Fig. 1. EVC-based optical hiding system: The extraction process

Fig. 2. Extended-visual-cryptographic-based optical hiding system: The hiding process

Fig. 3. Two shares (a), (b) are stacked together to recover the secret information (c)

Fig. 4. Examples of subpixel arrangements. Arranging two subpixels with and as the examples make , and

Fig. 5. A example of phase-keys generation algorithms: GS algorithm flow diagram

Fig. 6. (a), (b) The original images of shares; (c) the secret image; (d), (e) the shares; (f) the secret images decrypted by stacked (d), (e) together; (g), (h) the phase keys; (i), (j) the reconstructed shares using (g), (h); (k) the recover secret images by stacking (i), (j) together

Fig. 7. Optical setup for the extraction process of EVCOH

Fig. 8. (a), (b) The shares, (c) the secret images decrypted by stacked (a), (b) together; (d), (e) the phase keys; (f), (g) the reconstructed shares that were taken with a CCD camera; (h) the recover secret images by stacking (f), (g) together

Fig. 9. Noise analysis of EVCOH

Fig. 10. Noise analysis of EVCOH when the phase keys are four steps