Digital Image Processing

Digital image processing is the use of computer algorithms to perform image processing on digital images. As a subcategory or field of digital signal processing, digital image processing has many advantages over analog image processing. It allows a much wider range of algorithms to be applied to the input data and can avoid problems such as the build-up of noise and signal distortion during processing. Since images are defined over two dimensions (perhaps more) digital image processing may be modeled in the form of multidimensional systems.

History

Many of the techniques of digital image processing, or digital picture processing as it often was called, were developed in the 1960s at the Jet Propulsion Laboratory, Massachusetts Institute of Technology, Bell Laboratories, University of Maryland, and a few other research facilities, with application to satellite imagery, wire-photo standards conversion, medical imaging, videophone, character recognition, and photograph enhancement.[1] The cost of processing was fairly high, however, with the computing equipment of that era. That changed in the 1970s, when digital image processing proliferated as cheaper computers and dedicated hardware became available. Images then could be processed in real time, for some dedicated problems such as television standards conversion. As general-purpose computers became faster, they started to take over the role of dedicated hardware for all but the most specialized and computer-intensive operations. With the fast computers and signal processors available in the 2000s, digital image processing has become the most common form of image processing and generally, is used because it is not only the most versatile method, but also the cheapest.

Digital image processing technology for medical applications was inducted into the Space Foundation Space Technology Hall of Fame in 1994.[2]

In 2002 Raanan Fattel, introduced Gradient domain image processing, a new way to process images in which the differences between pixels are manipulated rather than the pixel values themselves.[3]

Tasks

Digital image processing allows the use of much more complex algorithms, and hence, can offer both more sophisticated performance at simple tasks, and the implementation of methods which would be impossible by analog means.

In particular, digital image processing is the only practical technology for:

Some techniques which are used in digital image processing include:


Digital image transformations

Filtering

Digital filters are used to blur and sharpen digital images. Filtering can be performed in the spatial domain by convolution with specifically designed kernels (filter array), or in the frequency (Fourier) domain by masking specific frequency regions. The following examples show both methods: [4]


Filter type Kernel or mask Example
Original Image Affine Transformation Original Checkerboard.jpg
Spatial Lowpass Spatial Mean Filter Checkerboard.png
Spatial Highpass Spatial Laplacian Filter Checkerboard.png
Fourier Representation Pseudo-code:

image = checkerboard

F = Fourier Transform of image

Show Image: log(1+Absolute Value(F))

Fourier Space Checkerboard.png
Fourier Lowpass Lowpass Butterworth Checkerboard.png Lowpass FFT Filtered checkerboard.png
Fourier Highpass Highpass Butterworth Checkerboard.png Highpass FFT Filtered checkerboard.png


Image padding in Fourier domain filtering

Images are typically padded before being transformed to the Fourier space, the highpass filtered images below illustrate the consequences of different padding techniques:


Zero padded Repeated edge padded
Highpass FFT Filtered checkerboard.png Highpass FFT Replicate.png

Notice that the highpass filter shows extra edges when zero padded compared to the repeated edge padding.

Filtering Code Examples

MATLAB example for spatial domain highpass filtering.

img=checkerboard(20);                           % generate checkerboard
% **************************  SPATIAL DOMAIN  ***************************
klaplace=[0 -1 0; -1 5 -1; 0 -1 0];             % Laplacian filter kernel
X=conv2(img,klaplace);                          % convolve test img with
                                                % 3x3 Laplacian kernel
figure
imshow(X,[])                                    % show Laplacian filtered 
title('Laplacian Edge Detection')

MATLAB example for Fourier domain highpass filtering.

img=checkerboard(20);                           % generate checkerboard
% **************************  FOURIER DOMAIN  ***************************
pad=paddedsize([m,n]);                          % get padding size
imgp=padarray(img,[pad(1),pad(2)],'both');      % set pad before & after
[p,q]=size(imgp);                               % get size of padded img
fftpad=fft2(imgp);                              % Fourier transform
F=fftshift(fftpad);                             % shift low freq to middle

Hlp=fftshift(lpfilter('btw',p,q,60));           % get butterworth filter
Hhp=1-Hlp;                                      % get highpass
HPimg=abs(ifft2(F.*Hhp));                       % apply filter and IFFT
figure(7)
imshow(Hhp,[])                                  % show the filter 
title('Highpass Butterworth')
figure(8)                                       % show result cropped
imshow(HPimg(round((p-n)/2):round(n+(p-n)/2),round((q-m)/2):round(m+(q-m)/2)),[])
title('FFT Highpass Filtered')

Affine transformations

Affine transformations enable basic image transformations including scale, rotate, translate, mirror and sheer as is shown in the following examples show:[5]

Transformation Name Affine Matrix Example
Identity Affine Transformation Original Checkerboard.jpg
Reflection Affine Transformation Reflected Checkerboard.jpg
Scale Affine Transformation Scale Checkerboard.jpg
Rotate Affine Transformation Rotated Checkerboard.jpg
Shear Affine Transformation Shear Checkerboard.jpg

Applications

Digital camera images

Digital cameras generally include specialized digital image processing hardware - either dedicated chips or added circuitry on other chips - to convert the raw data from their image sensor into a color-corrected image in a standard image file format

Film

Westworld (1973) was the first feature film to use the digital image processing to pixellate photography to simulate an android's point of view.[6]

See also

References

  1. ^ Azriel Rosenfeld, Picture Processing by Computer, New York: Academic Press, 1969
  2. ^ "Space Technology Hall of Fame:Inducted Technologies/1994". Space Foundation. 1994. Archived from the original on 4 July 2011. Retrieved 2010. 
  3. ^ Bhat, Pravin, et al. "Gradientshop: A gradient-domain optimization framework for image and video filtering." ACM Transactions on Graphics 29.2 (2010): 10.
  4. ^ Gonzalez, Rafael (2008). 'Digital Image Processing, 3rd'. Pearson Hall. ISBN 9780131687288. 
  5. ^ Gonzalez, Rafael (2008). 'Digital Image Processing, 3rd'. Pearson Hall. ISBN 9780131687288. 
  6. ^ A Brief, Early History of Computer Graphics in Film Archived 17 July 2012 at the Wayback Machine., Larry Yaeger, 16 August 2002 (last update), retrieved 24 March 2010

Further reading

  • R. Fisher; K Dawson-Howe; A. Fitzgibbon; C. Robertson; E. Trucco (2005). Dictionary of Computer Vision and Image Processing. John Wiley. ISBN 978-0-470-01526-1. 
  • Rafael C. Gonzalez; Richard E. Woods; Steven L. Eddins (2004). Digital Image Processing using MATLAB. Pearson Education. ISBN 978-81-7758-898-9. 
  • Milan Sonka; Vaclav Hlavac; Roger Boyle (1999). Image Processing, Analysis, and Machine Vision. PWS Publishing. ISBN 978-0-534-95393-5. 

External links


  This article uses material from the Wikipedia page available here. It is released under the Creative Commons Attribution-Share-Alike License 3.0.


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