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ISBN: 978-972-8939-25-0 © 2010 IADIS3.3 Image generating Fidelity CriteriaThe RIG generated images need to satisfy the most common regularity properties. Images that are containingrecognizable shapes are called regular images. So every image generated by RIG must be tested for itsregularity and amount of information. The Fourier spectrum can be used to detect the image regularity. Eachimage can be detected by calculating the Fast Fourier Transform (FFT) for it and then we check the energy ofthe selected image. The Discrete Fourier Transform (DFT) equation may be illustrated as:N 1 kF ( k)f ( n)Qn0For k =0, 1… N-1 (3.3)N 11kf ( n) F(n)QN n0For n = 0, 1… N-1 (3.4)Where 2k 2kQ k cos i sin N N (3.5)Where i 1For regularity property, if the energy of the spectrum is concentrated in the high frequency, then image isirregular, otherwise the image is regular; the amount of image information is measured by applying Entropytest. The following equation describes the entropy formula:L 1Entropy p i* log p i(3.6) i02( )Where p i : stands for the probability of the ith gray level = n k / N 2n k : stands for the total number of pixels with gray value k.L : stands for the total number of gray levels (e.g., 256 for 8 bits)4. THE PROPOSED TECHNIQUEThe proposed approach uses the structured images, which are generated using random image generation(RIG) algorithm. Because if photographs are used instead of the structured images; it is easier for theobserver to predict some information about secure embedded images, and another personal knowledge, thefollowing algorithm describes the technique of the proposed system:Step 1: Extracting the embedded image properties.Step 2: The first random digit is generated using LFSR and depends on extracted properties.Step 3: If the first digit matches one of the grammar numbers that is suited to the first generated digits, thenstore it in a temporary array. Otherwise return to step 2.Step 4: Scan for the applicable rule that the generated digit number matches its number.Step 5: Search the matching rule to count the number of arguments of its function.Step 6: If the number of arguments is not equal to zero and is less than 13 (since the maximum number ofnested functions can be 13), then do the following steps:i) Generate new digit using LFSR.ii) Search for the application rule that the generated digit number matches its rule number.iii) Scan the matching rule to count the number of arguments of its function.iv) If the number of arguments is equal to zero, then decrement one from the resisted number ofarguments of the previous function. Otherwise, increment the number of functions of the resultingstream of integers and return to step 6.Step 7: If the number of arguments is equal to zero then go to step 9.Step 8: Else, if the number of arguments is not equal to zero and the number of functions in the formula isgreater than 13 then generate random number using LFSR and decrement one from the number ofarguments. Repeat this process until the number of arguments becomes zero.Step 9: Replace the resulted stream of digits to the corresponding right-hand side of the production rule inthe RIG grammar.358