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<html>
<head>
<title>
IMAGE_DENOISE - Remove Noise from an Image
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
IMAGE_DENOISE <br> Remove Noise from an Image
</h1>
<hr>
<p>
<b>IMAGE_DENOISE</b>
is a MATLAB program which
uses the median filter to try to remove noise from an image.
</p>
<p>
In MATLAB, a black and white or gray scale image can be represented
using a 2D array of nonnegative integers over some range 0 to GMAX.
The value 0 indicates black, and GMAX white. Intermediate values
represent shades of gray in a natural way. Note, however, that
the eye has a nonlinear response to intensity, so that the value
GMAX/2 will not be perceived as halfway between 0 and GMAX. That
is a separate issue.
</p>
<p>
A color image can be represented using a set of three 2D arrays,
which can be thought of as R, G, and B, and which represent the
intensity of the red, green and blue signals that combine to
form the color image. A common maximum value is assumed, RGBMAX.
</p>
<p>
An image can be read into MATLAB using the <b>imread</b> function
in the Image Processing Toolbox.
</p>
<p>
In the example considered here, a good image is damaged by the addition of
"salt and pepper" noise; that is, a scattering of individual pixels have
been randomly reset to the lowest or highest possible values. In a
gray scale picture, such noise looks as though salt and pepper were
added to the picture.
</p>
<p>
<image src = "balloons_noisy.png" WIDTH=320 HEIGHT=240
ALT="A Noisy Image">
</p>
<p>
Since an image is fairly "smooth", each good pixel should actually be
fairly close to the values of good pixels nearby, while this will not
be true for the salt and pepper pixels. So one way to make the noise
go away, at the cost of some minor blurring, is to replace each pixel
by the median value of itself and its neighbors.
</p>
<p>
MATLAB provides a command <b>medfilt2</b> which will do this, with
the user allowed to specify the shape of a rectangular neighborhood
about each pixel. However, in this example, we go through some of
the lower level details "by hand", using finally the <b>median</b>
command to get what we want.
</p>
<p>
Along the way, it should become clear how to deal with grayscale
and RGB images. Moreover, some other issues arise, such as ensuring
that the processed data retains the "unsigned 8-bit integer" property
of the original data.
</p>
<p>
The functions <b>image_denoise_gray_news</b> and
<b>image_denoise_rgb_news</b> use the central pixel and its
north, east, west and south neighbors.
</p>
<p>
The functions <b>image_denoise_gray_3x3</b> and
<b>image_denoise_rgb_3x3</b> use the central pixel and the entire
layer of 8 pixels that lie at most one pixel away in the
north/south and east/west directions.
</p>
<h3 align = "center">
Licensing:
</h3>
<p>
The computer code and data files described and made available on this web page
are distributed under
<a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
</p>
<h3 align = "center">
Languages:
</h3>
<p>
<b>IMAGE_DENOISE</b> is available in
<a href = "../../c_src/image_denoise/image_denoise.html">a C version</a> and
<a href = "../../cpp_src/image_denoise/image_denoise.html">a C++ version</a> and
<a href = "../../f77_src/image_denoise/image_denoise.html">a FORTRAN77 version</a> and
<a href = "../../f_src/image_denoise/image_denoise.html">a FORTRAN90 version</a> and
<a href = "../../m_src/image_denoise/image_denoise.html">a MATLAB version</a>.
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../m_src/image_components/image_components.html">
IMAGE_COMPONENTS</a>,
a MATLAB library which
seeks the connected "nonzero" or "nonblack" components of an image or integer vector,
array or 3D block.
</p>
<p>
<a href = "../../m_src/image_contrast/image_contrast.html">
IMAGE_CONTRAST</a>,
a MATLAB program which
applies image processing techniques to increase the contrast in an image.
</p>
<p>
<a href = "../../m_src/image_denoise_spmd/image_denoise_spmd.html">
IMAGE_DENOISE_SPMD</a>,
a MATLAB program which
demonstrates the SPMD parallel programming feature for image operations;
the client reads an image, the workers process portions of it, and
the client assembles and displays the results.
</p>
<p>
<a href = "../../m_src/image_diffuse/image_diffuse.html">
IMAGE_DIFFUSE</a>,
a MATLAB library which
uses diffusion to smooth out an image.
</p>
<p>
<a href = "../../m_src/image_edge/image_edge.html">
IMAGE_EDGE</a>,
a MATLAB library which
demonstrates a simple procedure for edge detection in images.
</p>
<p>
<a href = "../../m_src/image_match_genetic/image_match_genetic.html">
IMAGE_MATCH_GENETIC</a>,
a MATLAB program which
tries to match a 256x256 JPEG image by blending 32 colored rectangles,
using ideas from genetic algorithms,
based on an example by Nick Berry.
</p>
<p>
<a href = "../../m_src/image_noise/image_noise.html">
IMAGE_NOISE</a>,
MATLAB programs which
add noise to an image.
</p>
<p>
<a href = "../../m_src/image_quantization/image_quantization.html">
IMAGE_QUANTIZATION</a>,
a MATLAB library which
demonstrates how the KMEANS algorithm can be used to reduce the number
of colors or shades of gray in an image.
</p>
<p>
<a href = "../../m_src/image_rgb_to_gray/image_rgb_to_gray.html">
IMAGE_RGB_TO_GRAY</a>,
MATLAB programs which
makes a grayscale version of an RGB image.
</p>
<p>
<a href = "../../m_src/image_threshold/image_threshold.html">
IMAGE_THRESHOLD</a>,
MATLAB programs which
make a black and white version of a grayscale image by setting all pixels
below or above a threshold value to black or white.
</p>
<h3 align = "center">
Reference:
</h3>
<p>
MathWorks documentation for the Image Processing Toolbox is available at
<a href = "http://www.mathworks.com/access/helpdesk/help/pdf_doc/images/images_tb.pdf">
http://www.mathworks.com/access/helpdesk/help/pdf_doc/images/images_tb.pdf</a>.
</p>
<p>
<ul>
<li>
Jonas Gomes, Luiz Velho,<br>
Image Processing for Computer Graphics,<br>
Springer, 1997,<br>
ISBN: 0387948546,<br>
LC: T385.G65.
</li>
<li>
William Pratt,<br>
Digital Image Processing,<br>
Second Edition,<br>
Wiley, 1991,<br>
ISBN13: 978-0471857662,<br>
LC: TA1632.P7.
</li>
</ul>
</p>
<h3 align = "center">
Source Code:
</h3>
<p>
<ul>
<li>
<a href = "image_denoise_gray_3x3.m">image_denoise_gray_3x3.m</a>,
applies the 3x3 neighborhood median filter to remove noise from
a grayscale image.
</li>
<li>
<a href = "image_denoise_gray_news.m">image_denoise_gray_news.m</a>,
applies the NEWS neighborhood median filter to remove noise from
a grayscale image.
</li>
<li>
<a href = "image_denoise_rgb_3x3.m">image_denoise_rgb_3x3.m</a>,
applies the 3x3 neighborhood median filter to remove noise from
an RGB image.
</li>
<li>
<a href = "image_denoise_rgb_news.m">image_denoise_rgb_news.m</a>,
applies the NEWS neighborhood median filter to remove noise from
an RGB image.
</li>
</ul>
</p>
<h3 align = "center">
Examples and Tests:
</h3>
<p>
<b>BALLOONS</b> is an RGB image of a couple holding balloons.
<ul>
<li>
<a href = "balloons.tif">balloons.tif</a>,
a TIF RGB image.
</li>
<li>
<a href = "balloons.png">balloons.png</a>,
a PNG version of the image.
</li>
<li>
<a href = "balloons_noisy.tif">balloons_noisy.tif</a>,
the noisy image.
</li>
<li>
<a href = "balloons_noisy.png">balloons_noisy.png</a>,
a PNG version of the image.
</li>
<li>
<a href = "balloons_3x3.png">balloons_3x3.png</a>,
the image after filtering with the 3x3 median.
</li>
<li>
<a href = "balloons_news.png">balloons_news.png</a>,
the image after filtering with the NEWS median.
</li>
</ul>
</p>
<p>
<b>GLASSWARE</b> is a grayscale image of a few glass vases.
<ul>
<li>
<a href = "glassware_noisy.pgm">glassware_noisy.pgm</a>,
the noisy image.
</li>
<li>
<a href = "glassware_noisy.png">glassware_noisy.png</a>,
a PNG version of the image.
</li>
<li>
<a href = "glassware_news.pgm">glassware_news.pgm</a>,
the image after filtering with the NEWS median.
</li>
<li>
<a href = "glassware_news.png">glassware_news.png</a>,
a PNG version of the image.
</li>
<li>
<a href = "glassware_3x3.pgm">glassware_3x3.pgm</a>,
the image after filtering with the 3x3 median.
</li>
<li>
<a href = "glassware_3x3.png">glassware_3x3.png</a>,
a PNG version of the image.
</li>
</ul>
</p>
<p>
You can go up one level to <a href = "../m_src.html">
the MATLAB source codes</a>.
</p>
<hr>
<i>
Last revised on 25 December 2010.
</i>
<!-- John Burkardt -->
</body>
</html>