<< Chapter < Page
  Image coding   Page 1 / 1
Chapter >> Page >
This module introduces the 2-dimensional DCT.

In the equation from our discussion of the Haar transform: y T x T and to invert: x T y T we saw how a 1-D transform could be extended to 2-D by pre- and post-multiplication of a square matrix x to give a matrix result y . Our example then used 2 2 matrices, but this technique applies to square matrices of any size.

Hence the DCT may be extended into 2-D by this method.

E.g. the 8 8 DCT transforms a subimage of 8 8 pels into a matrix of 8 8 DCT coefficients.

The 2-D basis functions, from which x may be reconstructed, are given by the n 2 separate products of the columns of T with the rows of T . These are shown for n 8 in (a) of as 64 subimages of size 8 8 pels.

The result of applying the 8 8 DCT to the Lenna image is shown in (b) of . Here each 8 8 block of pels x is replaced by the 8 8 block of DCT coefficients y . This shows the 8 8 block structure clearly but is not very meaningful otherwise.

Part(c) of shows the same data, reordered into 64 subimages of 32 32 coefficients each so that each subimage contains all the coefficients of a given type - e.g: the top left subimagecontains all the coefficients for the top left basis function from (a) of . The other subimages and basis functions correspond in the same way.

We see the major energy concentration to the subimages in the top left corner. (d) of is an enlargement of the top left 4 subimages of (c) of and bears a strong similarity to the group of third level Haar subimages in (b) of this figure . To emphasise this thehistograms and entropies of these 4 subimages are shown in .

Comparing with this figure , the Haar transform equivalent, we see that the Lo-Lo bands have identicalenergies and entropies. This is because the basis functions are identical flat surfaces in both cases. Comparing the other 3bands, we see that the DCT bands contain more energy and entropy than their Haar equivalents, which means less energy (and so hopefully less entropy) in the higher DCT bands (not shown) because the total energy isfixed (the transforms all preserve total energy). The mean entropy for all 64 subimages is 1.3622 bit/pel, which comparesfavourably with the 1.6103 bit/pel for the 4-level Haar transformed subimages using the same Q step 15 .

(a) Basis functions of the 8 8 DCT; (b) Lenna transformed by the 8 8 DCT; (c) reordered into subimages grouped by coefficient type; (d) top left 4 subimages from (c).
The probabilities p i and entropies h i for the 4 subimages from the top left of the 8 8 DCT ((d) of ).
(a) Mesh and (b) row plots of the entropies of the subimages of (c) of .
Lenna transformed by the 4 4 DCT (a) and 16 16 DCT (b).

What is the optimum dct size?

This is a similar question to: What is the optimum number of levels for the Haar transform?

We have analysed Lenna using DCT sizes from 2 2 to 16 16 to investigate this. shows the 4 4 and 16 16 sets of DCT subimages. The 2 2 DCT is identical to the level 1 Haar transform (so see (b) of ) and the 8 8 set is in (c) of .

and show the mesh plots of the entropies of the subimages in .

compares the total entropy per pel for the 4 DCT sizes with the equivalent 4 Haartransform sizes. We see that the DCT is significantly better than the rather simpler Haar transform.

As regards the optimum DCT size, from , the 16 16 DCT seems to be marginally better than the 8 8 DCT, but subjectively this is not the case since quantisation artefacts become more visible as the block sizeincreases. In practise, for a wide range of images and viewing conditions, 8 8 has been found to be the optimum DCT block size and is specified in most current coding standards.

(a) Mesh and (b) row plots of the entropies of the 4 4 DCT in (a) of .
(a) Mesh and (b) row plots of the entropies of the 16 16 DCT in (b) of .
Comparison of the mean entropies of the Haar transform of Lenna at levels 1 to 4, and of the DCT for sizes from 2 2 to 16 16 pels with Q step 15 .

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
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..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
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.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
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
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Image coding. OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10206/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Image coding' conversation and receive update notifications?

Ask