What is the Mat object in OpenCV.js and how to create and manage it?

Mat (Matrix) is the core data structure in OpenCV.js, used to store image and matrix data.

Basic Concepts of Mat

Mat is an n-dimensional array that can store:

• Single or multi-channel images (grayscale, RGB, RGBA)
• Matrix data
• Other numeric data types

Creating Mat Objects

javascript
// Create empty Mat
let mat = new cv.Mat();

// Create Mat with specified size (default black)
let mat = new cv.Mat(rows, cols, type);

// Common types
cv.CV_8UC1  // 8-bit unsigned single channel (grayscale)
cv.CV_8UC3  // 8-bit unsigned three channels (RGB)
cv.CV_8UC4  // 8-bit unsigned four channels (RGBA)
cv.CV_32FC1 // 32-bit float single channel

Creating Mat from HTML Elements

javascript
// Create from canvas
let canvas = document.getElementById('canvas');
let mat = cv.imread(canvas);

// Create from img element
let img = document.getElementById('image');
let mat = cv.imread(img);

Common Mat Operations

javascript
// Read and set pixel values
let pixel = mat.ucharAt(row, col);  // Read single channel pixel
let pixel = mat.data;  // Get all pixel data

// Copy Mat
let matCopy = mat.clone();

// Create Region of Interest (ROI)
let roi = mat.roi(new cv.Rect(x, y, width, height));

// Convert color space
cv.cvtColor(mat, mat, cv.COLOR_RGBA2GRAY);

// Release memory
mat.delete();

Memory Management Considerations

1. Manual Release: JavaScript doesn't have automatic garbage collection, so you must manually call delete() to release memory
2. Avoid Memory Leaks: Release memory immediately when Mat is no longer needed
3. Use try-finally: Ensure resources are released even in exceptional cases

javascript
try {
  let mat = new cv.Mat(100, 100, cv.CV_8UC3);
  // Process image
} finally {
  mat.delete();
}

Common Errors

javascript
// Error: Memory leak due to forgetting to release
let mat = new cv.Mat();
// No mat.delete() called after use

// Error: Double release
mat.delete();
mat.delete();  // Will throw error

// Correct: Using smart pointer pattern
function processImage() {
  let mat = new cv.Mat();
  try {
    // Processing logic
    return mat.clone();  // Return copy
  } finally {
    mat.delete();  // Release original Mat
  }
}