Contrast Adjustment

Adjust image contrast using linear scaling

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Libraries:
Computer Vision Toolbox / Analysis & Enhancement

Description

The Contrast Adjustment block adjusts the contrast of an image by linearly scaling the pixel values between specified upper and lower limits. Pixel values that are above or below the specified range are saturated to the upper or lower limit value, respectively. For more information, see Adjust Contrast of Image.

Examples

Adjust Contrast of Image

Adjust the contrast of an image by linearly scaling pixel values.

• Open Script

Ports

Input

I — Input image

matrix | array

Input image, specified as an M-by-N matrix or an M-by-N-by-C array, where C is the number of color channels in the input image.

Data Types: single | double | int8 | int16 | uint8 | uint16 | Boolean | fixed point

Output

Port_1 — Output image

matrix | array

Output image, returned as an M-by-N matrix or an M-by-N-by-C array, where C is the number of color channels.

Data Types: single | double | int8 | int16 | uint8 | uint16 | Boolean | fixed point

Parameters

Main

Adjust pixel values from — Input pixel value range definition

Full input data range [min max] (default) | User-defined range | Range determined by saturating outlier pixels

Specify how the block determines the pixel value range of the input image as one of these options:

• Full input data range [min max] — Range set to the minimum and maximum pixel values in the input image.

• User-defined range — Select this value to enable a Range [low high] parameter. Specify the Range [low high] parameter as a two-element vector of nonnegative values, in which the first element is the lower limit of the input pixel value range and the second element is the upper limit of the input pixel value range.

• Range determined by saturating outlier pixels — Range specified by saturating the outlier pixels. The block determines the range based on the Percentage of pixels to saturate [low high], Specify number of histogram bins (used to calculate the range when outliers are eliminated), and Number of histogram bins parameters. For more information, see Determine Contrast Adjustment Range by Saturating Outlier Pixels.

Range [low high] — Input pixel value range

[0 255] (default) | two-element vector of nonnegative numbers

Specify the input pixel value range as a two-element vector of nonnegative numbers. The first and second elements of the vector specify the lower and upper limit of the input pixel value range, respectively.

Dependencies

To enable this parameter, set the Adjust pixel values from parameter to User-defined.

Percentage of pixels to saturate [low high] — Maximum percentages of pixels to saturate

[1 1] (default) | two-element vector of nonnegative numbers

Specify the maximum percentages of pixels to saturate as a two-element vector of nonnegative numbers. Each element must be in the range 0 to 100. The block calculates the lower limit of the input pixel value range such that the percentage of pixels in the input image with values less than the lower limit is at most the value of the first element. Similarly, the block calculates the upper limit of the input pixel value range such that the percentage of pixels in the input image with values greater than the upper limit is at most the value of the second element.

Dependencies

To enable this parameter, set the Adjust pixel values from parameter to Range determined by saturating outlier pixels.

Specify number of histogram bins (used to calculate the range when outliers are eliminated) — Custom number of histogram bins

on (default) | off

Select this parameter to enable the Number of histogram bins parameter, which enables you to specify a custom number of histogram bins for the block to use to calculate the input pixel value range after outliers are eliminated.

Dependencies

To enable this parameter, set the Adjust pixel values from parameter to Range determined by saturating outlier pixels.

Number of histogram bins — Number of histogram bins

256 (default) | positive integer

Specify the number of histogram bins to use to calculate the range for scaling the input pixel values.

Dependencies

To enable this parameter, set the Adjust pixel values from parameter to Range determined by saturating outlier pixels, and select the Specify number of histogram bins (used to calculate the range when outliers are eliminated) parameter.

Adjust pixel values to — Output pixel value range definition

Full data type range (default) | User-defined range

Specify how the block determines the pixel value range of the output image as one of these options:

• Full data type range — Specifies the output range of pixel values as from the minimum to the maximum value of the input data type.

• User-defined range — Select this value to enable a Range [low high] parameter. Specify the Range [low high] parameter as a two-element vector of nonnegative values, in which the first element is the lower limit of the output pixel value range and the second element is the upper limit of the output pixel value range.

Range [low high] — Output pixel value range

[0 255] (default) | two-element vector of nonnegative numbers

Specify the output pixel value range as a two-element vector of nonnegative numbers. The first and second elements of the vector specify the lower and upper limit of the output pixel value range, respectively.

Dependencies

To enable this parameter, set the Adjust pixel values to parameter to User-defined range.

Data Types

For details on the fixed-point block parameters, see Specify Fixed-Point Attributes for Blocks (DSP System Toolbox).

Lock data type settings against change by the fixed-point tools — Data type override

off (default) | on

Select this parameter to prevent the fixed-point tools from overriding the data types you specify in this block. For more information, see Lock the Output Data Type Setting (Fixed-Point Designer).

Block Characteristics

Data Types	double | fixed point | integer | single
Multidimensional Signals	no
Variable-Size Signals	yes

More About

The following diagram shows the data types used in the Contrast Adjustment block for fixed-point signals:

Algorithms

Adjust Contrast of Image

The contrast adjustment operation is defined by this equation.

$$O_i=\left\{\begin{array}{c}{l}_{\text{out}}\begin{array}{cc},& I_i\le l_{\text{in}}\end{array}\\ l_{\text{out}}+(I_i-l_{\text{in}})\frac{h_{\text{out}}-l_{\text{out}}}{h_{\text{in}}-l_{\text{in}}}\begin{array}{cc},& l_{\text{in}}<I_i<h_{\text{in}}\end{array}\\ h_{\text{out}}\begin{array}{cc},& I_i\ge h_{\text{in}}\end{array}\end{array}\right\},$$

where I_i and O_i are the input and output image pixels, the input pixel value range is [l_in h_in] and the output pixel value range is [l_out h_out].

Determine Contrast Adjustment Range by Saturating Outlier Pixels

When you set the Adjust pixel values from parameter to Range determined by saturating outlier pixels, the block determines the input pixel value range using these steps.

1. Finds the minimum input pixel value min_in and maximum input pixel value max_in.

2. Scales the pixel values in the range [min_in, max_in] to [0, num_bins–1], where num_bins is the number of histogram bins specified by the Number of histogram bins parameter. Then, the block calculates the histogram of the scaled input. For more information on image histograms, see the 2-D Histogram block.

3. Determines the lower input pixel value limit such that the percentage of pixels in the input image with values less than the lower limit is at most the value of the first element of the Percentage of pixels to saturate [low high] parameter. Similarly, find the upper input pixel value limit such that the percentage of pixels in the input image with values greater than the upper limit is at most the value of the second element of the parameter.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2006b