quantization of dct image for steganography

Question

I hav a greyscale image. I did 8x8 blocks and computed each of their DCTs. I want to quantize the DCT coefficients and then replace their LSBs with my secret message bits. How exactly do I quantize the coefficients? Should I use the quantization matrix used by JPEG? How to determine the values of such a quantization matrix?

Answer 1

You will probably want to set the quality level to the highest (smallest values in the quantization matrix) so that the modified LSB of each coefficient perturbs the image data the least.

For encoding:
You will need access to the DCT values after quantization and before entropy coding. There you can modify the LSB's. You should probably only modify the non-zero coefficient values or you will make the compressed image file much larger and more distorted. This way, you will probably be able to encode 20-30 bits per DCT block.

For decoding:
You will need to do the reverse and get access to the DCT values immediately after the entropy decode and before the dequantization step.

To calculate the total number of bits available for your message, use the following example:

For a VGA sized image (640x480) which is encoded as 4:2:0 (subsampled color in both dimensions), you will have 40 x 30 = 1200 MCUs. Each MCU has 6 DCT blocks (4Y, 1Cr, 1Cb). This is a total of 7200 DCT blocks. If each block encodes an average of 25 coefficients (a reasonable quality level), then your message can be a total of 7200x25 = 180000 bits.