ALPHA_MIX HLS C Kernel Creation - 2023.1 English

Vitis Tutorials: Hardware Acceleration (XD099)

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2023.1 English

According to the design specification, the alpha_mix kernel will include following functions:

  • Receive monochrome (one channel 8-bit depth) digital clock images via AXI Stream slave interface

  • Read the background image from global memory via AXI master interface

  • Resize the digital clock image to designated scale

  • Color the clock digit characters as well as the background layer for the clock images

  • Mix the clock digit, clock background layer and the background image with opacity setting

  • Output the mixed image via AXI Stream master interface

Following code block is the top level function declaration for alpha_mix kernel. You can see that: the hardware AXI stream port (master or slave) is represented by hls::stream data type; the hardware AXI master port (read or write) is represented by ap_uint (actually array pointer) data type; those kernel arguments are represented by normal int data type, which will be grouped up to map to an AXI slave interface.

void alpha_mix(hls::stream<ap_axiu<64, 0, 0, 0>> &time_img_input,   // time image input
               ap_uint<256> *bgr_img_input,                         // background image input
               hls::stream<ap_axiu<64, 0, 0, 0>> &mix_img_output,   // mixed image output
               int     time_img_rows_in,                            // input time image height
               int     time_img_cols_in,                            // input time image width
               int     time_img_rows_rsz,                           // resized time image height
               int     time_img_cols_rsz,                           // resized time image width
               int     time_img_pos_row,                            // resized time image position - Y
               int     time_img_pos_col,                            // resized time image position - X
               ap_uint<32> time_char_color,                         // [31:0] = [xRGB]
               ap_uint<32> time_bgr_color,                          // [31:0] = [xRGB]
               int     time_bgr_opacity,                            // time image background opacity,[7:0] used
               int     bgr_img_rows,                                // background image height
               int     bgr_img_cols                                 // background image width

Following is the sub-function and data flow diagram for alpha_mix kernel.

In the diagram, the sub-functions filled with red are from Vitis Vision Library, and those with blue are hand-written. xf::cv::Mat is the counterpart for cv::Mat in OpenCV software library, it is very useful for handling image data. In the hardware implementation, if we want to handle the image with in-order pixel level (no need to randomly access the pixel data), we can use #pragma HLS stream to indicate the compiler to map the xf::cv::Mat object to array. This is the case for our alpha_mix kernel. Many functions in Vitis Vision Library support xf::cv::Mat as the input and output data. In the HLS C code, we can use for-loop to process data in xf::cv::mat stream easily, you can refer to the source code of sub-function mixing for alpha-mixing operation applied to the input and output xf::cv::Mat objects.

We use Vitis v++ command to compile the HLS C source code to kernel file (xo). For example, use following command to compile alpha_mix.c to hardware kernel with xilinx_u50_gen3x16_xdma_201920_3 platform.

v++ --platform xilinx_u50_gen3x16_xdma_201920_3                                                     \
    --target hw                                                                                     \
    --kernel alpha_mix                                                                              \
    --include ./include                                                                             \
    --advanced.prop kernel.alpha_mix.kernel_flags="-std=c++0x -D__SDSVHLS__ -DHLS_NO_XIL_FPO_LIB"   \
    --compile                                                                                       \
    --output alpha_mix.xo                                                                           \

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