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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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800 Chapter 24 The Fourier transform

f(0, 0)

f(124, 124)

Figure24.21

Source imageford.c.t.

application.

We use the transform given above to produce an N ×M matrix containing the

real d.c.t. values,F[k,l]. Figure 24.22 is a visualization ofF[k,l]. Dark pixels represent

large values, white pixels represent zero or close to zero elements. Notice the

concentration of largevalues close toF[0,0], which isinthe top-left corner.

F(0, 0)

Figure24.22

Thed.c.t. matrix.

F(124, 124)

Inthenextstepwecompresstheimagebysettingthearrayelementstozeroatthe

right-handsideandbottom.Sincethecoefficientsaresmallinthisregiontheeffects

on the image after inversion are minor, unless a significant number of values on the

right-hand sideand bottom are set tozero.

The results of applying the inverse transform are shown in Figure 24.23. We can

seethattheimageontherightwhichcontainsthelowestnumberofd.c.t.coefficients

hasmoredistortionduetothecompression.Suchdistortionsareoftentermedvisual

artefacts. The advantage of having a highly compressed file is that it requires less

space for data storage.

In practice, the image is often broken into smaller blocks with the d.c.t. being

carried out on these blocks. If a highly compressed image is viewed on a computer

atalowresolutionitisoftenpossibletoseetheseblocks,whichtypicallymeasure8

by 8 pixels.

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