the purpose of image processing?
in image processing. Correction of histogram.
Сonvolution methods. Binarization.
in Adobe Photoshop
The Photoshop Curves tool
Lightness channel and adjustments curves
The Exposure tool
Black & White film
What is the purpose of image processing?
Image processing has multiple purpose.
• To improve the quality of an image in subjective way, usually by increasing its contrast. This is called
• To use az few bits as possible to represent the image, with minimum deterioration in its quality. This is
called image compression.
• To improve an image in an objective way, for axample by reducing its blurring. This is called image
• To make explicit certain characteristics of the image which can be used to identify the contents of the
image. This is called feature extraction.
Methods in image processing
Correction of histogram
The definition of a histogram is:
A bar graph of a frequency distribution in which the
widths of the bars are proportional to the classes into
which the variable has been divided and the heights of
the bars are proportional to the class frequencies.
Convolution is the modification of a pixel’s value on the basis of the value of neighbouring pixels.
One of the most powerful techniques in all of image processing is convolution.
Convolution is the modification of a pixel's value on the basis of the value of neighboring pixels.
Images are convolved by multiplying each pixel and its neighbors by a numerical matrix, called a kernel. This matrix
is essentially moved over each pixel in the image, each pixel under the matrix is multiplied by the appropriate matrix
value, the total is summed and normalized, and the central pixel is replaced by the result.
Cx,y = å(å(Pi,j*Mi,j))/(å(å(Mi,j))
Binary images are images that have been quantised to two values, usually denoted 0 and 1, but often with pixel values 0
and 255, representing black and white.
Binary images are used in many applications since they are the simplest to process, but they are such an impoverished
representation of the image information that their use is not always possible. However, they are useful where all the in-
formation you need can be provided by the silhouette of the object and when you can obtain the silhouette of that object
Figure 1: The ideal histogram of a light object on a
darker plain background.
Figure 2: The histogram of an image showing the frequen-
cy of occurrence of each grey scale value.
What are brightness and
Photoshop allows you to adjust
the brightness and contrast of
your image. If an image is too
light or too dark, you can use the
brightness and contrast controls
to improve the picture. Below is
an example of an image before and
after the brightness and contrast
for more information :
Levels is a tool in Photoshop and other image editing programs which can move and stretch the brightness
levels of an image histogram. It has the power to adjust brightness, contrast, and tonal range by specify-
ing the location of complete black, complete white, and midtones in a histogram. Since every photo’s
histogram is unique, there is no single way to adjust the levels for all your photos. A proper understanding
of how to adjust the levels of an image histogram will help you better represent tones in the final image.
The levels tool can move and
stretch brightness levels in a
histogram using three main
components: a black point, white
point and midtone slider. The
position of the black and white
point sliders redefine the histo-
gram’s “Input Levels” so they are
mapped to the “Output Levels”
(default is black (0) or white
(255), respectively), whereas the
midtone slider redefines the loca-
tion of middle gray (128). Each
slider is shown below as they
How it wokrs ?
appear in Photoshop’s levels tool,
with added blue labels for clarity: http://www.cambridgeincolour.com/tutorials/levels.htm
The Photoshop Curves tool
The Photoshop curves tool is perhaps
the most powerful and flexible image
transformation, yet it may also be one of
the most intimidating.
Lightness channel Using curves
and adjustments curves
Performing curves to just the light-
ness/luminosity channel - either
in LAB mode or as an adjustment Original
layer can help reduce changes in
hue and color saturation. Move
your mouse over each of the im-
ages below to see what would
have happened if this curve had Using Levels
been applied to the RGB channel.
The Exposure tool
Calibrated in stops, it increases brightness throughout the image but affects the shadow areas far less than
the highlights. Increase the Exposure sufficiently though and the image will turn solid white. Set it low
enough and you will end up with solid black.
The oddly named Off- set slider seems to
primarily do the opposite, stretching the ton-
al range downward to dark- en the shadows
without overly influencing the highlights. Af-
ter adjusting the Exposure, Offset can be used
to tweak the shadow and quarter-tone areas
of the image while only moderately affect-
ing the rest of the image.
The Gamma slider works in a similar fash-
ion to the Brightness slider in Adobe Camera
Raw. Moving it to the right increases the mid-
tone brightness of the image, while sliding it to the left will darken the overall appearance of the image
Under exposure Over exposure
h t t p : / / w w w. b a i r a r t e d i t i o n s . c o m / p a g e s / t u t o r i a l s / p h o t o s h o p / e x p o s u r e . h t m l
One of the new
features in Pho-
toshop CS4 is
a Vibrance adjust-
ment, a feature
borrowed from light-
module. It's fair-
ly intuitive to use,
but have you ever
handy little ad-
Original 50% Vibrance 100% Vibrance
Vibrance: Adjusts the saturation so that clipping is minimized as colors approach full saturation. This setting change
the saturation of all lower-saturated colors with less effect on the higher-saturated colors. Vibrance also prevents skin
tones from becoming oversaturated.
for more information :
Hue is another
name for color.
When an im-
age’s hue is ad-
justed, the base
color of that im-
age changes in
the color wheel.
fers to the inten-
sity of its colors.
Original 50% 100%
Colors In photography and im-
age processing, color balance is
the global adjustment of the in-
tensities of the colors (typically
red, green, and blue primary
colors). An important goal of
this adjustment is to render spe-
cific colors – particularly neutral
colors – correctly; hence, the gen-
eral method is sometimes called
gray balance, neutral balance,
or white balance. Color balance
changes the overall mixture of col-
ors in an image and is used for col-
or correction; generalized versions
of color balance other than neutrals
to also appear correct or pleasing
http://www.geofflawrence.com/photoshop_tutorial_color.php Black & White