Does Mat in hls:Mat really represents a matrix? - fpga

Question

I am working on Vivado HLS. I am reading an image via stream and storing it in hls:mat. I want to perform an element-wise operation on this mat. Does mat really represent a matrix? Is there a way in which I can access it like a Matrix i.e. A[rows][columns]?
Method A.at<double>(0,0) is not working.

Answer 1

No, according to Xilinx application note XAPP1167:
A second limitation is that the hls::Mat<> datatype used to model
images is internally defined as a stream of pixels, using the
hls::stream<> datatype, rather than as an array of pixels in external
memory. As a result, random access is not supported on images, and the
cv::Mat<>.at() method and cvGet2D() function have no corresponding
equivalent function in the synthesizable library.
So you can only stream data to/from hls::Mat and you cannot access a random element.

Answer 2

I found the answer using Sobel code (XAP1167)
void created_window(MY_IMAGE& src, MY_IMAGE& dst, int rows, int cols)
{
MY_BUFFER buff_A;
MY_WINDOW WINDOW_3x3;
for(int row = 0; row < rows+1; row++){
for(int col = 0; col < cols+1; col++){
#pragma HLS loop_flatten off
#pragma HLS dependence variable=&buff_A false
#pragma HLS PIPELINE II = 1
// Temp values are used to reduce the number of memory reads
unsigned char temp;
MY_PIXEL tempx;
//Line Buffer fill
if(col < cols){
buff_A.shift_down(col);
temp = buff_A.getval(0,col);
}
//There is an offset to accommodate the active pixel region
//There are only MAX_WIDTH and MAX_HEIGHT valid pixels in the image
if(col < cols && row < rows){
MY_PIXEL new_pix;
src >> new_pix;
tempx = new_pix;
buff_A.insert_bottom(tempx.val[0],col);
}
//Shift the processing window to make room for the new column
WINDOW_3x3.shift_right();
//The processing window only needs to store luminance values
//rgb2y function computes the luminance from the color pixel
if(col < cols){
WINDOW_3x3.insert(buff_A.getval(2,col),2,0);
WINDOW_3x3.insert(temp,1,0);
WINDOW_3x3.insert(tempx.val[0],0,0);
}
MY_PIXEL conn_obj;
//The operator only works on the inner part of the image
//This design assumes there are no connected objects on the boundary of the image
conn_obj = find_conn(&WINDOW_3x3);
//The output image is offset from the input to account for the line buffer
if(row > 0 && col > 0) {
dst << conn_obj;
}
}
}
}
void create_window(AXI_STREAM& video_in, AXI_STREAM& video_out, int rows, int cols)
{
//Create AXI streaming interfaces for the core
#pragma HLS INTERFACE axis port=video_in bundle=INPUT_STREAM
#pragma HLS INTERFACE axis port=video_out bundle=OUTPUT_STREAM
#pragma HLS INTERFACE s_axilite port=rows bundle=CONTROL_BUS offset=0x14
#pragma HLS INTERFACE s_axilite port=cols bundle=CONTROL_BUS offset=0x1C
#pragma HLS INTERFACE s_axilite port=return bundle=CONTROL_BUS
#pragma HLS INTERFACE ap_stable port=rows
#pragma HLS INTERFACE ap_stable port=cols
MY_IMAGE img_0(rows, cols);
MY_IMAGE img_1(rows, cols);
#pragma HLS dataflow
hls::AXIvideo2Mat(video_in, img_0);
created_window(img_0, img_1, rows, cols);
hls::Mat2AXIvideo(img_0, video_out);
}