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Histogram-based processing.

Image Histograms
Graphic representation by means of vertical bars, where each bar represents the number of pixels present in the image for a given intensity of the gray scale (or color channel). Wikipedia definition.

Among the examples in Processing , one finds the code Histogram that overlap an image to its own histogram.

The histogram offers a synthetic representation of images, in which one looses the information concerning the pixel positions and considers only the chromatic aspects. This provides information about the Tonal Gamma of an image (gray intensity that are present) and about the Dynamics (extension of the Tonal Gamma). The image of a chess board, for example, has a Tonal Gamma that includes only two gray levels (black and white) but it has a maximal dynamics (since white and black are the two extremity of the representable gray levels).

The histogram is the starting point for various processing effects aiming at balancing or altering the chromatic contents of an image. In general,the question is building a map g o f g i for the gray levels (or color-channel levels) that can be applied to each pixel. The histogram can drive the definition of this map.

Translation and expansion of an histogram

If the map is of the kind g o g i k the histogram is translated in the sense of a higher or lower brightness, according to the sign of k . On the other side, if the map is of the kind g o k g i the histogram will be expanded or compressed, for values of k smaller or greater than 1 , respectively.

The contrast stretching is one of the operations of this kind of linear scaling that tries to extend the dynamic range of an image. The intervalby means of which one performs the scaling is set on the basis of the histogram, neglecting, for example, thetails of the distribution corresponding to 10 % of the darkest and brightest pixels.

Non linear scaling

More in general, the map g o f g i can be non linear, and this allows a greater flexibility in the manipulation of the histogram. A usefulinstrument is the one that allows to manipulate interactively the scaling map and to see the results on the image and/oron the histogram in real time. The instrument Color Tools/Curves of the image processing software Gimp does this, using an interpolating spline. In Processing it is possible to build a similar instrument, as reported in [link] .

Equalization of an histogram

The non linear scaling is the tool to equalize the histogram, that is to shape it in a desirable way. An image has a balanced tonal gamma, if all of the gray levels arerepresented and if the distribution is approximately uniform. In other words, one aims at a flat histogram. Without entering too much into the mathematicaldetails, one can say that the non linear map to be used for the equalization is obtained from the cumulated distribution of the histogram of the image f g i k 0 g i h k , where h k is the frequency, properly scaled by means of a normalization constant, with which the k -th gray level appears .

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Media processing in processing. OpenStax CNX. Nov 10, 2010 Download for free at http://cnx.org/content/col10268/1.14
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