Processing black objects (deprecated, available till v10.2)
Black and tinted black (Gray) objects in PDF files can be rendered in different ways.
CMYK color space with 4 channels
C, M and Y are 0 %, where only the black (K) channel is used
Color space with 1 channel
Uses only the K channel of the process colors for output
There also is a special form of black in the following color spaces:
Additionally there are the following color spaces:
ICCbasedGray, ICCbasedCMYK, ICCbasedRGB
When processing black it is recommended to ignore ICC profiles, since this would lead to a conversion of black using the PCS of the ICC profile resulting in rich black defined in CMYK (assuming the destination profile to be a CMYK profile).
Depending on the color space the black is defined in, it will normally be processed like all the other colors in this color space; e.g. a black in DeviceCMYK will be processed like the other CMYK values.
Within the scope of print production though it is preferable to treat every black the same, no matter how it is defined internally. To process black elements properly according to the settings in the gray conversion, the following parameters are available:
Please keep in mind that some of the parameters mentioned in this article have to be supplemented with a prefix (e.g. "RGBImage-..." or "CMYKGraph-...") before they can be used in the policy file to ensure correct processing.
Details can be found in the article: Policies
The color conversion engine uses a small tolerance when detecting black objects. So the objects considered as "black" might have a slightly different colorant value, but as their visual appearance will be gray when printed, they'll be handled the same way as if there colorant values were completely the same (also called "gray balance").
The used tolerances for the different colorspaces are:
Tolerance = 1%
((|L - a| <= 0.01) & (|L - b| <= 0.01)) = Gray
RGB and calRGB:
Tolerance = 3%
((|R - G| <= 0.03) & (|G - B| <= 0.03) & (|R - B| <= 0.03)) = Gray
Tolerance = 0%
(C == 0) & (M == 0) & (Y == 0) = Gray
In the usual cases it is desirable, that black/gray elements are only represented in the K channel after any conversion. Therefore you may use the option "Preserve Black" in the fixup "Convert colors". This guarantees that all black objects are converted according to the gray treatment defined in the color policy file.
The parameters set by the option "Preserve Black" are:
- SetGrayColorSpace Separation_Black
- C_eq_M_eq_Y_is_Black Separation_Black
The parameters set in the "Standard" color policy file (overruling the parameters set by the option "Preserve Black") are:
- SetGrayColorSpace NoChange
- Gray_AdvancedColorConversion AdjustDotGain
Elements defined in DeviceGray remain DeviceGray after conversion. Gray elements defined in DeviceCMYK/Separation Black or DeviceN Black are mapped to Separation Black after the conversion.
The parameters set by the option "Preserve Black" are:
- R_eq_G_eq_B_is_Black Separation_Black
- a_eq_b_eq_0_is_Black Separation_Black
The option "Preserve Black" converts any RGB and/or Lab black to Separation Black, if objects of this color space are converted to CMYK. This conversion applies for images as well as for text and vector objects. During this conversion RGB gray scales are mapped 1:1 to Separation Black. This means, a 50% RGB gray (127/127/127) will be 50% Separation Black after conversion.
If files are converted amongst different printing conditions the different dot gain might be counterbalanced. The following parameters – seen as defined in the policy "Standard" – allow to compensate the different dot gain:
- GrayGraph-AdvancedColorConversion AdjustDotGain
- GrayImage-AdvancedColorConversion AdjustDotGain
The difference in dot gain between the gray source ICC profile and the destination ICC profile will be compensated. 100% Black remains unchanged.
By changing the color policy file it is possible to define the destination color space of gray elements with the following parameters:
||Element will be converted according to the settings for the corresponding color space
||Element will be converted according to the settings for the color space "Gray" and stored in DeviceGray
||Element will be converted according to the settings for the color space "Gray" and stored in DeviceCMYK – Black channel only
||Element will be converted according to the settings for the color space "Gray" and stored in Separation Black
- Attention: In DeviceGray, a defined Overprinting flag will have no influence on subjacent CMYK elements. These will always be knocked out.
Some special transparency effects are based on the interaction of color channels and therefore, a conversion of objects using a gray balance of a process color (e.g. RGB, CMYK or Lab) to pure gray scale may result in visually changed results. As these effects differ from the "construction", the used BlendMode and color space of a PDF a general solution is not possible and excluding certain BlendModes depending on the input files is necessary.
All BlendModes can be excluded, which means they are converted to a process color space. Normally the gray balance is preserved by converting it to grayscale, but this can result in a changed visual appearance with some special usage of certain BlendModes (e.g. ColorDodge).
Possible values are: Normal, Compatible, Multiply, Screen, Overlay, Darken, Lighten, ColorDodge, ColorBurn, HardLight, SoftLight, Difference, Exclusion, Hue, Saturation, Color or Luminosity.
If more than one BlendMode shall be excluded, the values have to be separated either by a space, a comma or a semicolon.