Specify Contrast Adjustment Limits
You can optionally specify the range of the input values and the output values using
imadjust
. You specify these ranges in
two vectors that you pass to imadjust
as arguments. The first
vector specifies the low- and high-intensity values that you want to map. The second
vector specifies the scale over which you want to map them.
Note
You must specify the intensities as values between 0 and 1 regardless of the class
of I
. If I
is uint8
, the
values you supply are multiplied by 255 to determine the actual values to use; if
I
is uint16
, the values are multiplied by
65535. To learn about an alternative way to set these limits automatically, see
Set Image Intensity Adjustment Limits Automatically.
Specify Contrast Adjustment Limits as Range
This example shows how to specify contrast adjustment limits as a range using the imadjust
function. This example decreases the contrast of an image by narrowing the range of the data.
Read an image into the workspace.
I = imread('cameraman.tif');
Adjust the contrast of the image, specifying the range of values used in the output image. In the example below, the man's coat is too dark to reveal any detail. imadjust
maps the range [0,51]
in the uint8
input image to [128,255]
in the output image. This brightens the image considerably, and also widens the dynamic range of the dark portions of the original image, making it much easier to see the details in the coat. Note, however, that because all values above 51 in the original image are mapped to 255 (white) in the adjusted image, the adjusted image appears washed out.
J = imadjust(I,[0 0.2],[0.5 1]);
Display the original image and the contrast-adjusted image.
imshowpair(I,J,'montage')
Set Image Intensity Adjustment Limits Automatically
For a more convenient way to specify limits, use the stretchlim
function. (The imadjust
function
uses stretchlim
for its simplest syntax,
imadjust(I)
.)
This function calculates the histogram of the image and determines the adjustment
limits automatically. The stretchlim
function returns these
values as fractions in a vector that you can pass as the [low_in
high_in]
argument to imadjust
; for
example:
I = imread("rice.png");
J = imadjust(I,stretchlim(I),[0 1]);
By default, stretchlim
uses the intensity values that
represent the bottom 1% (0.01) and the top 1% (0.99) of the range as the adjustment
limits. By trimming the extremes at both ends of the intensity range,
stretchlim
makes more room in the adjusted dynamic range
for the remaining intensities. But you can specify other range limits as an argument
to stretchlim
.