EP O 1 _9 291 B1 gefofe deScfibing the ffst embod_iment of the present inventìon, the two-d.imensional coding SCheme of the CCITT Recommendation T.4, as a typical coding scheme, is deSCfibed hefe__naftef. _ The followjng Table 1 shows the nNo-d_imensional code table of the CCITT ReCOmmendatiOn T.4. _ _ It is noted that. C 5A 5C 6A d 6C d 5 - _in the case that the f_irst and second scan lineS in the Fi9S. 3AI 4A, 4, , C, danC COFreSPdOnt to the Fefefence and coding lines.in Fig. 2,.respectiveIY, the Chan9in9 elementS 1 an 2 COf FeSPOn ' ,Jb,'' and ''a,'', respectively, and C A 5C 6A d 6C d - _in the caSe that the second and third SCan l__neS in the Fi9S. 3A, 4A, 4, 5, , an COFreSPdOn to the fefe Fence and cod_ing l_ines in Fig. 2, respectively, the changin9 elementS C2 and Cs COffeSPOn tO Io ''b,'' and ''a,'', reSpe CtiVeIY. f d. _ b _ th h . l nt Nen e.g. referr.ing to F_ig. 3A, the inventiOn deteCtS a iFSt _StanCe 12 e een e C and9dl_9 e em_e C, in the f_i Fst scan l_ine and the changing element C2 in the SeCOnd SCan line, and aISO a SeCOn i StanCe 2S be_een the changing element C, in the second scan line and the Chan9in9 element Cs in the thi Fd SCan l.ine. The above ope Fation is Shown in blocks 1 through 5 in Fig. 1. The OPe FatiOn Sta FtS at bIOCk 1. BIOCk 2 _5 sto Fes thfee Scan lines into a thfee line buffer not shown. Block 3 dete CtS the bOUnda_ 31 betWeen the changing element C, and the nen white element. Block Q detects the boundafY 32 be_een the Chan9in9 element C, and the nen white element. The boundary 31 relates to the bOUnda_ 32. AISO, bIOCk 4 dete CtS the distance _,,. glock 5 detects the boundary 33 which _is related to the bOUnda_ 32. AISO, bIOCk 5 dete CtS the di StanCe _z3. . h h th f l 3 < __ _ < AND 3 < 2o The operation proCeedS to block 6, The bIOCk 6 detefmineS W et eF e OfmU ad 1z. h Kh d . d __,,l < x is satisfied, or not. As explained below the vaIUe Of ''X'' iS Sele Cted __n aCCOfda6nCCehW_t t e e'f''t'h .image f.idelity. If l_,2l _ 3 or l_23l _ 3, aS iS the CaSe in the FigS. 4A, 4C, 5A, 5C, 6A an I t e anSWef O e block 6 _is NO and the operat_ion proceedS to bIOCk 8. d f h The distance l_,,l as well as l_23l in the Figs. 4A, 4C, 5AI 5C, 6A and 6_ C C_OFFe_SPO_n S tO One dO t e 'aSe'f 2_ _a,b,_ = 1, la,b, l = 2 and la,b,l = 3 shown in the Table 1 D lf the di StanCe _,2 Or _2s COFfeSPOn.S tO ''d' 't the th Fee cases, the __nvention, at bIOCk 8, Fe-a FfangeS Of COnVeftS the Of_19_lnal blt Pa_efnS OCF 16mga9ed 6a Da shown _in the F.igs. 4A, 4C. 5A, 5C, 6A and 6C to the bit patterns ShoWn in F__9S. 4B, 4D, 5g, 5, an , respectively. The nen Table 2 ShoWS the b._t COnVefSiOn pe_Ormed in bIOCk 8. 30 TBb_e Z __ D_etance _ l 2 or _23 ' _ew D1etance 35 l $ O (_IGS. _A-_D) _ 2 $ l (_IGS _ _A-__) '' - 3 _ _$ _ _ _. (_IGg_,__6A-6D) . The reasons for perform__ng the above convefsions are. _ (a) The numbers of bìts of the code words for the cases a,b, = O and a,b, = 1 are ''one b_it'' and ''three bitS'', FeSpe CtiVely, aS ShoWn in Table 1, Wh._le the nUmber Of bitS Of COde WOfdS fOF the CaSeS a,b, _ 2 and a,b, -- 3 a Fe ''S._X b._tS'' and ''SeVen b._tS'', and (bl The above conversion of the distanc in accordance with the _invention does not adverselv a_ect the image fidel__ty of the original image data with a f_ine resolution, e.g., of 8 pelslmm. In other words, the _o or._ginal._mage._s ma._ntained even._f the distance is converted. The Figs. QA through 6D will be described in more detail later on. If the formula 3 < l_,2l < x AND 3 < l_z3l < x is satisfiedl the operation proceeds to block J via the YES path. The value ofx is selected in advance,._n accordance w_ith the _image fidelity which one des_ires. In other words. the value x is the upper lim'it for do._ng the pre-process. _i.e. for not necessarily keep._ng the orig._nal _5 image._nformation with.in the converted image data. In the exemplary case, the value of x is selected to be ''6''. F__g. 3A ShoWS one CaSe p FodUCing_a YE_ S fFOm bloCk 6_, Wh_ efein the VaIUe Of _,2 iS ''4'' and the VaIUe Of _23.is '_5''. Thenl block 7 determ.ines if _,2 is larger than _23. For the exemplary bit pattern shown in Fig. 3A. the block 7 produces a NO outeut. and the operation proceeds to block 1 O. Then the orig._nal._mage data Of Fig. 3A._S Fe-affanged to fedUCe the d__Stan Ce _,2 to the VaIUe ''3''. The Fe-a Ffanged image data iS ShoWn __n 60 F._g. 3B. It._S n Oted _hat the tOtal length Of _,2 and _23 in the Original image data iS mainta__ned after the . conversion, as._t should be to keep the image fidelitv of the or_iginal data. _ When l_,2l is Iarger than l_23_I the YES ou_put _is producedl and the operati_on p_ roceeds to block 9. NO eXample fOf the prO_eSS inCIUd_Ing bloCk 9 iS ShOWn_ in t_he fig_ureS_. In bIOCk 91 _23 Of the OFiginal bi_level image data iS reduced to ''3''I but the total length of _,2 and _23 _in the original image data is ma_inta_ined 65 in the re-arranged._mage data. 4 .