IMAGE_CONTRAST
Enhance Contrast in an Image

IMAGE_CONTRAST, a MATLAB program which carries out a simple contrast enhancement operation on an image.

Note that MATLAB's image processing toolbox includes functions such as nlfilter() 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.

For a gray-scale image, the function has the form:

function gray_enhanced = contrast_gray ( gray, s )

• gray is an m by n uint8 (short integers, 0 to 255) array containing the gray scale values; MATLAB's imread() command can be used to create such a dataset from a graphics image file.
• s is the sharpness parameter. Normally, s is greater than 1, and larger values increase the contrast.
• gray_enhanced is an m by n uint8 (short integer) array containing the gray scale values after the contrast has been enhanced.

The corresponding function contrast_rgb() operates on RGB images.

Licensing:

The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.

Languages:

IMAGE_CONTRAST is available in a MATLAB version.

Related Data and Programs:

CONTRAST_SPMD, 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.

CONTRAST2_SPMD, 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.

IMAGE_COMPONENTS, a MATLAB library which seeks the connected "nonzero" or "nonblack" components of an image or integer vector, array or 3D block.

image_contrast, matlab programs which apply image processing techniques to increase the contrast in an image.

image_contrast_test

IMAGE_DECIMATE, a MATLAB library which compresses an image by dropping the even rows and columns of data.

IMAGE_DENOISE, a MATLAB program which demonstrates a simple technique for removing noise from an image.

IMAGE_DOUBLE, a MATLAB library which doubles the height and width of an image by repeating each row and column.

IMAGE_EDGE a MATLAB library which demonstrates a simple procedure for edge detection in images.

IMAGE_MATCH_GENETIC, 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.

IMAGE_NOISE, MATLAB programs which add noise to an image.

IMAGE_QUANTIZATION, 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.

IMAGE_RGB_TO_GRAY, MATLAB programs which makes a grayscale version of an RGB image.

IMAGE_THRESHOLD, 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.

Reference:

MathWorks documentation for the Image Processing Toolbox is available at http://www.mathworks.com/access/helpdesk/help/pdf_doc/images/images_tb.pdf.

• Jonas Gomes, Luiz Velho,
  Image Processing for Computer Graphics,
  Springer, 1997,
  ISBN: 0387948546,
  LC: T385.G65.
• William Pratt,
  Digital Image Processing,
  Second Edition,
  Wiley, 1991,
  ISBN13: 978-0471857662,
  LC: TA1632.P7.

Source Code:

• contrast_gray.m, applies the contrast enhancement procedure to a gray-scale image.
• contrast_rgb.m, applies the contrast enhancement procedure to an RGB image.
• timestamp.m, prints the current YMDHMS date as a timestamp;

Examples and Tests:

• image_contrast_test.m, calls all the tests.
• image_contrast_test.sh, runs all the tests.
• image_contrast_test.txt, the output file.

SURFSUP is a grayscale image.

• surfsup.png, the original image, somewhat blurry.
• surfsup_contrast_0.25.png, the image, with contrast S = 0.25. The result is blurrier.
• surfsup_contrast_5.00.png, the image, with contrast S = 5.00.

UNDERWATER_BMX 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.

• underwater_bmx.png, the original image, somewhat blurry.
• underwater_bmx_contrast_5.00.png, the image, with contrast S = 5.00.
• underwater_bmx_contrast_10.00.png, the image, with contrast S = 10.00.