Sobel filter

The xFSobel function Computes the gradients of input image in both x and y direction by convolving the kernel with input image being processed.

  • For Kernel size 3x3
    • GradientX:

    • GradientY:

  • For Kernel size 5x5
    • GradientX:

    • GradientY:

  • For Kernel size 7x7
    • GradientX:

    • GradientY:

API Syntax

template<int BORDER_TYPE,int FILTER_TYPE, int SRC_T,int DST_T, int ROWS, int COLS,int NPC=1>
void xFSobel(xF::Mat<SRC_T, ROWS, COLS, NPC> & _src_mat,xF::Mat<DST_T, ROWS, COLS, NPC> & _dst_matx,xF::Mat<DST_T, ROWS, COLS, NPC> & _dst_maty)

Parameter Descriptions

The following table describes the template and the function parameters.

Table 1. xFSobel Function Parameter Descriptions
Parameter Description
FILTER_TYPE Filter size. Filter size of 3(XF_FILTER_3X3), 5(XF_FILTER_5X5) and 7(XF_FILTER_7X7) are supported
BORDER_TYPE Border Type supported is XF_BORDER_CONSTANT
SRC_T Input pixel type. Only 8-bit, unsigned, 1 channel is supported (XF_8UC1)
DST_T Output pixel type. Only 16-bit, signed, 1 channel is supported (XF_16SC1) for filter size 3 and 5. Only 32 bit, signed 1 channel is supported for filter size 7.
ROWS Maximum height of input and output image (must be a multiple of 8)
COLS Maximum width of input and output image (must be a multiple of 8)
NPC Number of pixels to be processed per cycle; possible options are XF_NPPC1 and XF_NPPC8 for 1 pixel and 8 pixel operations respectively.
_src_mat Input image
_dst_matx X gradient output image.
_dst_maty Y gradient output image.

Resource Utilization

The following table summarizes the resource utilization of the kernel in different configurations, generated using Vivado HLS 2017.1 tool for the Xilinx Xczu9eg-ffvb1156-1-i-es1 FPGA, to process a grayscale HD (1080x1920) image.

Table 2. xFSobel Function Resource Utilization Summary
Operating Mode Filter Size

Operating Frequency

(MHz)

 Utilization Estimate
BRAM_18K DSP_48Es FF LUT

CLB
1pixel 3x3 300 3 0 609 616 135
5x5 300 5 0 1133 1499 308
7x7 300 7 0 2658 3334 632
8pixel 3x3 150 6 0 1159 1892 341
5x5 150 10 0 3024 5801 999

Performance Estimate

The following table summarizes the performance of the kernel in different configurations, as generated using Vivado HLS 2017.1 tool for the Xilinx Xczu9eg-ffvb1156-1-i-es1, to process a grayscale HD (1080x1920) image.

Table 3. xFSobel Function Performance Estimate Summary
Operating Mode

Operating Frequency

(MHz)

 Filter Size Latency Estimate

(in ms)
1pixel 300 3x3 7.5
300 5x5 7.5
300 7x7 7.5
8pixel 150 3x3 1.7
150 5x5 1.71