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Lapped Biorthogonal Transform

The LOT does not decay smoothly to zero at the boundaries, as shown in the figures of the previous section. Thus, discontinuities still occur in images reconstructed using the LOT, although they should be more bland than the discontinuities evident by using unlapped transforms. Smoothing out the decay, should result in less blocking effects, since there should be less sharp discontinuities due to quantization errors.

A lapped transform should still allow for perfect reconstruction of the image with no quantization. There should be a separate forward and backward transform, that are not related simply by conjugate transposition as in the orthogonal transform. A biorthogonal transform can be represented in matrix form, with a forward analyses matrix tex2html_wrap_inline326 and a backward synthesis matrix tex2html_wrap_inline328 . Orthogonality must still hold in that

displaymath312

The asymmetry of the biorthogonal transform allows for more flexibility in the shape of the basis functions. A careful modification of the original LOT tex2html_wrap_inline250 matrix allows us to reshape the poorly behaved LOT basis functions. These new synthesis functions decay closer to zero in a smoother fashion than the LOT basis functions do.

The analyses (synthesis) transformation is changed by multiplying (dividing) the each first odd DCT coefficient by tex2html_wrap_inline332 . This leads to an asymmetric transform, which is equivalent to pre-multiplying all tex2html_wrap_inline334 terms in the original LOT matrix by a scaling matrix

displaymath313

and the backwards scaling matrix

displaymath314

This leads to the following synthesis matrix:

displaymath315

and analysis matrix

displaymath316

Now,

align60

where c is the contribution from adjacent blocks. Notice that the overlap condition is valid, in that

displaymath317

where W is the shift matrix, defined by

displaymath318

Thus, the entire transformation process is truly invertible, with proper selection of the boundary matrices, tex2html_wrap_inline340 to give entire matrices

gather69

The transformation relationships then become

align83




next up previous
Next: LBT Experimental Results Up: EE368b Final Report Performance Previous: LOT Experimental Results

Daniel Veiner
Wed Nov 29 20:06:15 PST 2000