HoughLines - 2023.2 English

Vitis Libraries

Release Date
2023.2 English

The HoughLines function here is equivalent to HoughLines Standard in OpenCV. The HoughLines function is used to detect straight lines in a binary image. To apply the Hough transform, edge detection preprocessing is required. The input to the Hough transform is an edge detected binary image. For each point (xi,yi) in a binary image, we define a family of lines that go through the point as:

rho= xi cos(theta) + yi sin(theta)

Each pair of (rho,theta) represents a line that passes through the point (xi,yi). These (rho,theta) pairs of this family of lines passing through the point form a sinusoidal curve in (rho,theta) plane. If the sinusoids of N different points intersect in the (rho,theta) plane, then that intersection (rho1, theta1) represents the line that passes through these N points. In the HoughLines function, an accumulator is used to keep the count (also called voting) of all the intersection points in the (rho,theta) plane. After voting, the function filters spurious lines by performing thinning, that is, checking if the center vote value is greater than the neighborhood votes and threshold, then making that center vote as valid and other wise making it zero. Finally, the function returns the desired maximum number of lines (LINESMAX) in (rho,theta) form as output.

The design assumes the origin at the center of the image i.e at (Floor(COLS/2), Floor(ROWS/2)). The ranges of rho and theta are:

theta = [0, pi)
rho=[-DIAG/2, DIAG/2), where DIAG = cvRound{SquareRoot( (COLS*COLS) + (ROWS*ROWS))}

For ease of use, the input angles THETA, MINTHETA and MAXTHETA are taken in degrees, while the output theta is in radians. The angle resolution THETA is declared as an integer, but treated as a value in Q6.1 format (that is, THETA=3 signifies that the resolution used in the function is 1.5 degrees). When the output (rho, ? theta) is used for drawing lines, you should be aware of the fact that origin is at the center of the image.

API Syntax

void HoughLines(xf::cv::Mat<SRC_T, ROWS, COLS, NPC, XFCVDEPTH_IN_1> & _src_mat,float outputrho[MAXLINES],float outputtheta[MAXLINES],short threshold,short linesmax)

Parameter Descriptions

The following table describes the template and the function parameters.

Table 589 Table . HoughLines Parameter Description
Parameter Description
RHO Distance resolution of the accumulator in pixels.
THETA Angle resolution of the accumulator in degrees and Q6.1 format.
MAXLINES Maximum number of lines to be detected
MINTHETA Minimum angle in degrees to check lines.
MAXTHETA Maximum angle in degrees to check lines
DIAG Diagonal of the image. It should be cvRound(sqrt(rows*rows + cols*cols)/RHO).
SRC_T Input Pixel Type. Only 8-bit, unsigned, 1-channel is supported (XF_8UC1).
ROWS Maximum height of input image
COLS Maximum width of input image
NPC Number of Pixels to be processed per cycle; Only single pixel supported XF_NPPC1.
XFCVDEPTH_IN Depth of the input image.
_src_mat Input image should be 8-bit, single-channel binary image.
outputrho Output array of rho values. rho is the distance from the coordinate origin (center of the image).
outputthe ta Output array of theta values. Theta is the line rotation angle in radians.
threshold Accumulator threshold parameter. Only those lines are returned that get enough votes (>threshold).
linesmax Maximum number of lines.

Resource Utilization

The table below shows the resource utilization of the kernel for different configurations, generated using Vivado HLS 2019.1 version tool for the Xczu9eg-ffvb1156-1-i-es1 to process a grayscale HD (1080x1920) image for 512 lines.

Table 590 Table . Houghlines Function Resource Utilization Summary
Name Resource Utilization
BRAM_18K 542
DSP48E 10
FF 60648
LUT 56131

Performance Estimate

The following table shows the performance of kernel for different configurations, generated using Vivado HLS 2019.1 version tool for the Xczu9eg-ffvb1156-1-i-es1 to process a grayscale HD (1080x1920) image for 512 lines.

Table 591 Table . Houghlines Function Performance Estimate Summary
Operating Mode Operating Frequency (MHz) Latency Estimate
Max (ms)
THETA=1, RHO=1 300 12.5