The approach detailed in this subcl

The approach detailed in this subclause provides additional diagnostic information and enables allowance to be made for the effect of error correction in masking less than perfect attributes of the symbol that influence symbol quality, by applying an overlay technique as described in Annex F. It enables the Decodability, Defects and Modulation parameters of scan reflectance profiles covering the entire data region of the symbol to be graded in accordance with ISO/IEC 15416.This approach uses the following procedure for the assessment of each of the three parameters.The entire symbol shall be scanned until 0.9n codewords(where n has the same meaning as in 6.2.3)have been decoded ten times or until it is certain that each codeword has been scanned at least once without inter- row interference.In each scan, the Decodability, Defects and Modulation parameters shall be measured in each symbol character in accordance with ISO/IEC 15416.The calculation of all three parameters shall be based on the value of Symbol Contrast obtained from Rmax and Rmin in that scan line.The interim codeword grade of each parameter(Modulation, Defects and Decodability)for each codeword is the highest codeword grade for that parameter obtained on any scan for that codeword.Where the rows include overhead characters(other than the Start and Stop, or equivalent patterns), for example Row Indicators in PDF417 symbols, that are not included in the error correction calculation, these overhead characters shall be assessed first for each row together with the corresponding characters from the rows immediately above and below the row being considered.The highest interim codeword grade for any of these six(or four, in the case of the top or bottom row)characters shall be the overhead grade used to moderate the interim codeword grades for the codewords in the row.If a data codeword's interim codeword grade is higher than the grade obtained by the overhead characters, the data codeword's interim codeword grade shall be reduced to the overhead grade.The interim parameter grades so obtained shall then be modified to allow for the influence of error correction, as described below.For each parameter, the cumulative number of symbol characters achieving each grade from 4 to 0 or a higher grade, and those not decoded, shall be counted, and the counts shall be compared with the error correction capacity of the symbol as follows:For each grade level, assuming that all symbol characters not achieving that grade or a higher grade are erasures, derive a notional grade for Unused Error Correction as described in 6.2.4, based on the percentage thresholds shown in Table 3.The codeword parameter grade shall be the lower of the grade level and the notional UEC grade.NOTE 1 This notional grade is not related to, and does not affect, the UEC grade for the symbol as calculated according to 6.2.4, but is a means of compensating for the extent to which error correction can mask imperfections in a symbol.If one symbol has higher error correction capacity than another symbol, then the former symbol can tolerate a greater number of codewords with poor values for the parameter in question than the latter.See Annex F for a fuller description of the approach.The final codeword parameter grade for the symbol shall be the highest codeword interim grade for all grade levels.Table 4 shows an example of grading one parameter in a symbol containing 100 symbol characters(codewords)with an error correction capacity of 32 codewords.The 100 codewords consist of 68 data codewords, 3 error correction codewords reserved for error detection, and 29 error correction codewords to be used for correcting erasures or errors, giving an erasure correction capacity of 29.