Friday 19 July 2013

Convolution Matrix for image processing (Android).

Posted By:Hamad | At:09:00 Be the first to comment!
It’s not easy to explain about Convolution, so I just pick up several article links which would be helpful for you to understand convolution better.and you can use it for references.

GIMP Documentation on Convolution Matrix <— really detailed on explanation.

  I write this post just to introduce about Convolution and prepare for the next several posts on image processing, cool effects!
At the very first, you need a base for computing Convolution Matrix on Android, don’t you?
so here is the code:

  1.code of convolution matrix:

package pete.android.study;
 
import android.graphics.Bitmap;
import android.graphics.Color;
 
public class ConvolutionMatrix
{
    public static final int SIZE = 3;
 
    public double[][] Matrix;
    public double Factor = 1;
    public double Offset = 1;
 
   //Constructor with argument of size
    public ConvolutionMatrix(int size) {
        Matrix = new double[size][size];
    }
 
    public void setAll(double value) {
        for (int x = 0; x < SIZE; ++x) {
            for (int y = 0; y < SIZE; ++y) {
                Matrix[x][y] = value;
            }
        }
    }
 
    public void applyConfig(double[][] config) {
        for(int x = 0; x < SIZE; ++x) {
            for(int y = 0; y < SIZE; ++y) {
                Matrix[x][y] = config[x][y];
            }
        }
    }
 
    public static Bitmap computeConvolution3x3(Bitmap src, ConvolutionMatrix matrix) {
        int width = src.getWidth();
        int height = src.getHeight();
        Bitmap result = Bitmap.createBitmap(width, height, src.getConfig());
 
        int A, R, G, B;
        int sumR, sumG, sumB;
        int[][] pixels = new int[SIZE][SIZE];
 
        for(int y = 0; y < height - 2; ++y) {
            for(int x = 0; x < width - 2; ++x) {
 
                // get pixel matrix
                for(int i = 0; i < SIZE; ++i) {
                    for(int j = 0; j < SIZE; ++j) {
                        pixels[i][j] = src.getPixel(x + i, y + j);
                    }
                }
 
                // get alpha of center pixel
                A = Color.alpha(pixels[1][1]);
 
                // init color sum
                sumR = sumG = sumB = 0;
 
                // get sum of RGB on matrix
                for(int i = 0; i < SIZE; ++i) {
                    for(int j = 0; j < SIZE; ++j) {
                        sumR += (Color.red(pixels[i][j]) * matrix.Matrix[i][j]);
                        sumG += (Color.green(pixels[i][j]) * matrix.Matrix[i][j]);
                        sumB += (Color.blue(pixels[i][j]) * matrix.Matrix[i][j]);
                    }
                }
 
                // get final Red
                R = (int)(sumR / matrix.Factor + matrix.Offset);
                if(R < 0) { R = 0; }
                else if(R > 255) { R = 255; }
 
                // get final Green
                G = (int)(sumG / matrix.Factor + matrix.Offset);
                if(G < 0) { G = 0; }
                else if(G > 255) { G = 255; }
 
                // get final Blue
                B = (int)(sumB / matrix.Factor + matrix.Offset);
                if(B < 0) { B = 0; }
                else if(B > 255) { B = 255; }
 
                // apply new pixel
                result.setPixel(x + 1, y + 1, Color.argb(A, R, G, B));
            }
        }
 
        // final image
        return result;
    }
}

  2. note that:

  • You can find more articles on Internet for details. However, you can see that Convolution is applied very widely in image processing for using filters.
  • Some image effects are better to implement using Convolution Matrix method like: Gaussian Blur, Sharpening, Embossing…

  3. conclusion:

  • Some deep information about Convolution Matrix.
  • Know what is convolution matrix and where it can be used.

  4. about the post:

  •  The code seems to explain itself due to comments, and is very easy to understand.
  •  Don’t mind to write a comment whatever you like to ask, to know,to suggest or recommend.
  •  Hope you enjoy it!
  • Hope you like and comment on it!

  5. Source Code:
        you can download the source code here


Cheers,
Hamad Ali Shaikh
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