Data Amount Reduction in Mosaic Image Transmission Techniques for Digital Interactive Television Applications.

Several images are used as a part of the interactive data in the Nipo-Brazilian digital TV system that require good subjective quality while using the lowest possible bandwidth but the well-known traditional compression systems are already applied by default to these images. The concept of a mosaic image (several images forming one) has been formerly used in the steganography application field. The mosaic image is obtained by reordering the image-blocks of a secret image disguised as another image, the so-called target image, and then feeding both to a near reversible color-transformation algorithm. Its use is a possible solution to this need in digital interactive television, for which the main challenge is to achieve this by using less bandwidth for transmission. We propose here a procedure to reduce the amount of data needed to recover the secret image from the mosaic image, as well as a criterion to select the target image and therefore improve the quality of the recovered secret image in interactive data applications. The main objective is to efficiently transmit two images as one using a lower bandwidth. On the one hand, the number of bits needed to recover a given secret image is highly reduced by modeling the image-block standard deviation statistical distribution. On the other hand, the entropy of the image-block means and standard deviations per color component are used to identify the most convenient target image among the images set present in the interactive application of interest. A series of experiments with a set of 20 mostly natural images showed a reduction in the number of bits close to 3-to-1 with respect to techniques of reference. The proposed method allows us to improve the bandwidth use by reducing the number of bits needed to recover the secret image, it preserves the subjective quality of recovered secret images, and it gives the possibility to determine the best target images available in several multimedia and digital terrestrial television applications.