About Gradation Curves
In GMG OpenColor, you can use gradation curves for the following purposes:
- To specify the tone value curve of an ink if only solid ink measurements are available. This allows you to predict the ink behavior in the proof when the design uses tints. The factory-default PANTONE inks, for example, are all linked to one of the ISO gradation curves listed in the Database. (It is mandatory to specify a gradation curve if only solid ink measurements are available.)
- To change the tone value gradation of an ink if the target printing process is different from the characterization. This allows you to flexibly reuse existing characterization data to proof similar printing processes with different tone value curves.
- As a RIP compensation curve.
In GMG OpenColor, make sure to use the same calculation method that is used at the printing press to ensure a print-to-proof match.
The two calculation methods are not compatible with each other. If you have imported or created a gradation curve based on the Murray-Davies equation, you cannot simply change it to SCTV in GMG OpenColor. If you want to use an SCTV curve, you will have to import SCTV data or create a new curve. However, GMG OpenColor allows you to view a Murray-Davies curve in SCTV mode and vice versa, so that you can see how it would look like.
Switching the Calculation Method from Murray-Davies (MD) to Spot Color Tone Value (SCTV) in the Color Corrections dialog box is possible only if the spot color characterization includes tint values and if no correction has been applied so far. This means, if either a gradation curve was loaded as a correction curve or if the calculated gradation curve was edited manually, the Calculation Method drop-down list will be disabled. This also means that if you are using a factory-default GMG spot color library such as PANTONE PLUS, the Calculation Method drop-down list will be disabled, as these libraries include only full tone values. However, you can load a custom gradation curve created with the Spot Color Tone Value (SCTV) calculation method to use the new method for these spot colors.
|Spot Color Tone Value (SCTV)||ISO 20654:2017 defines a metric for assessing intermediate tones of a spot ink. Instead of density, it works based on color measurements. It is able to produce approximately uniform visual spacing of tones between substrate and solid.|
|Murray-Davies (MD)||The Murray-Davies equation. is a traditional method to calculate tone values of CMYK inks from density measurements. This method has its limitations when it is applied to spot inks.|
- From the software used to calibrate / standardize the printing press.
- From measurements of the sample print.
- Use the default Offset gradation curves listed in the GMG OpenColor Database. These curves are ISO curves which conform to the PSO (Process Standard Offset). For Flexo printing, it is recommended to create your own gradation curves.
- Create one yourself. Since the values between the fulcrums are interpolated in a linear way, it is important to define a curve with more than just two or three fulcrums.
You can create a gradation curve either by selecting and duplicating an existing curve or by creating a gradation from scratch, as described in the following.
- On the Database tab, click New Gradation.
A new gradation curve is displayed as a straight line.
- Enter a name for the gradation.
- Select the Type. In case of a Tone Value Curve, select the Calculation Method.
- Left-click on any point of the curve to add fulcrums. (Remove fulcrums by right-clicking on a fulcrum.)
- Click-and-drag fulcrums, or enter values into the gradation table to shape the curve.
- Save the gradation to the Database.
Values in-between fulcrums are interpolated by linear regression. We recommend to add fulcrums in 10% or even smaller steps.