【正文】 53Step 7. To embed bit ‘1,’ the required condition is given byR1greaterorequalslant R2. (8)If (8) is not satisfied, then Z is modified using the modification procedure, as explained below, until it is satisfied.Step 8. To embed bit ‘0,’ the required condition is:R1 R2. (9)If (9) is not satisfied, then Z is modified using the modification procedure, as explained below, until it is satisfied.Step 9. After deciding on the values R1and R2, using CRT (5) bine them with M1and M2to get Zprime.Step 10. Combine the value of Y with Zprimeto get Xprime, the new watermarked pixel value.Step 11. Reconstruct the block with the new watermarked pixel value Xprime.Step 12. Repeat Steps 1–11 for all the blocks until all the watermark bits are embedded.There were some considerations to be made for embedding a watermark bit. Firstly, we are using only the 6 LSBs and notthe full 8 bits, because we do not want to modify the pixel values to a large extent, which can cause some visual distortion.Our focus in this scheme is to keep distortion minimal. Secondly, we have selected the pairwise coprime numbers M1and M2, as 6 and 11, respectively. According to CRT, the product of the pairwise coprime numbers should be greater thanthe possible range of numbers under consideration, which in our case is 0–63. Another reason behind choosing 6 and 11instead of other numbers is that the maximum possible pixel intensity, 63 is close to 66. This allows minimum distortionto the watermarked pixel.We have mentioned that to embed bits ‘1’ and ‘0’ some conditions are to be satisfied. If these are not fulfilled, the valueof Z is to be systematically modified. This is carried as follows.1) Modification procedure to embed bit ‘1’:First check whether the condition (8) is satisfied. If it satisfied, then there is no need to modify the value of subtract 1 from Z. Going back to Step 6, check whether condition (8) is satisfied. If it is satisfied then select new valueof Z. Otherwise, add 1 to Z, continue with Step 6 and check whether (8) is satisfied. If adding or subtracting 1 doesnot yield the expected result, then carry on subtracting or adding 1 to Z until (8) is satisfied. Thereafter, the modifiedvalues of R1and R2are used by CRT (5) to get Zprime.2) Modification procedure to embed ‘0’:In this case, the modification procedure is the same as that of bit ‘1’ except that the condition to be checked is (9).A flowchart illustrating the above embedding procedure is shown in Fig. 2.. Extraction procedureThe extraction procedure is reverse of the embedding procedure. We need to know only the following information toextract the watermark from the watermarked image.1. Watermarked image.2. Size of the watermark.3. Seed of the PRNG.4. The pairwise coprime numbers M1and M2.With the knowledge of PRNG, the pixel of the block in which watermark is embedded is selected and its pixel valueZ is found out. Thereafter, using M1, M2and Z, R1and R2are determined using (7). After which, a parison is madebetween R1and R2.IfR1greaterorequalslant R2, bit ‘1’ would be extracted, otherwise bit ‘0’ would be extracted. These steps are repeatedfor every consecutive block to extract all the watermark bits.5. Experimental resultsIn order to carry out the parison between the proposed and the SVDbased techniques, few performance measuresare defined below. The extent of tampering of the extracted watermark is puted using Tampered Assessment Function(TAF) [22]. Considering the size of the watermark as m n, the TAF in percentage is defined asTAF =1mnbracketleftBiggm?1summationdisplayn?1summationdisplayw(i, j) ⊕ w(i, j)bracketrightBigg100, (10)i=0 j=0. Patra et al. / Digital Signal Processing 20 (2020) 442–453 447Fig. 2. Flowchart to embed a watermark bit.where, w(i, j) and w(i, j) represent the original and extracted watermarks at position, (i, j) respectively, and ⊕ is anexclusiveOR operator. The acceptance level of TAF is 5% because, above this value, the extracted watermark will not berecognizable.Peak SignaltoNoise Ratio (PSNR) measures the quality between two images. Usually we would pare the modifiedsignal against the original signal, ., in this case, the watermarked image against the original host image. The value of PSNRusually ranges from 20 dB (low quality) to 40 dB (high quality). Since the host images used for the experiments are in 8bitgrayscale format, the peak value of the image is taken to be 255. The PSNR in dB is given byPSNR = 10log10bracketleftBigg25521M1NsummationtextM?1i=0summationtextN?1j=0[A(i, j) ? A(i, j)]2bracketrightBigg, (11)where, A represents the host image, A represents the watermarked image and M and N represent the dimensions of thetwo images.In this study we carry out performance parison among the three schemes considering the following performancecriteria:1. Computational plexity of the embedding and extracting procedures.2. Quality of the watermarked images.3. Robustness of the schemes to different attacks.4. Further increase in embedding capacity.448 . Patra et al. / Digital Signal Processing 20 (2020) 442–453Fig. 3. The three host images (256 256): (a) Lenna, (b) Baboon, (c) Airplane, and (d) the watermark image, Panda (64 64).Fig. 4. Comparison of embedding and extracting times for the three schemes: (a) embedding time, (b) extraction time.. Computational plexityThe putational time for embedding and extraction is an important performance measure, especially when the watermarking is to be carried out for online applications, ., video or audio broadcasting. Several experiments were conductedto evaluate performance of the proposed scheme against the two SVDbased schemes. The experiments were carried outin a laptop PC with Intel P