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 \
alpha_mix.c
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