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<html>
<head>
<title>
IMAGE_CONTRAST - Enhance Contrast in an Image
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
IMAGE_CONTRAST <br> Enhance Contrast in an Image
</h1>
<hr>
<p>
<b>IMAGE_CONTRAST</b>
is a MATLAB function which
carries out a simple contrast enhancement operation on an image.
</p>
<p>
Note that MATLAB's image processing toolbox includes a functions such
as <b>nlfilter</b> which can do a more sophisticated treatment of images.
But the purpose of this example is to demonstrate the process of image
enhancement in a way that is clear enough to understand quickly.
</p>
<p>
For a gray-scale image, the function has the form:
<blockquote>
function <i>gray_enhanced</i> = <b>contrast_gray</b> ( <i>gray</i>, <i>s</i> )
</blockquote>
where
<ul>
<li>
<i>gray</i> is an <i>m</i> by <i>n</i> <b>uint8</b>
(short integers, 0 to 255) array containing the gray scale values;
MATLAB's <b>imread()</b> command can be used to create such a
dataset from a graphics image file.
</li>
<li>
<i>s</i> is the sharpness parameter. Normally, <b>s</b> is
greater than 1, and larger values increase the contrast.
</li>
<li>
<i>gray_enhanced</i> is an <i>m</i> by <i>n</i> <b>uint8</b>
(short integer) array containing the gray scale values
after the contrast has been enhanced.
</li>
</ul>
</p>
<p>
The corresponding function <b>contrast_rgb()</b> operates on RGB images.
</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_CONTRAST</b> is available in
<a href = "../../m_src/image_contrast/image_contrast.html">a MATLAB version</a>.
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../m_src/contrast_spmd/contrast_spmd.html">
CONTRAST_SPMD</a>,
a MATLAB program which
demonstrates the SPMD parallel programming feature for a image operations;
the client reads an image, the workers process portions of it to enhance
the contrast, and the client assembles and displays the results.
</p>
<p>
<a href = "../../m_src/contrast2_spmd/contrast2_spmd.html">
CONTRAST2_SPMD</a>,
a MATLAB program which
demonstrates the SPMD parallel programming feature for image operations;
this improves the contrast_spmd program by allowing the workers to share some
data; this makes it possible to eliminate artificial "seams" in the processed
image.
</p>
<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_denoise/image_denoise.html">
IMAGE_DENOISE</a>,
a MATLAB program which
demonstrates a simple technique for removing noise from 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 = "contrast_gray.m">contrast_gray.m</a>,
applies the contrast enhancement procedure to a gray-scale image.
</li>
<li>
<a href = "contrast_rgb.m">contrast_rgb.m</a>,
applies the contrast enhancement procedure to an RGB image.
</li>
</ul>
</p>
<h3 align = "center">
Examples and Tests:
</h3>
<p>
<b>SURFSUP</b> is a grayscale image.
<ul>
<li>
<a href = "surfsup.png">surfsup.png</a>,
the original image, somewhat blurry.
</li>
<li>
<a href = "surfsup_contrast_0.25.png">surfsup_contrast_0.25.png</a>,
the image, with contrast S = 0.25. The result is blurrier.
</li>
<li>
<a href = "surfsup_contrast_5.00.png">surfsup_contrast_5.00.png</a>,
the image, with contrast S = 5.00.
</li>
</ul>
</p>
<p>
<b>UNDERWATER_BMX</b> is an RGB image. Note that attempts to increase the
blurriness of this image using a value of S less than 1 didn't seem to have
much effect.
<ul>
<li>
<a href = "underwater_bmx.png">underwater_bmx.png</a>,
the original image, somewhat blurry.
</li>
<li>
<a href = "underwater_bmx_contrast_5.00.png">underwater_bmx_contrast_5.00.png</a>,
the image, with contrast S = 5.00.
</li>
<li>
<a href = "underwater_bmx_contrast_10.00.png">underwater_bmx_contrast_10.00.png</a>,
the image, with contrast S = 10.00.
</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>