This document discusses various unitary transforms that can be used to decompose images, including the discrete Fourier transform (DFT), discrete cosine transform (DCT), Karhunen-Loève transform (KLT), Hadamard transform, and wavelet transforms. Unitary transforms have desirable properties like energy conservation, orthonormal bases, and de-correlation of image elements. The KLT provides optimal energy compaction and de-correlation but relies on signal statistics. Practical transforms like the DCT approximate the KLT while having fast implementations and being signal-independent. Transforms are widely used for applications like image compression, feature extraction, and pattern recognition.