.25);
Select a seed location.
R = 70; C = 216; hold on; plot(C, R, 'r.', 'LineWidth', 1.5, 'MarkerSize',15); title('Original Image with Seed Location')
Segment the image using the weight array.
thresh = 0.1; [BW, D] = imsegfmm(W, C, R, thresh); figure, imshow(BW) title('Segmented Image') hold on; plot(C, R, 'r.', 'LineWidth', 1.5, 'MarkerSize',15);
Geodesic distance matrix
D can be thresholded using different thresholds to get different segmentation results.
figure, imshow(D) title('Geodesic Distances') hold on; plot(C, R, 'r.', 'LineWidth', 1.5, 'MarkerSize',15);
I— Grayscale image
Grayscale image, specified as a numeric matrix.
sigma— Standard deviation for derivative of Gaussian
1.5(default) | positive number
Standard deviation for derivative of Gaussian, specified as a positive number.
comma-separated pairs of
the argument name and
Value is the corresponding value.
Name must appear inside quotes. You can specify several name and value
pair arguments in any order as
W = gradientweight(I,1.5,'RolloffFactor',3,'WeightCutoff',0.25);
'RolloffFactor'— Output weight roll-off factor
3(default) | positive scalar
Output weight roll-off factor, specified as the comma-separated
pair consisting of
'RolloffFactor' and a positive
scalar of class
double. Controls how fast weight
values fall as a function of gradient magnitude. When viewed as a
2-D plot, pixel intensity values might vary gradually at the edges
of regions, creating a gentle slope. In your segmented image, you
might want the edge to be more well-defined. Using the roll-off factor,
you control the slope of the weight value curve at points where intensity
values start to change. If you specify a high value, the output weight
values fall off sharply around the edges of smooth regions. If you
specify a low value, the output weight has a more gradual fall-off
around the edges. The suggested range for this parameter is
'WeightCutoff'— Threshold for weight values
0.25(default) | positive number in the range
Threshold for weight values, specified as the comma-separated pair consisting of
'WeightCutoff' and a positive
number in the range
If you use this parameter to set a threshold on
weight values, it suppresses any weight values
less than the value you specify, setting these
pixels to a small constant value (1e-3). This
parameter can be useful in improving the accuracy
of the output when you use the output weight array
W as input to Fast Marching
Method segmentation function,
W— Weight array
Weight array, returned as a numeric array of the same size as the input image,
I. The weight array is of class
in which case it is of class
gradientweight uses double-precision
floating point operations for internal computations for all classes
I, except when
I is of class
in which case
gradientweight uses single-precision
floating point operations internally.