imgradientxy

Directional gradients of an image

Syntax

[Gx,Gy] = imgradientxy(I)
[Gx,Gy] = imgradientxy(I,method)

Description

example

[Gx,Gy] = imgradientxy(I) returns the directional gradients, Gx and Gy of the grayscale or binary image I.

You optionally can compute the directional gradients using a GPU (requires Parallel Computing Toolbox™). For more information, see Image Processing on a GPU.

example

[Gx,Gy] = imgradientxy(I,method) returns the directional gradients using the specified method.

Examples

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Read an image into workspace.

I = imread('coins.png');

Calculate the x- and y-directional gradients using the Prewitt gradient operator.

[Gx, Gy] = imgradientxy(I,'prewitt');

Display the directional gradients.

figure
imshowpair(Gx, Gy, 'montage');
title('Directional Gradients: x-direction, Gx (left), y-direction, Gy (right), using Prewitt method')

Read an image into workspace.

I = imread('coins.png');

Calculate the x- and y-directional gradients. By default, imgradientxy uses the Sobel gradient operator.

[Gx,Gy] = imgradientxy(I);

Display the directional gradients.

imshowpair(Gx,Gy,'montage')
title('Directional Gradients Gx and Gy, Using Sobel Method')

Calculate the gradient magnitude and direction using the directional gradients.

[Gmag,Gdir] = imgradient(Gx,Gy);

Display the gradient magnitude and direction.

imshowpair(Gmag,Gdir,'montage')
title('Gradient Magnitude (Left) and Gradient Direction (Right)')

Input Arguments

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Input image, specified as a 2-D grayscale image or 2-D binary image.

Data Types: single | double | int8 | int32 | uint8 | uint16 | uint32 | logical

Gradient operator, specified as one of the following values.

MethodDescription
'sobel'

Sobel gradient operator. The gradient of a pixel is a weighted sum of pixels in the 3-by-3 neighborhood. In the vertical (y) direction, the weights are:

[ 1  2  1 
  0  0  0 
 -1 -2 -1 ]
In the x direction, the weights are transposed.

'prewitt'

Prewitt gradient operator. The gradient of a pixel is a weighted sum of pixels in the 3-by-3 neighborhood. In the vertical (y) direction, the weights are:

[ 1  1  1 
  0  0  0 
 -1 -1 -1 ]
In the x direction, the weights are transposed.

'central'

Central difference gradient. The gradient of a pixel is a weighted difference of neighboring pixels. In the y direction, dI/dy = (I(y+1) - I(y-1))/2.

'intermediate'

Intermediate difference gradient. The gradient of a pixel is the difference between an adjacent pixel and the current pixel. In the y direction, dI/dy = I(y+1) - I(y).

Data Types: char | string

Output Arguments

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Horizontal gradient, returned as a numeric matrix of the same size as image I. The horizontal (x) axis points in the direction of increasing column subscripts. Gx is of class double, unless the input image I is of class single, in which case Gx is of class single.

Data Types: single | double

Vertical gradient, returned as a numeric matrix of the same size as image I. The vertical (y) axis points in the direction of increasing row subscripts. Gy is of class double, unless the input image I is of class single, in which case Gy is of class single.

Data Types: single | double

Tips

  • When applying the gradient operator at the boundaries of the image, values outside the bounds of the image are assumed to equal the nearest image border value.

Algorithms

The algorithmic approach is to compute directional gradients with respect to the x-axis and y-axis. The x-axis is defined along the columns going right and the y-axis is defined along the rows going down.

imgradientxy does not normalize the gradient output. If the range of the gradient output image has to match the range of the input image, consider normalizing the gradient image, depending on the method argument used. For example, with a Sobel kernel, the normalization factor is 1/8, and for Prewitt, it is 1/6.

Extended Capabilities

Introduced in R2012b