# What does the negative sign mean in the image frequency domain?

When I transform an image from the spatial domain to the frequency domain, some frequencies are negative. What is the meaning of the negative sign?

For example:

• The numbers in the matrix 'b' are not frequencies but amplitudes at specific frequencies. – pichenettes Jan 5 '14 at 16:19
• so, what does it mean?! – Mohammad Hyari Jan 5 '14 at 16:36
• a phase shift of 180° for the corresponding frequency – sellibitze Jan 5 '14 at 22:04

The matrix b is the same size as a and contains the discrete cosine transform coefficients indicating the correlation between the cosine function at two specific frequencies. They are not necessary to be positive values. The dimension, 4*4 in your case, represents the different frequencies.