README.md

Sliding Filter For AWGN Denosing

This package contains the sliding filter for removal of additive stationary white Gaussian noise (AWGN) from 2D/3D images using the sliding window approach. Unlike a traditional way of implementation using for-loops for local filtering, this implementation does not use any for-loop, making the code way faster!

NOTE: The sliding filter accepts a variety of sparsifying/decorrelating transforms for the local transform-domain filtering step. This transforms, which are separable, can be selected from the same family or composed of different families (see the details of transform_type in the syntax section for more information).

Background 📖

The sliding filtering of a noisy image $z=x+\eta$, where $\eta\sim\mathcal{N}(0,\sigma^2)$, proceeds as the following:

Create two null-arrays $\hat{x}^w$ and $w$ as size as $z$, respectively, representing the accumulated denoised blocks/cubes and accumulated weights (buffers)

For each block extracted from $z$ do

Apply a transform-domain filtering to the $i$-th extracted block/cube

$$\hat{x}_i^{w} =\mathcal{T}_{d\textrm{D}}^{-1}\Big(\Upsilon\big(\mathcal{T}_{d\textrm{D}}(z_i),~\lambda\sigma\big)\Big)\,,$$

where $\mathcal{T}_{d\textrm{D}}$ is a $d$-dimensional sparsifying/decorrelating transform ($d=2$ for blocks and $d=3$ for cubes), $\Upsilon$ is a nonlinear shrinkage operator (e.g., soft- or hard-shrinkage), and $\lambda>0$ is a thresholding factor.
Create a buffer block/cube (i.e. array of all ones)

$$w_i \equiv 1$$

Add $\hat{x}_i^{w}$ to its corrsponding position in $\hat{x}$
Add $\hat{w}_i$ to its corrsponding position in $w$

Compensating the effect of accumulation

$$\hat{x} = \hat{x}^{w}\oslash{w}$$

Syntax 📜

function interface

    sliding_filter(noisy_img, noise_std, transform_type=None, partition_size=None, step_size=None, shrinkage_type=None)
    """
    Args:
    - noisy_img (ndarray): 2D/3D noisy image (2D: monochromatic; 3D: RGB, hyperspectral, ...)

    - nois_std   (float) : noise standard deviation    
      
    - transform_type (list/tuple of str) : sparsifying/decorrelating transform
              the string can be 'dct', 'fft', 'hadamard', or anything that is listed by
              'pywt.wavelist()' (e.g., 'haar', 'db', 'sym', 'coif', 'bior', 'rbio', 'dmey'),
              identity transform, 'DCrand'-- an orthonormal transform with a DC, and
              all the other basis elements of random nature. The transform in the list/tuple
              of transform_type can be different (combination of separable transforms), e.g., 
                    transform_type = ('bior1.5','haar','dct')[:noisy_img] 
              or the same, e.g.,
                    transform_type = ('dct',)*len(noisy_img)
              
    - partition_size (list/tuple of int): size of each extracted block/cube.
               Note that for a chosen wavelet as transform_type, its corresponding partition 
               size must be dyadic 2^K.       

    - step_size (str, or list/tuple of int),               
              if the type is either 'str' , then step_size takes the following options:
              - "sliding"  : filter operates on "fully"-overlapped blocks/cubes (i.e. the
              two adjacent blocks/cubes have one pixel slide toward a dimension)                
              - "distinct" : filter operates on non-overlapping parsing blocks/cubes
              if the type is list or tuple, then step_size indicates the step-size (stride)
               toward each dimension.
               
    - shrinkage_type (str) : type of shrinkage operator, which can be either of the following:
              - "h" or "hard"   : hard-shrinkage (default)
              - "s" or "soft"   : soft-shrinkage

              
    Returns:
    - denoised_img (ndarray): 2D/3D denoised image.
    """

Denoising demo 🔍

simply run the following command to see a denoising demo

    python3 sliding_filter.py 

Demo of the traditional procedure vs. the implemented one 💡

Traditional sliding filtering approach proceeding block-by-block

demo_filtering_typical.mp4

Implemented approach using sliding filter, where extraction/aggregation of blocks is done by imreshape

demo_filtering_no_forloop.mp4

Disclaimer ©️

Copyright © 2023 Nasser Eslahi. All right reserved.

This project is licensed under the MIT License - see the LICENSE file for details.

Feedback 🗣️ 📬

If you have any comment, suggestion, or question, please do contact Nasser Eslahi ⛄
nasser.eslahi@tuni.fi
nasser.eslahi@gmail.com