How to set a matrix region with opencv4nodejs

Question

Question:

What is the fastest method to set a matrix area in opencv4nodejs?

Problem:

I am overlaying a source image into a larger destination image with dimensions in the low thousands.

In Python I'd get/set regions of matching size with:

destination[y:y+h, x:x+w] = source[:,:]

But I'm not sure how to do this in Javascript.

I've tried a few methods but even the fastest is prohibitively slow.

---

In example I have:

• source_mat cols:2929, rows:2049
• dest_mat cols:3000, rows:6000
• offset_x:150, offset_y:150

casting matrix to array, looping rows and cols, setting dest pixels takes about 12 seconds

let debug_overlay_start = new Date().valueOf();
let source_pixels = source_mat.getDataAsArray();
for (let row_index = 0, l_1 = source_pixels.length; row_index < l_1; row_index++) {
    if(row_index + offset_y < 0) continue;
    if(row_index + offset_y >= dest_mat.rows) continue;
    let this_col = source_pixels[row_index];
    for (let col_index = 0, l_2 = this_col.length; col_index < l_2; col_index++) {
        if(col_index + offset_x < 0) continue;
        if(col_index + offset_x >= dest_mat.cols) continue;
        dest_mat.set(row_index + offset_y, col_index + offset_x, source_pixels[row_index][col_index]);
    }
}
let debug_overlay_end = new Date().valueOf();
console.log(`overlay method took ${((debug_overlay_end - debug_overlay_start)/1000).toFixed(2)}`); // overlay method took  11.63
return dest_mat;

Casting both to arrays, looping through rows, splicing cols in takes a massive 82 seconds:

let debug_overlay_end = new Date().valueOf();
let source_pixels = source_mat.getDataAsArray();
let new_dest_mat = dest_mat.getDataAsArray();
for (let row_index = 0, l_1 = source_pixels.length; row_index < l_1; row_index++) {
    if(row_index + offset_y < 0) continue;
    if(row_index + offset_y >= dest_mat.rows) continue;
    let this_col = source_pixels[row_index]; // entire column of source pixels
    new_dest_mat[row_index + offset_y].splice(offset_x, this_col.length, ...this_col);
}
let debug_overlay_end = new Date().valueOf();
console.log(`overlay method took ${((debug_overlay_end - debug_overlay_start)/1000).toFixed(2)}`); // 82 seconds
return new cv.Mat(new_dest_mat, dest_mat.type);

Replacing region didn't work at all, throwing a lifecycle error with no additional logging:

let debug_overlay_end = new Date().valueOf();
let area_to_replace = dest_mat.getRegion(new cv.Rect(x, y, source_mat.cols, source_mat.rows));
area_to_replace = source_mat.getRegion(new cv.Rect(0, 0, source_mat.cols, source_mat.rows)); // lifecycle error
console.log(`overlay method took ${((debug_overlay_end - debug_overlay_start)/1000).toFixed(2)}`);
return dest_mat;

Using setAt() and atRaw() is fastest so far at 8 seconds:

let debug_overlay_start = new Date().valueOf();
for(let row_index = 0, row_length = source_mat.rows; row_index < row_length; row_index++){
    if(row_index + offset_y < 0) continue;
    if(row_index + offset_y >= dest_mat.rows) continue;
    for(let col_index = 0, col_length = source_mat.cols; col_index < col_length; col_index++){
        if(col_index + offset_x < 0) continue;
        if(col_index + offset_x >= dest_mat.cols) continue;
        dest_mat.set(row_index + offset_y, col_index + offset_x, source_mat.atRaw(row_index, col_index));
    }
}
let debug_overlay_end = new Date().valueOf();
console.log(`overlay method took ${((debug_overlay_end - debug_overlay_start)/1000).toFixed(2)}`); // 8.09
return dest_mat;

I've had a look at the docs and I'm surprised to see this isn't a common use case. Is the node/electron environment slowing down an otherwise fast operation?

Answer 1

Managed to get down to 1.1 seconds using buffer arrays. Unsure how else to speed up.

/**
 * Places a source matrix onto a dest matrix.
 * Note: This replaces pixels entirely, it doesn't merge transparency
 * @param {cv.Mat} source_mat matrix being copied
 * @param {cv.Mat} dest_mat matrix being pasted into
 * @param {number} x horizontal offset of source image
 * @param {number} y vertical offset of source image
 */
function overlayOnto(source_mat, dest_mat, x, y){
    if(source_mat.channels != dest_mat.channels) throw new Error('src and dst channel counts must match');
    let source_uint8 = new Uint8Array( source_mat.getData() ); // WARNING 4 CHANNELS
    let dest_uint8 = new Uint8Array( dest_mat.getData() ); // WARNING 4 CHANNELS
    let dest_width = dest_mat.cols;
    let x_count = 0; // set counters
    let y_count = 0; // set counters
    let channel_count = source_mat.channels;
    for (let i = 0; i < source_uint8.length; i += channel_count) { // WARNING 4 CHANNELS
        let dest_x = x_count + x; // add offset
        let dest_y = y_count + y; // add offset

        if( !( (dest_x < 0 || dest_x > dest_mat.cols-1) || (dest_y < 0 || dest_y > dest_mat.rows-1) ) ){ // pixel does not fall outside of dest mat
            // write into buffer array
            let dest_i = (dest_x + dest_width * dest_y); // (x + w * h) to get x/y coord in single-dimension array
            let dest_buffer_i = dest_i * channel_count;
            if(channel_count >= 1)  dest_uint8.fill(source_uint8[i+0], dest_buffer_i+0 , dest_buffer_i+0+1);
            if(channel_count >= 2)  dest_uint8.fill(source_uint8[i+1], dest_buffer_i+1 , dest_buffer_i+1+1);
            if(channel_count >= 3)  dest_uint8.fill(source_uint8[i+2], dest_buffer_i+2 , dest_buffer_i+2+1);
            if(channel_count >= 4)  dest_uint8.fill(source_uint8[i+3], dest_buffer_i+3 , dest_buffer_i+3+1);
        }
        x_count++; // increase col
        x_count = x_count % source_mat.cols; // end of col? move to start
        if(x_count == 0) y_count++; // started new col? increase row 
    }
    return new cv.Mat(dest_uint8, dest_mat.rows, dest_mat.cols, dest_mat.type);
}