BSTJ 60: 3. March 1981: An Extension of the CCITT Facsimile Codes for Dithered Pictures. (Johnsen, O.; Netravali, A.N.)

8.7 MB·English

An Extension of the CCITT Facsimile Codes for
Dithered Pictures.

{By ©. JOHNSEN and AN. NETRAVALL
‘Manuscript received Sestonber 24, 1080)

‘The Internationad Telegraph and Telephone Consultative Com:
itt (CCTPT) has recendty recommended two redundancy reduction
odes for the digital transmission af two-level facsimute. It is of
tnerest to transmit other topes of mill pletures using these codes. In
this paper ce develop @ movil fr bilevel dithered pictures, wherein
Dieturce are preprocesed reversty 0 permit efficent coding hy the
CCITT niles, Phe preprocessing consints of eal rearrangement of
Dicture elements to obtain large contiguous black and white areas
‘The main advantage ofthese schemes over the existing ones is that
they require the addition vf « simple processor to a stendardized
fcsimile-cader. One of the schemas has the additional adcantage
‘that wen in the absence of the postprocessor at the receiver,
‘igiortel but recognizable picture te obtained. A compression rat of
41S to 35 ty obdaiued with these methods, which is comparable with
‘other coding schrmes.

‘The Intemational Telegraph and Telephone Consultative Commit
tee (CCITT) hve recently recommended two redundancy resluction
odes for he digital transiiaeion of ro-leve acral over the general
Seitehad celephone necworks, In he fsture, most facsimile apparatus
ting redundancy reduetionlnchniquee will us these codes. The fst.
‘de one-dimensional rus length code called the modifi Hufinan
ford (ouuc). ‘The sieond Pola ie two-dimensional extension of the
modified Huffman code, ealled she modifiod uzan code (MRC),

Dithering is ant image processing technique which creates a two.
level pitore tht gives the Ulusion of x mallevel picture by appro
printely controling the spatial demsily of black abd whice plore
Clements?” es useful in eyeteme that use ino bilevel displays
(eg, plasma pone). Dithering allows a pictur to bo transmitted as
1 two-level picture, chus greatly reducing the number of bits to be
‘naalted

"The bigh frequency components of a ditered pleture prevent the
tae of CCLIT codes aa an efficient redundaney reduction rchene
Jdiee” Neeravali et al,” and Johnsen’ have proposed several coding
techniques for dithered pictures. Judice*has proposed sbi interne
ing cher vehi regroup pela wich the same oF sear Unreshod
level thus allowing the use of rur-lengch coding

‘Were interested in coding schemes fr dithered pictures that would
require only a aight modifeation tothe standardized one-dimensional
tnd two-dimensional CCITT codes. Quai into inchate only simple
[preprocecor at che transmitter and a postprocessor atthe receiver
‘The preprocessor would include dithering provssaor which trane-
forms the analog sienal from ch facsimile seanner into che binary data
stream corresponding to the dithered image and a procedcr which
modifies the clithered image in such a way that Ik ean be efficiently
ded by the aneimnsonal or two-dimensional rods. We nly
consider severeble transformnatins. The postprocescor of the receiver
‘mus. make the inverse transformation. One of the preprocesing
‘chon that we propone hne an interesting property thatthe ordered
picture i visually kimilar to the orignal dithered piezre. Therefore,
2 facsimile receiver not concaining the pasiprooetne wil be ale ta
"eproduce a pitare that isin most cane recognizable

"The dithering technique consists of comparing « uaultilvel image
with 8 position dependent threshold and taming only thoes pictur
clement “on™ (oe "1") here ve input signal exerts the terol
‘alue. "The equare matrix of threshold values (ealled the “diher
periodically repeated over the entire picture to provide the
thaeahold patter fr the whole image.

‘Several dither matrices can bo used. They have been judged on the
basis of subjoctve fidelity of reproduction. A clas of dithor matrices
of special intrest are the “ordered dither matrices.” A dithor matrix
of sige m Xn simulates n? +1 brightnea lvela where mia a power of
2. Thus, 17 brightness levels can be simulaced with w 4 x 4 ordered
dither matrix and 66 brightness lovee with a 6% W maatrix. AA 4
‘mabx, with 16 threshold levels, shown in ea. (1

o 8 2 wl
hz 4 gf
aug ”
15 7 13 3

Figure 1 shows dithered pices, "Karen." “Rngincering drawing,

‘hese pictures are 10 en

392. THE BELL SYSTEM TECHNICAL JOURNAL, MAFICH 1981
by 10 em and are scanned to genoraue un neray of 512 by 612, The
picture incensityv Tinesriy quantizod lo & bits (256 levels) and then
fithered. The following dither matrix, bssined from oy. (2 by ex
Danvlng the threshold level i cnver the entice span of pleture inen-
ily ese:

19640168)
eon 72 252 104
Bi Tht 24182
m4 13 2168)

e

‘We mention thatthe chee pictures of Fig ave made from the sane
originals as wel in Rela 4 and 6, but they are not the same digitized

1M. REORDERED PEL CODING

Since the cules tn be used are alway kaw, the goal of the
edeving is Lo Lasers the picture to minimize the coding longi. In
the one-dimensional gee, the nurs aut he as long as posible Tn the

CCITT CODES FOR DITHERED PIcTURES 398
gb Dither texte of Rogesrng dain

two-dimensional ent, the vertical coreelation hetween ruts mus also
be maximized.

"The pel reorderoy ix related o th bit interleaving method propaced
by Judice The main diference i hl now the reordering ofthe pel
is done locally. The ordering is made within afew 4% 4 blocks wt a
time, typically 1 to 4 blocks. ‘The ditference ean be seen from the
‘elation Between neighboring pls of tho ordened pictures, ln Jude's
schemes, neighboring pela are pls will similar threshold levele of
‘adjacent blocks, while Inthe Incl ordering scheme, neighboring pela
fre pela of nearly the same threshald ltl ofthe samo or neighboring
blocks "Therefore, bit interleaving exploits better the correlation
among pels with sinilae Unentold level side pel ordering exploita
bettor the correlation among the neighboring pele,

‘We propane two clesely related kinds of pel reordering. One of ehern
is optimized for rur-ength coding, while the other is optimized forthe
two-dimensional codes. They have both hoon conoeived for 4% 4
ardered dither matrices, ul in the esse of larger dither macrces,
simple modifications can be made

304 THE BELL SYSTEM TECHMICAL JOURNAL, MARCI 98)
2.1 Pal reordeting for rundonath coon

‘Let us conidor the 4% 4 dither matrix uf oy, (1). To have as few
rung as ponsible, the pels with neazly the me threshold level should
be contiguous on the warn ine. One wer to reduce the number of runs,
in to reorder the dichered pele for (wo consecutive blocks slightly
differently, for example,

Block A Block B
o rz 3) 3210
4567/7 85 4 o
heoulnw 9 8
RuKB bu

Another possibility sto reorder a4 4 tel into a 2% 8 block. ‘Two
blocks muel Uneretare he reordered the same time. Hor veusons of

lficieney four blocks 42, ane eowdered es shown in Table I.
Fiqures 2 and 3 show thy untered pitare of Karen with 4» 4 aed 2
>t onrdering. Ie can bu ser Unt these reordered pictures can Ie

CCITT CODES FOR DITHERED PICTURES 395
‘Table Reordering of four blocks $22

zun-ength coded efficiently been: there wre many long ana, Also
note thatthe reordered pictures are wally degraded version ofthe
‘original eithered pictures, The 1 % 4 reordering gives smaller degre
‘lation then the 2 % 8 ordering.

12.2 Pel reardaring fr two-dimensional coding
Pol reordering for two-dimensional coding should increase the ver-
Uicel correlation between rans, Le, the reordered picture abould hay
mostly vertical black end waive tips. This is accomplished within
‘ch block by moving the pels most Likely to be white to the lef side
td the pele mort likely co be black tn the eight side and reversing thi

ig. Kaze ter ¢ error ret ol

396 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1981
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prions forthe next block, ‘The soordering of ovo blocks, A, would
be

Block A Block B
046 MT
15 9m Ww 6 2 a
260 3 9 5 1

nb Bs 40

Further improvements are posi but the coding efficiency with 4
4 reordering slow compared i the ner reordering schemes,
Improvement in he ence effcieney a obtained by 8x 2 reordering.
“This type of reordering slghly eeduces the vetccel redundancy be
frrcen the runs, bu that is largely compensated by the reduction in
{he mnber of rune, To exploit she maximum amount of correlation,
two groups of four blocks of 4 % 4 pels mnt be considered. These
‘blocks, named A, 14, €, D, A, BC’, and D’, are shown belo

ave le p
a Bc’

(CCITT CODES FOR DITHERED PICTURES 397
“Table —Rocrdored configuration of oa. (6)

able II shows the reerdered configuration,

“Another type of reordering is the 16% 1 reordering, Now the ple
are arranged inorder of inerousing or decroasing threshold level Hight
{De blocs shown below,

ajalolpl Rl] lala],

are transformed in

AQl Wi bu. tar
Bol wi wu TOF
Col. wm bu.1o@ @
Dol wi 10H

A light knproveuent ean be slisined by reversing every samond yop,
ff Tur linea, Figure <a 3 show che pit "Karen” carafe by
>< 2 and 16 91 s0ordering, seepectivels

33 Implementation considerations
All he above rearing aches ane imple to implement, xcept
in the ease of 2 B reordering for one dimensional coding, only lour
Sines witha maximum of 16 pels per line are processed aca ime. The
‘basic operation isto address the brs in different sequence. “Ihe
reordering should add very lite to the coat of the ender and decode
certainly leas than the dithering operation which isl is quite simple

Iv, COMPARISON OF PERFORMANCES

“The ening Iength in bits per pel have heen measured fo the varione
reordering sehenes and for the bi interleaving schumes of dle?
"The three teat piclres of Fig | were ured. Only the one-dimensional
snd the two-dimensional codes commended by the CCITT for stand
frdizaion' are considered, The end of line codeword has been sup
preasod in the comparisons. In the two-dimensional ease, all lines,
eae the fst me, are ended with the twins end, Nate
‘aL al the reoedeeing schemes are Lsnvdimennianal, even when a ne

dimensional eade is used afterward

90 THE BELL SYSTEM TECHNICAL JOURNAL MARCH 1963,
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