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We note how aliasing can be produced by down-sampling (or decimating) a sampled image. Starting from a discrete-spaceimage, we can select only a subset of samples arranged in a regular grid. This will determine the periodic repetition ofthe spectral images, that will end up overlapping.

In order to explore the concepts of sampling, down-sampling, and aliasing, run the applet drawing ellipses . With the keyboard arrow you can double or halve the horizontal and vertical sampling steps.

A simple introduction to the first elements of image processing is found in Digital Image Processing Basics .


With the adjective "digital" we indicate those systems that work on signals that are represented by numbers, with the(finite) precision that computing systems allow. Up to now we have considered discrete-time and discrete-space signals as ifthey were collections of infinite-precision numbers, or real numbers. Unfortunately, computers only allow to representfinite subsets of rational numbers. This means that our signals are subject to quantization.

For our purposes, the most interesting quantization is the linear one, which is usually occurring in the process ofconversion of an analog signal into the digital domain. If the memory word dedicated to storing a number is made of b bits, then the range of such number is discretized into 2 b quantization levels. Any value that is found between two quantization levels can be approximated by truncation orrounding to the closest value. The [link] shows an example of quantization with representation on 3 bits in two's complement .

Sampling and quantization of an analog signal

The approximation introduced by quantization manifests itself as a noise, called quantization noise . Often, for the analysis of sound-processing circuits, such noise isassumed to be white and de-correlated with the signal, but in reality it is perceptually tied to the signal itself, in such anextent that quantization can be perceived as an effect.

To have a visual and intuitive exploration of the phenomenon of quantization, consider the applet that allows to vary between 1 and 8 the number of bits dedicated to the representation of each of the RGB channels representing color. The same number of bitsis dedicated to the representation of an audio signal coupled to the image. The visual effect that is obtained by reducingthe number of bits is similar to a solarization .

Extend the code of the applet [link] to add some interaction features:

  • Make the fundamental frequency of the automatically-generated sound changing randomly at eachframe (see random() ).
  • Make the framerate (and the metronome for generating notes) dependent on the horizontal position of the mouse( mouseX ).
  • Make the number of harmonics of the sound (i.e., its brightness) dependent on the vertical position of themouse ( mouseY ).
  • Paint the window background in such a way that moving from left to right the tint changes from blue tored. In this way, a blue color will correspond to a slow tempo, and a red color to a fast tempo.
  • Make the color saturation of the background dependent on the vertical position of the mouse. In thisway a sound with a few harmonics (low brightness) will correspond to a grayish color, while a sound rich ofharmonics (high brightness) will correspond to a saturated color.
  • Add a control to stop the computation and display of the spectrum and create an effect of image freezing, whilesound continues to be generated (for instance by keeping the mouse button pressed).
  • Add a control to cancel the dependence of tempo, brightness, and color saturation on mouse position (forinstance by pressing a key).
  • Add a control that, in case of image freezing (mouse button pressed), will stop the generation of new noteswhile "freezing" the current note.
  • Finally, add a control that, in case of image freezing, will stop the sound generation and make theapplication silent.

The proposed extensions are implemented in the Processing code .

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Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
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.
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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