In this paper we propose a method to adapt the quantization tables of typical
block-based transform codecs when the input to the encoder is a panoramic image
resulting from equirectangular projection of a spherical image. When the visual
content is projected from the panorama to the viewport, a frequency shift is
occurring. The quantization can be adapted accordingly: the quantization step sizes
that would be optimal to quantize the transform coefficients of the viewport image
block, can be used to quantize the coefficients of the panoramic block. As a proof
of concept, the proposed quantization strategy has been used in JPEG compression.
Results show that a rate reduction up to 2.99% can be achieved for the same
perceptual quality of the spherical signal with respect to a standard quantization.