How to get Mat with a drawable input in Android using OpenCV

Introduction

OpenCV is a powerful computer vision library that allows you to perform a wide range of image and video processing tasks. In Android, you can use OpenCV to process images loaded from drawables. This article will guide you through the steps on how to obtain a Mat object from a drawable input in your Android application.

Steps

1. Set up the Environment

• Include OpenCV library in your project.
• Add the necessary dependencies to your build.gradle file.

2. Load the Drawable

Drawable drawable = getResources().getDrawable(R.drawable.your_image);

3. Convert Drawable to Bitmap

Bitmap bitmap = ((BitmapDrawable)drawable).getBitmap();

4. Create Mat from Bitmap

Mat mat = new Mat();
Utils.bitmapToMat(bitmap, mat);

5. Process the Mat object

Now that you have a Mat object, you can use OpenCV functions to process the image data.

// Example: Convert image to grayscale
Mat grayMat = new Mat();
Imgproc.cvtColor(mat, grayMat, Imgproc.COLOR_BGR2GRAY);

Example Code

import android.graphics.Bitmap;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import org.opencv.android.Utils;
import org.opencv.core.Mat;
import org.opencv.imgproc.Imgproc;

// ...

public void processDrawable(Drawable drawable) {
    // 1. Load Drawable
    // 2. Convert Drawable to Bitmap
    // 3. Create Mat from Bitmap
    Bitmap bitmap = ((BitmapDrawable) drawable).getBitmap();
    Mat mat = new Mat();
    Utils.bitmapToMat(bitmap, mat);

    // 4. Process the Mat object
    Mat grayMat = new Mat();
    Imgproc.cvtColor(mat, grayMat, Imgproc.COLOR_BGR2GRAY);

    // 5. Convert Mat back to Bitmap (if needed)
    Bitmap grayBitmap = Bitmap.createBitmap(grayMat.cols(), grayMat.rows(), Bitmap.Config.ARGB_8888);
    Utils.matToBitmap(grayMat, grayBitmap);
}

Explanation

In this example, we perform the following steps:

• Load a drawable resource using getResources().getDrawable().
• Convert the drawable to a bitmap using ((BitmapDrawable)drawable).getBitmap().
• Create a Mat object from the bitmap using Utils.bitmapToMat().
• Perform image processing operations on the Mat object using OpenCV functions, such as converting the image to grayscale using Imgproc.cvtColor().
• Optionally convert the Mat object back to a bitmap using Utils.matToBitmap().

Conclusion

By following these steps, you can successfully convert a drawable to a Mat object in Android using OpenCV and perform any desired image processing operations. This allows you to leverage the powerful capabilities of OpenCV for image manipulation directly within your Android application.