# How left out down-sampling in 3D-DWT just on the Z direction

I am working on image processing by a 3D wavelet transform.

I have a problem with the classification size of the wavelet coefficients. As you know, when we apply 3D_DWT on the image (for example: Hyperion of the area by 256*256*128 size) the sub-band size would be decreased (for LLL in 1l level the size is 128*128*64).

How I could apply down sampling just for the Z direction (spectral direction)?

It means this size of the sub-band should be 256*256*62 after one level wavelet applying. I want to do it in the Matlab script (dwt3.m & wavedec3.m), but when I removed the down sampling, it was applied in all three directions.

You are applying a 3D DWT scheme whose down-sampling is coherent across the three directions. Nothing forbids you from decomposing bands the way you want, and the literature is full of so-called isotropic, Tensor, Separable, Square or Standard implementations (vocabulary: separable/non-separable, standard/non-standard, S-form/NS-form, rectangular/square, anisotropic/isotropic, tensor/Mallat, hyperbolic/isotropic, separated/combined wavelet transform)

To do that, it is more appropriate to perform 1D wavedec passes your own way on each direction separately.

• Thanks for your replying, I will try to apply 1D_DWT on each direction and for 4 level, it should be repeated for LLL sub bands. – Mehdi Nov 2 '17 at 14:47