CODEWITHSUNDEEP
javaimagejpegsteganographydct
Emil Grigore
Emil Grigore

Reputation: 949

Java JPG DCT coefficients alter by themselfs after I modify them

I have a program that changes the DCT coefficients of a JPG image. This is the code that gives me the DCT coefficients

    public int[] quantizeBlock(double inputData[][], int code) {
    int outputData[] = new int[blockSize * blockSize];
    int i, j;
    int index;
    index = 0;
    for (i = 0; i < 8; i++) {
        for (j = 0; j < 8; j++) {
            // The second line results in significantly better compression.

            outputData[index] = (int) (Math.round(inputData[i][j]
                    * (((double[]) (Divisors[code]))[index])));
            // outputData[index] = (int)(((inputData[i][j] * (((double[])
            // (Divisors[code]))[index])) + 16384.5) -16384);
            index++;
        }
    }

    return outputData;
}

This is a DCT matrix before modifications

     -43 7 0 0 0 0 0 0 
     -8 1 2 -1 0 0 0 0 
     -1 -1 -1 1 0 0 0 0
     -2 1 0 -1 0 0 0 0
      6 0 0 0 0 0 0 0
     -2 0 1 0 0 0 0 0
     -1 0 0 0 0 0 0 0
      0 0 0 0 0 0 0 0

This is after the modifications

      -42 8 0 0 0 0 0 0
      -7 1 3 0 0 0 0 0
       0 0 0 1 0 0 0 0 
      -1 1 0 0 0 0 0 0 
       7 0 0 0 0 0 0 0 
      -1 0 1 0 0 0 0 0
       0 0 0 0 0 0 0 0

After I save the image using image Buffer,I use the created image to get back the modified DCT from it but all I get is:

      -41 9 0 0 0 0 0 0
      -6 1 4 0 0 0 0 0 
       0 0 0 1 0 0 0 0
       0 1 0 0 0 0 0 0 
       8 0 0 0 0 0 0 0 
       0 0 1 0 0 0 0 0
       0 0 0 0 0 0 0 0
       0 0 0 0 0 0 0 0

I've seen a question where the user using a library in IOS did the same thing and had the same problem.Apparently the library recopressed the image and the hidden message was destroyed.

I don't know if this is the case for me.I use Image Buffer to create the image.

Upvotes: 0

Views: 773

Answers (1)

user3344003
user3344003

Reputation: 21627

A couple things off the top that could be happening. The first is rounding errors. The JPEG process introduces small errors. All your values are one off. This could come from rounding.

The second is quantization. Your values may be quantized (divided). Your example does not indicate the compression stages that may be taking place in between your examples.

Upvotes: 1

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