Video and Image Processing Blockset™

Hough Transform - Find lines in images

Library

Transforms

Description

Use the Hough Transform block to find lines in an image. The block maps points in the Cartesian image space to curves in the Hough parameter space using the following equation:

The block outputs a parameter space matrix whose rows and columns correspond to the rho and theta values, respectively. Peak values in this matrix represent potential lines in the input image. The theta values range from -pi/2 radians to pi/2 radians with a step-size determined by the Theta resolution (radians) parameter.

Port	Input/Output	Supported Data Types	Complex Values Supported
BW	Matrix that represents a binary image	Boolean	No
Hough	Parameter space matrix	Double-precision floating point Single-precision floating point Fixed point (unsigned, fraction length equal to 0) 8-, 16-, 32-bit unsigned integer	No
Theta	Vector of theta values	Double-precision floating point Single-precision floating point Fixed point (signed) 8-, 16-, 32-bit signed integer	No
Rho	Vector of rho values	Same as Theta port	No

If the output of the Hough port is floating point, the outputs of the Theta and Rho ports are the same data type.

Use the Theta resolution (radians) parameter to specify the spacing of the Hough transform bins along the theta-axis.

Use the Rho resolution parameter to specify the spacing of the Hough transform bins along the rho-axis.

If you select the Output theta and rho values check box, the Theta and Rho ports appear on the block. The block outputs vectors of theta and rho values at these ports.

Use the Output data type parameter to specify the data type of your output signal.

Examples

See Finding Lines in Images and Measuring an Angle Between Lines in the Video and Image Processing Blockset™ User's Guide.

Fixed-Point Data Types

The following diagram shows the data types used in the Hough Transform block for fixed-point signals.

You can set the sine table, rho, product output, accumulator, Hough output, and Theta output data types in the block mask as discussed in the next section.

Dialog Box

The Main pane of the Hough Transform dialog box appears as shown in the following figure.

Theta resolution (radians): Specify the spacing of the Hough transform bins along the theta-axis.
Rho resolution: Specify the spacing of the Hough transform bins along the rho-axis.
Output theta and rho values: If you select this check box, the Theta and Rho ports appear on the block. The block outputs vectors of theta and rho values at these ports.
Output data type: Specify the data type of your output signal.

The Fixed-point pane of the Hough Transform dialog box appears as shown in the following figure.

Rounding mode

Select the rounding mode for fixed-point operations.

Overflow mode

Select the overflow mode for fixed-point operations.

Sine table

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 are always saturated and rounded to Nearest.

Rho

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. The bias of all signals in Video and Image Processing Blockset is 0.

Product output

As depicted in the previous figure, the output of the multiplier is placed into the product output data type and scaling. Use this parameter to specify how to designate this 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. The bias of all signals in the Video and Image Processing Blockset is 0.

Accumulator

As depicted in the previous figure, inputs to the accumulator are cast to the accumulator data type. The output of the adder remains in the accumulator data type as each element of the input is added to it. Use this parameter to specify how to designate this accumulator word and fraction lengths:

When you select Same as product output, these characteristics match those 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. The bias of all signals in the Video and Image Processing Blockset is 0.

Hough output

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 is always 0.
When you select Slope and bias scaling, you can enter the word length, in bits, of the Hough output. The slope is always 0. The bias of all signals in the Video and Image Processing Blockset is 0.

Theta output

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. The bias of all signals in the Video and Image Processing Blockset is 0.