Edge Detection

Find edges of objects in images using Sobel, Prewitt, Roberts, or Canny method

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Libraries:
Computer Vision Toolbox / Analysis & Enhancement

Description

The Edge Detection block finds edges of objects in an input image. The block supports four methods: Sobel, Prewitt, Roberts, and Canny. The first three methods find the edges by approximating the gradient magnitude of the image. The block convolves the input matrix with the Sobel, Prewitt, or Roberts kernel respectively, and outputs the resulting gradient components of the image. Additionally, the block outputs a binary image of the edge detections by thresholding the gradient magnitude.

The Canny method computes the gradient of input image using the derivative of the Gaussian filter. It then finds edges by looking for the local maxima of the gradient. This method uses two thresholds to detect the strong and weak edges. The weak edges are part of the output only if they are connected to strong edges. The block outputs a binary image of the edge detections for this method.

Examples

Edge Detection of Intensity Image

Detect edges in an intensity image using the Canny edge detector.

• Open Model

Ports

Input

I — Input image

matrix

Input image, specified as a matrix of intensity values.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Note

The Canny method does not support 8-bit, 16-bit, or 32-bit signed fixed-point integers or 8-bit, 16-bit, or 32-bit unsigned integers.

Th — Threshold

scalar | vector

Specify a threshold as a scalar or a two-element vector to detect edges from the gradient. The data type of this value must match the input image data type. For the Canny edge detection method, the threshold must be a two-element vector in the range [0 1]. For the other methods, the threshold is a scalar.

Dependencies

To enable this port:

• Set the Method parameter to Canny. Alternatively, set the Method parameter to Sobel, Prewitt, or Roberts, and also set the Output type parameter to Binary image or Binary image and gradient components.

• Select the User-defined threshold.

• Set the Threshold source parameter to Input port.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Note

The Canny method does not support 8-bit, 16-bit, or 32-bit signed fixed-point integers or 8-bit, 16-bit, or 32-bit unsigned integers.

Output

Edge — Edge detections

matrix

Edge detections, returned as a binary image. The pixels with the value of 1 represent the edge pixels.

Dependencies

This port is always enabled when you set the Method parameter to Canny. To enable this port for the Sobel, Prewitt, or Roberts methods, set the Output type parameter to Binary image or Binary image and gradient components.

Data Types: Boolean

Gv — Gradient responses to vertical edges

matrix

Matrix of gradient responses to the vertical edges.

Dependencies

To enable this port, set these parameters:

• Method — Sobel or Prewitt

• Output type — Gradient components or Binary image and gradient components

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Gh — Gradient responses to horizontal edges

matrix

Matrix of gradient responses to the horizontal edges.

Dependencies

To enable this port, set these parameters:

• Method — Sobel or Prewitt

• Output type — Gradient components or Binary image and gradient components

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

G45 — Gradient responses to edges at 45 degrees

matrix

Matrix of gradient responses to edges at 45 degrees.

Dependencies

To enable this port, set these parameters:

• Method — Roberts

• Output type — Gradient components or Binary image and gradient components

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

G135 — Gradient responses to edges at 135 degrees

matrix

Matrix of gradient responses to edges at 135 degrees.

Dependencies

To enable this port, set these parameters:

• Method — Roberts

• Output type — Gradient components or Binary image and gradient components

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point

Parameters

Main

Method — Edge detection method

Sobel (default) | Prewitt | Roberts | Canny

Specify a method to detect edges in the input image. You can choose from Sobel, Prewitt, Roberts or Canny.

Output Type — Output image type

Binary image (default) | Gradient components | Binary image and gradient components

Specify the type of output image from one of these options:

• Binary image — The block outputs a Boolean matrix at the Edge port. The elements of the matrix labeled 1 represent the edge pixels.

• Gradient components — The block outputs the gradient components. For the Sobel and Prewitt methods, the gradient components correspond to the horizontal and vertical edge responses at the Gh and Gv ports respectively. For the Roberts method the gradient components correspond to 45 degree and 135 degree edge responses at the G45 and G135 ports respectively.

• Binary image and gradient components — The block outputs both the binary image and the gradient components of the image

Dependencies

To enable this parameter, set the Method parameter to Sobel, Prewitt, or Roberts.

User-defined threshold — User-defined threshold

off (default) | on

Specify a desired threshold value via dialog or the Th port.

Dependencies

This parameter is always enabled when you set the Method parameter to Canny. To enable this parameter for the Sobel, Prewitt, or Roberts methods, set the Output type parameter to Binary image or Binary image and gradient components.

Threshold source — Threshold source

Input port (default) | Specify via dialog

Select the source to enter the threshold value from one of these options:

• Input port — Specify a threshold value using the Th port. The data type of this value must match the input image data type.

• Specify via dialog — Enter the desired threshold value in the dialog box.

Dependencies

To enable this parameter:

• Set the Method parameter to Canny. Alternatively set the Method parameter to Sobel, Prewitt, or Roberts, and also set the Output type parameter to Binary image or Binary image and gradient components.

• Select the User-defined threshold.

Threshold — Threshold

20 (default) | scalar

Specify the threshold value that is within the range of your input data. This threshold applies to Sobel, Prewitt, and Roberts edge detection methods.

Dependencies

To enable this parameter, set these block parameters:

• Method — Sobel, Prewitt, or Roberts

• Output type — Binary image or Binary image and gradient components

• User-defined threshold — On.

• Threshold source — Specify via dialog

Threshold, [Low High] — Threshold for Canny edge detection

[0.25 0.60] (default) | vector

Specify the threshold for the Canny edge detection as a two-element vector in the range [0 1]. The elements of the vector specifies the low and high threshold values for the weak and strong edges respectively.

Dependencies

To enable this parameter, set these block parameters:

• Method — Canny

• User-defined threshold — On.

• Threshold source — Specify via dialog

Threshold scale factor:(used to automatically calculate threshold value) — Threshold scale factor

4 (default) | scalar

Specify the threshold scale factor to automatically calculate threshold value. The block computes the automatic threshold using the mean of the gradient magnitude squared image and multiplies the value of this parameter with the automatic threshold value to determine a new threshold value.

Dependencies

To enable this parameter, set these block parameters:

• Method — Sobel, Prewitt, or Roberts

• Output type — Binary image or Binary image and gradient components

• User-defined threshold — Off

Edge Thinning — Edge thinning

off (default) | on

Select this parameter to reduce the thickness of the edges in your output images. This option requires additional processing time and memory resources.

Tip

This block is most efficient in terms of memory usage and processing time when you clear the Edge thinning check box and use the Threshold parameter to specify a threshold value.

Dependencies

To enable this parameter, set these block parameters:

• Method — Sobel, Prewitt, or Roberts

• Output type — Binary image or Binary image and gradient components

Approximate percentage of weak edge and nonedge pixels (used to automatically calculate threshold values) — Approximate percentage of weak edge and nonedge pixels

70 (default) | scalar

Specify the approximate percentage of weak edge and nonedge image pixels. The block computes the automatic threshold values using this value.

Dependencies

To enable this parameter, set these block parameters:

• Method — Canny

• User-defined threshold — Off

Standard deviation of Gaussian filter — Standard deviation of Gaussian filter

1 (default) | scalar

Specify the standard deviation of Gaussian filter used for Canny edge detection algorithm.

Dependencies

To enable this parameter, set the Method parameter to Canny.

Data Types

For information about the fixed-point block parameters, see Specify Fixed-Point Attributes for Blocks (DSP System Toolbox).

Block Characteristics

Data Types	double | fixed point | integer | single
Multidimensional Signals	no
Variable-Size Signals	yes

References

[1] Gonzales, Rafael C. and Richard E. Woods. Digital Image Processing. 2nd ed. Englewood Cliffs, NJ: Prentice Hall, 2002.

[2] Pratt, William K. Digital Image Processing, 2nd ed. NY: John Wiley & Sons, 1991.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Fixed-Point Conversion

Design and simulate fixed-point systems using Fixed-Point Designer™.

When you select the Normalized output check box, the block input cannot be fixed point.

Version History

Introduced before R2006a