Main Content

Hough Transform

Find lines in images

expand all in page
  • Hough Transform

Libraries:
Computer Vision Toolbox / Transforms

Description

Use the Hough Transform block to find straight lines in an image. The block outputs the Hough space matrix and, optionally, the rho-axis and theta-axis vectors. Peak values in the matrix represent potential straight lines in the input image. You can use the output of the Hough Transform block to find straight lines in the image by using the Hough Lines block. Alternatively, you can use a custom algorithm to identify potential straight lines by locating peaks in the Hough space matrix.

Examples

Plot Hough Transform of Image

Plot Hough Transform of Image

Plot the Hough Transform of an image.

Ports

Input

expand all

Binary image, specified as a matrix of Boolean values.

Data Types: Boolean

Output

expand all

Parameter space matrix, returned as a numeric matrix. The values in the parameter space matrix are in the (rho, theta) space.

Data Types: single | double | fixed point

Theta values, returned as a numeric vector. Each element represents the rotation angle of the corresponding line, in radians.

Dependencies

To enable this port, you must select the Output theta and rho values parameter.

Data Types: single | double | fixed point

Rho values, returned as a numeric vector. Each element represents the distance of the corresponding line from the top-left point of the image.

Dependencies

To enable this port, you must select the Output theta and rho values parameter.

Data Types: single | double | fixed point

Parameters

expand all

Main

Specify the spacing of the Hough transform bins along the theta-axis. This value must be less than pi/2.

Specify the spacing of the Hough transform bins along the rho-axis.

Select this parameter to enable the Theta and Rho output ports. The block outputs the theta-axis and rho-axis vectors, respectively, at these ports.

Specify the data type of your output signal. If you select double or single, all the outputs are of the specified data type. If you select Specify via Fixed-point tab, you can specify the word length of each output in the Data Types tab.

Data Types

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

Select this parameter to prevent the fixed-point tools from overriding the data types you specify in this block. For more information, see Lock the Output Data Type Setting (Fixed-Point Designer).

Block Characteristics

Data Types

Boolean | doublea | fixed point | integer | singlea

Multidimensional Signals

no

Variable-Size Signals

yes

a This data type is supported at the output port(s).

Algorithms

The Hough Transform block implements the Standard Hough Transform (SHT). The SHT uses the parametric representation of a line:

Hough line equation

Hough line on an image grid, with a line labeled rho extending from the upper-left corner to a point on the Hough line and a curve labeled theta measuring the angle from the horizontal axis to the rho line.

The block uses the upper-left corner pixel of the image as the origin, assigning it the xy-value of (0, 0).

The rho value indicates the perpendicular distance from the origin to the Hough line.

The theta value indicates the angle of inclination of the normal line from the x-axis. The range of theta is −π2≤θ<+π2, with a step-size determined by the Theta resolution (radians) parameter. The SHT measures the angle of the line clockwise, with respect to the positive x-axis.

The Hough Transform block creates an accumulator matrix. The (rho, theta) pair represent the location of a cell in the accumulator matrix. Every valid (logical true) pixel of the input binary image represented by (row, col) produces a rho value for all theta values. The block quantizes the rho values to the nearest number in the rho vector. The rho vector depends on the size of the input image and the user-specified rho resolution. The block increments a counter (initially set to zero) in those accumulator array cells represented by the (rho, theta) pairs found for each pixel. This process validates that the point (row, col) is on the line defined by (rho, theta). The block repeats this process for each logical true pixel in the image. The Hough Transform block outputs the resulting accumulator matrix.

Extended Capabilities

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

Version History

Introduced before R2006a

See Also

| | | |

Topics