Similar to the JPEG image compression standard, the JPEG XR image
compression solely controls the image quality loss and hence the output rate
by means of the quantizer bucket sizes; a precise rate control mechanism
like the EBCOT rate allocation algorithm in JPEG 2000 is not specified, and
hence rate-distortion optimality of the quantizer is, in general, not given.
In this work, a simple rate-control mechanism for JPEG XR is introduced that
allows an efficient control of the quantizer towards rate-distortion
optimality. One possibility to implement this quantizer control would be to
use the spatial variable quantization feature of JPEG XR, but it was seen in
an earlier work that the additional side information required to transmit
the quantization setting almost compensates the PSNR gain of variable
quantization and complicates the rate allocation process by requiring an
additional quantizer allocation step.
However, while JPEG XR defines the image reconstruction process completely,
an encoder still has the freedom to select the deadzone size of the
quantizer; this mechanism has the additional advantage that no additional
side information needs to be transmitted and that the deadzone size is not,
unlike the quantizer bucket size, constrained to a set of pre-defined
values specified in the standard.
It is found that the image quality of JPEG XR can be improved by about 0.2
to 0.4 dB by performing a rate-distortion optimal selection of the deadzone;
this gain is seen to be comparable to the PSNR loss of a JPEG 2000 codec
where, for experimental reasons, EBCOT rate control has been turned off.