  An efficient two-layer coding method using the histogram packing technique
with the backward compatibility to the legacy JPEG is proposed in this paper.
The JPEG XT, which is the international standard to compress HDR images, adopts
two-layer coding scheme for backward compatibility to the legacy JPEG. However,
this two-layer coding structure does not give better lossless performance than
the other existing single-layer coding methods for HDR images. Moreover, the
JPEG XT has problems on determination of the lossless coding parameters;
Finding appropriate combination of the parameter values is necessary to achieve
good lossless performance. The histogram sparseness of HDR images is discussed
and it is pointed out that the histogram packing technique considering the
sparseness is able to improve the performance of lossless compression for HDR
images and a novel two-layer coding with the histogram packing technique is
proposed. The experimental results demonstrate that not only the proposed
method has a better lossless compression performance than that of the JPEG XT,
but also there is no need to determine image-dependent parameter values for
good compression performance in spite of having the backward compatibility to
the well known legacy JPEG standard.
