  Compressing the sign information of discrete cosine transform (DCT)
coefficients is an intractable problem in image coding schemes due to the
equiprobable characteristics of the signs. To overcome this difficulty, we
propose an efficient compression method for the sign information called "sign
retrieval." This method is inspired by phase retrieval, which is a classical
signal restoration problem of finding the phase information of discrete Fourier
transform coefficients from their magnitudes. The sign information of all DCT
coefficients is excluded from a bitstream at the encoder and is complemented at
the decoder through our sign retrieval method. We show through experiments that
our method outperforms previous ones in terms of the bit amount for the signs
and computation cost. Our method, implemented in Python language, is available
from https://github.com/ctsutake/dsr.
