Heat map for «Out of gamut» visualization (englisch)

This feature compares the currently opened PDF page based on its actual or assigned source color space/output intent profile against a color converted version of that page in an out of gamut heat map view.  This makes it possible to quickly find out what would happen if the current were color converted to a different colorspace – were on the page colors would suffer from the conversion, and how much. This feature makes it easier to deal with situations where the actual printing process is not known beforehand, and the PDF file has  been prepared for a certain standard printing condition.

The "Out of gamut" visualization belongs to the group of "Visualizer" functions in pdfToolbox, and may be invoked from the menu entry "Visualize out of gamut" under the "View" menu.

Sample file: »drupa visitor brochure« for drupa 2021 from Messe Düsseldorf

The sample file used for this article can be downloaded from the "drupa visitor brochure" page on the drupa website. For the purpose of this article it has been converted to PDF/X-6 for the printing condition "PSO Coated v3" (FOGRA51).

This title page of the sample file uses a lot of vibrant colors, which will print quite nicely on high quality coated paper. Printing on uncoated paper or substrates or printing systems of lesser quality will obviously come at the price of color shifts and loss of saturation. The "out of gamut" visualization can be used to determine wether the result on for example uncoated paper could still be considered acceptable.

Open the menu item "Visualize out of gamut" from the "View" menu

Setting up the «Out of gamut» heat map view

The two most important settings for the "out of gamut" visualization are:

  1. Current color space determines how colors in the current PDF file are to be interpreted. If the PDF file contains an OutputIntent, this will be used by default. In all other cases, it is necessary to select a suitable ICC profile here. This will define as the color space into which the current PDF is rendered and it could be any ICC profile, not only CMYK but also RGB or grayscale (n-channel profiles are currently not supported).
  2. Target color space means that rendition create based on the current color space is converted to the target color space. This implies  that the PDF itself will remain untouched throughout.

Further settings can be used to fine tune the color conversion to the target color space and how the heat map is derived:

  1. Use black point compensation is often used in many color conversion scenarios, but should usually be left off in this context as it introduces shifts on the light dark axis.
  2. Simulate overprinting should always be left on, as otherwise page content that makes use of overprinting will not be rendered in the same way as if it would be printed.
  3. Delta E method  defines which method to use for computing the color difference between the PDF in the current color space and in the target color space. Available methods are:
    • Delta E 2000 (recommended)
    • Delta E 1976
    • Delta C 2000 (same as Delta E 2000, but not taking the L axis into account. This can make it easier to visualize shifts in the color angle – e.g. a green color shifting towards blue or towards yellow – and to disregard shifts in brightness – e.g. disregard that a given green becomes a bit lighter or darker. The human eye seems to be more sensitive to shiftss in color angle than to shifts in lightness.)
    • Delta C 1976 (same as Delta E 1976, but not taking the L axis into account.)
  4. Threshold (Delta E between current and target colorspace) defines from which threshold value on a difference shall be visualized in the heat map. A value of 2.5 is the recommended default value.
  5. The "temperature scale" in the heat map  "grows" from yellow (for the threshold value) over orange to red and violet (in increments of 2.5).

 

Inspecting the heat map

Now it is possible to look at the heat map overlay and find out where colors would suffer the most. Hovering over the image it is also possible, exactly what the color at a given point of the page would be before and after conversion, ands how large the color difference is:

  1. Current mouse position
  2. Values at the current mouse position; three lines consisting of a color patch an various values provide interesting details
    • the top most patch reflects the heat map temperature at that position, accompanied by the color difference value
    • the patch in the middle reflects the current color of the PDF, accompanied by the respective Lab value
    • the patch at th bottom reflects the color of the PDF after conversion to the target colorspace, accompanied by the respective Lab value

As the heat map – especially when there is a lot of color difference – obstructs the regular page view, there area the following methods to easily switch between views:

  1. Show/hide heat map: Turn the heat map overlay off and – on clicking again – back on
  2. Switch view: clicking on this button toggles the view between the PDF in the current color space and in the target colorspace
  3. Switch views continuously: clicking on this button once will trigger a sequence of views switching between the heat map overlay and – depending on which was last displayed – the PDF in the current color space or in the target colorspace (staying ca. 2 seconds at each view, with switching done 10 times)

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