Calibrating, Printing and Proofing by the G7™ Method

Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006
Appendix D: Fractional Percentage
Calculations
D.1 The problem with integer percentages
Because most integer (whole-number) percentage values have no exact equivalent in an 8-bit image,
file values expressed as integer percentages will often result in slightly different values when rendered
to an 8-bit/ channel image (assuming no error diffusion or 'dither' is applied). In some RIPs or software,
this can lead to significant calculation errors.
To eliminate this potential error, GRACoL 7 expresses halftone dot percentages as if they had been
rendered into an 8-bit/channel TIFF image without dither. The formula (1) converts integer percentage
to nearest integer 8-bit (0 - 255), then (2) converts integer 8-bit back to nearest fractional percentage,
with optional rounding. You can perform these conversions in Microsoft Excel using the following code;
D.2 Converting integer percentage to integer 8-bit (Excel)
Int_8bit = ROUND(255 - int_% * 2.55, 0);
D.3 Converting integer 8-bit to fractional percentage (Excel)
Frac_% = ROUND(100 - Int_8bit / 2.55, n);
(Where 'n' is the required number of decimal places)
D.4 Converting integer percentage to fractional percentage
Frac_% = ROUND(100 - ROUND(255 - int_% * 2.55, 0) / 2.55, n)
(Where 'n' is the required number of decimal places)
For an example of some common percentages converted with various levels of precision, see the chart
(next page).
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