Apply gradation curve to select objects

pdfToolbox (8.3 onwards) provides an opportunity to apply filter tone curves to specific objects. This tutorial demonstrates:

  • Set up the appropriate filter.
  • Configure a level for special requirements.
  • Perform levels.
  • And finally check the result.

The purpose of this tutorial is to make a narrow black line with a tone value of 35% a little bit darker. The line that has to be correct uses a particular type of color coding, know as "Separation Black". Other lines in the PDF file are thinner or thicker and has a different tone value.

Inspect the PDF file

Upon inspecting (View>Inspect) the middle dashed line in the attached PDF, you will find that the dashed line appears too light in the print. The tone has to be changed from 35% to 55%.

Create a Fixup

  1. Create a Fixup with type "Adjust dot gain".
  2. The setting for the tone value has to be changed to a customised setting (explained later).
  3. Here you have to specify a filter that determines which objects have to be 'fixed' (explained next).
  4. Click on the "plus" button to open a new customised Check.

Add and configure the Checks for the 'Apply to' filter

  • Check property: "Object uses Black"
  • Check property: "Is Separation color space"
  • Check property: "Is line"
  • Check property: "Color value 1 for stroke"
    • Number: 0,35
      Plus/minus: 0,01
  • Check property: "Line width"
    • Number: 1
      Plus/minus: 0,01

Finally, name the Check 'Separation black line, 35%, 1pt'

Apply customised Check in the 'Adjust dot gain' Fixup

  1. In the 'Apply to' filter, select "Separation black line, 35%, 1pt".

Open the dot gain curve setting folder

  1. Open the "Dot gain curve setting list".
  2. In the list select "Open folder with configuration files".

Modify existing dot gain curve setting file

Modify existing dot gain curve setting file
  1. Select the dot gain curve setting file "010_25perc.crv".

Modify existing dot gain curve setting file

Modify existing dot gain curve setting file
  1. Make a copy of the dot gain curve setting file "010_25perc.crv".

Modify existing dot gain curve setting file

Modify existing dot gain curve setting file
  1. Change the name to "Change to 55 percent".

Review existing dot gain curve setting data

Open the dot gain curve setting file "010_25perc.crv" in a text editor to review the data. Most text editors are free. For example on Mac you have "TextEdit" and "TextWrangler"; on Windows "Notepad" and "Notepad ++".

Modify existing dot gain curve setting data

Open the dot gain curve setting file "Change to 55 percent.crv" in a text editor to modify.

NOTE: When you edit or change the curve setting file, you have to make absolute sure to maintain the existing gaps between the values as tabs, and not to replace them with single tabs.

  1. If you want to specify a different name, this value must be set from 1 to 0.
  2. The name must be set to an appropriate value that is used. Here "FEATURE_AdjustDotGainMinus25Perc_long" should be "Change to 55 percent".
  3. The desired target values for levels must be registered in the rows affected by tabs. In this case the target value should be 55% (written as 0.55).

Save the curve setting file as "Change to 55 percent.crv".

Apply dot gain curve setting

NOTE: After saving the new dot gain curve setting file, the setting is not in the tone configuration list. You have to close the dialog and open again to see the new setting in the list.

  1. In the list click "Change to 55 percent".

Rename the Fixup: "Set 35% line to 55%"

Apply Fixup on PDF file and save the Resultant PDF

Apply Fixup on PDF file

Review the preflight report

Review the preflight report

The 35% line tone is changed to 55%.

Inspect the processed PDF file "Left_Demo file_Processed"

  1. Select the dashed line.

The tone of the dashed line is changed from 35% to 55%.

Review the processed PDF file "Left_Demo file_Processed.pdf"

At the top is the processed PDF file where the tone of the dashed line is changed to 55%. At the botton the original dashed line with the lighter tone of 35%. In both PDF files the remaining lines are unchanged.

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