影像隱藏是將重要影像隱藏於掩護影像中，進而產生內含重要資訊的偽裝影像技術。由於偽裝影像與掩護影像近乎相同，因此利用這門技術可以將內含重要影像的偽裝影像透過網際網路安全傳送到對方，而不會察覺到裡面含有重要影像。進而達到有效保護資訊的目的。在先前的影像隱藏文獻中，雖有不錯的影像品質(image quality)，但能嵌入的灰階影像容量卻有一定的限制，而且大多嵌入之影像不能還原回原始影像。 近年來，許多學者提出新的不同可逆式影像隱藏方法，如直方圖修改法(histogram modification)和像素差值法(pixels difference)，在可逆式資料隱藏中則要求在嵌入資料後，具有高資訊隱藏量及可逆性且無失真，不破壞原始影像的視覺效果等條件。 因此本文提出一套新的影像隱藏（image hiding）技術，將欲嵌入灰階秘密影像利用錯誤擴散法（error diffusion）轉成二位元之半色調影像（halftone image），再使用Kim等人提出的直方圖嵌入法嵌入於原始影像中。萃取方面，將半色調隱藏資訊從已嵌入之影像取出，利用逆半色調(LUT inverse halftone, LIH)演算法將半色調影像恢復成灰階影像，由於Kim等人提出的直方圖嵌入法具可逆性，因此可將已嵌入影像還原。實驗證明本文技術能嵌入高容量之灰階秘密影像，嵌入後影像仍保持非常好的影像品質。萃取時，除仍擁有高品質灰階秘密資訊外，也可讓嵌入之影像還原。 Data hiding technique is an important vector of network information communication and plays a role in security. This technique embeds secret information called a mark into host image to provide different purposes such as copyright protection, proof, and so on. Reversible data hiding technique provides not only secret data but also images being definitely in the decoding stage. This is highly desired in quality-sensitive imagery where even the minimal distortion introduced by embedding data is unacceptable since the reversibility helps to mark right decision during image analysis. In recent years, researchers have developed some different reversible image hiding theories and methods based on image histogram modification and pixels difference. But high capacity of digital images is low and the loss of host information happens in traditional reversible image hiding method. This study proposes a novel image hiding method based on histogram hiding and halftoning technique. This method uses reversible data hiding to embed error diffusion halftone image into original image for image secrecy. In image extract process, we use reversible data hiding to extract the halftone information, and hidden image is achieved from LUT inverse halftone. Experimental result shows that the performance of our method can hide high capacity of digital images, the stego-image is almost the same as the cover-image, and it can be reversible.
