Documentation |
Find lines in images
Use the Hough Transform block to find 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 lines in the input image. Generally, the Hough Transform block precedes the Hough Lines block which uses the output of this block to find lines in an image. You can instead use a custom algorithm to locate peaks in the Hough space matrix in order to identify potential lines.
Port | Input/Output | Supported Data Types | Supported Complex Values |
---|---|---|---|
BW | Matrix that represents a binary image | Boolean | No |
Hough | Parameter space matrix |
| No |
Theta | Vector of theta values |
| No |
Rho | Vector of rho values | Same as Theta port | No |
The Main pane of the Hough Transform dialog box appears as shown in the following figure.
Specify the spacing of the Hough transform bins along the theta-axis.
Specify the spacing of the Hough transform bins along the rho-axis.
If you select this check box, the Theta and Rho ports appear on the block. The block outputs theta and rho-axis vector values at these ports.
Specify the data type of your output signal.
The Data Types pane of the Hough Transform block dialog appears as shown in the following figure. The Data Types pane will not show fixed-point parameters when Output data type parameter is set to double or single.
Select the rounding mode for fixed-point operations.
Select the overflow mode for fixed-point operations.
Choose how to specify the word length of the values of the sine table:
When you select Binary point scaling, you can enter the word length of the sine table values, in bits.
When you select Slope and bias scaling, you can enter the word length of the sine table values, in bits.
The sine table values do not obey the Rounding mode and Overflow mode parameters; they always saturate and round to Nearest.
Choose how to specify the word length and the fraction length of the rho values:
When you select Binary point scaling, you can enter the word length and the fraction length of the rho values, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the rho values. All signals in Computer Vision System Toolbox™ blocks have a bias of 0.
. Use this parameter to specify how to designate the product output word and fraction lengths:
When you select Binary point scaling, you can enter the word length and the fraction length of the product output, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the product output. All signals in Computer Vision System Toolbox blocks have a bias of 0.
See Multiplication Data Types for illustrations depicting the use of the product output.
Use this parameter to specify how to designate this accumulator word and fraction lengths:
When you select Same as product output, these characteristics match the characteristics of the product output.
When you select Binary point scaling, you can enter the word length and the fraction length of the accumulator, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the accumulator. All signals in Computer Vision System Toolbox blocks have a bias of 0.
See Multiplication Data Types for illustrations depicting the use of the accumulator data type in this block.
Select this parameter to prevent the fixed-point tools from overriding the data types you specify on the block mask. For more information, see fxptdlg, a reference page on the Fixed-Point Tool in the Simulink^{®} documentation.
Choose how to specify the word length and fraction length of the Hough output of the block:
When you select Binary point scaling, you can enter the word length of the Hough output, in bits. The fraction length always has a value of 0.
When you select Slope and bias scaling, you can enter the word length, in bits, of the Hough output. The slope always has a value of 0. All signals in Computer Vision System Toolbox blocks have a bias of 0.
Choose how to specify the word length and fraction length of the theta output of the block:
When you select Binary point scaling, you can enter the word length and the fraction length of the theta output, in bits.
When you select Slope and bias scaling, you can enter the word length, in bits, and the slope of the theta output. All signals in Computer Vision System Toolbox blocks have a bias of 0.
The Hough Transform block implements the Standard Hough Transform (SHT). The SHT uses the parametric representation of a line:
The variable rho indicates the perpendicular distance from the origin to the line.
The variable theta indicates the angle of inclination of the normal line from the x-axis. The range of theta is $$-\frac{\pi}{2}\le \theta <+\frac{\pi}{2}$$ 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 (R,C) 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 (rho, theta) pairs found for each pixel. This process validates the point (R,C) to be on the line defined by (rho, theta). The block repeats this process for each logical true pixel in the image. The Hough block outputs the resulting accumulator matrix.
See Detect Lines in Images and Measure Angle Between Lines in the Computer Vision System Toolbox User Guide.