stretchlim

Find limits to contrast stretch image

Syntax

Low_High = stretchlim(I)

Low_High = stretchlim(I,Tol)

Low_High = stretchlim(RGB,Tol)

Low_High = stretchlim(gpuarrayI,___)

Description

Low_High = stretchlim(I) returns Low_High, a two-element vector of pixel values that specify lower and upper limits that can be used for contrast stretching image I. By default, values in Low_High specify the bottom 1% and the top 1% of all pixel values. The gray values returned can be used by the imadjust function to increase the contrast of an image.

Low_High = stretchlim(I,Tol) returns Low_High, a two-element vector of pixel values that specify lower and upper limits that can be used for contrast stretching image I. where Tol specifies the fraction of the image to saturate at low and high pixel values.

Low_High = stretchlim(RGB,Tol) returns Low_High, a two-element vector of pixel values that specify lower and upper limits that can be used for contrast stretching truecolor image RGB.

Low_High = stretchlim(gpuarrayI,___) performs the operation on a GPU. This syntax requires the Parallel Computing Toolbox™.

Examples

Find Limits to Stretch Contrast in Grayscale Image

Read grayscale image into the workspace and display it.

I = imread('pout.tif');
figure
imshow(I)

Adjust the contrast in the image using stretchlim to set the limits, and display the result. The example uses the default limits [0.01 0.99], saturating the upper 1% and the lower 1%.

J = imadjust(I,stretchlim(I),[]);
figure
imshow(J)

Find Limits to Stretch Contrast in Grayscale Image on a GPU

Read grayscale image, creating a gpuArray.

gpuarrayI = gpuArray(imread('pout.tif'));
figure, imshow(gpuarrayI)

Adjust the contrast in the image using stretchlim to set the limits. Display the result.

gpuarrayJ = imadjust(gpuarrayI,stretchlim(I),[]);
figure, imshow(gpuarrayJ)

Input Arguments

I — Grayscale image

real, nonsparse, numeric array

Grayscale image, specified as a real, nonsparse, numeric array.

Example: I = imread('pout.tif'); lohi = stretchlim(I);

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical

Tol — Fraction of the image to saturate

[0.01 0.99] (default) | numeric scalar or two-element vector in the range [0 1]

Fraction of the image to saturate, specified as a numeric scalar or two-element vector [Low_Fract High_Fract] in the range [0 1].

Value	Description
Scalar	If Tol is a scalar, Low_Fract = Tol, and High_Fract = 1 - Low_Fract, which saturates equal fractions at low and high pixel values.
0	If Tol = 0, Low_High = [min(I(:)); max(I(:))].
Default	If you omit the Tol argument, [Low_Fract High_Fract] defaults to [0.01 0.99], saturating 2%.
Too big	If Tol is too big, such that no pixels would be left after saturating low and high pixel values, stretchlim returns [0 1]

Example: lohi = stretchlim(I,[.02 .80]);

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

RGB — Truecolor image

real, nonsparse, numeric array

Truecolor image, specified as a numeric array.

Example: RGB = imread('peppers.png'); lohi = stretchlim(RGB);

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32

gpuarrayI — Input image

gpuArray

Input image, specified as a gpuArray.

Output Arguments

Low_High — Lower and upper limits for contrast stretching

two-element vector of pixel values

Lower and upper limits for contrast stretching, returned as a two-element vector of pixel values

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• This function supports the generation of C code using MATLAB^® Coder™. Note that if you choose the generic MATLAB Host Computer target platform, the function generates code that uses a precompiled, platform-specific shared library. Use of a shared library preserves performance optimizations but limits the target platforms for which code can be generated. For more information, see Understand Code Generation with Image Processing Toolbox.