GB1527394A - Apparatus for encoding images - Google Patents

Abstract

1527394 Facsimile systems; data compression INTERNATIONAL BUSINESS MACHINES CORP 23 Jan 1976 [27 Jan 1975] 2614/76 Heading H4P Grey levels of elements of a raster scanned picture are represented by different matrix patterns of bits and adjacent patterns are run-length encoded. Each picture element may thus appear as a matrix of dots the number of which determines the grey level of the reproduced picture. The patterns may be such that they can be efficiently run length encoded. For text images a logarithmic code is preferable but for drawings, charts, &c. whose run lengths do not include any distinct maxima a linear code is advantageous. After scanning at 20 individual analogue grey values are measured by unit 21 and assigned a digital value at 22 for encoding at 23 as a matrix pattern of bits. Two possible series of matrix patterns are illustrated in Figs. 1 and 2 (not shown), a completely white matrix not being included. Run length encoding is effected in unit 24 and the result stored in 25 for subsequent transmission display or printing. Control of the various units is performed by 26. Run lengths of low probability may be assigned a long code word and of high probability a short code word (Huffman distribution). Fig. 6 (not shown) gives an arrangement for generating patterns for the grey values given in Figs. 1 and 2. The 8 grey values require a triplet of bits which is fed in parallel through lines (1-3) and adder 4 to the first stage, of a triple N section shift register where N = number of areas. The shift register has cyclic feedback since the grey value of an area is required several times during the generation of patterns, hence as each matrix has four rows, the content of shift register has to be cycled four times via lines (8-10) through circuit (4). The triplets are clocked through the register and arrive at an adder 6 also receiving a signal whose value depends on which image element pair (see Fig. 5, not shown) is to be generated from the grey value of the image area. The sum is examined at (7) to determine whether the result is smaller than the number of possible amplitude intervals (in the embodiment = 9) and if so the corresponding image element pair is printed in black. If not it is printed in white. This information is stored and code optimized. In an alternative arrangement adder (6) and compare circuit (7) are replaced by an arrangement which for each predetermined grey value of an area performs a table look up operation containing all possible image element patterns for the various grey values.