By Max Derhak
There are two approaches to mixing image elements together that are often confused when rendering compound document formats like EPS/PS/PDF – transparency and overprint. Transparency rendering was previously discussed. In this article we will discuss the concept of overprint, how it is rendered, and problems that can occur when trying to render with overprint.
Introduction to Overprint
Unlike transparency, overprint is intimately connected with the final stages of rendering in the RIP. In figure 1 is shown the Colour rendering workflow in an EPS/PS/PDF RIP. Documents are first interpreted into a list of drawing instructions (in a display list). During the process of interpreting, a set of separation canvases (one for each output ink channel) is set up. After a page is interpreted, the render then repeatedly reads from the drawing instructions to draw into each separation canvas as directed. After the separation canvases are rendered for a page they are then saved and later combined to create a full Colour output image for each page.
|Figure 1 – Colour Rendering Workflow|
As was previously discussed, transparency processing is performed before rendering is performed to the separation canvases. Background and foreground image elements are separately rendered and then combined (AKA transparency flattening) into a single image element that is then rendered to the separation canvases. In some cases transparency flattening is performed by the creative application with the flattened portion placed as an image element in the document.
Overprint rendering is performed as part of the final stage of rendering to the separation canvases when pixels are actually placed on the canvas. Unlike transparency, overprint rendering is a binary state that can be set by the creative application. Overprint rendering can either be turned on or turned off. When overprint mode is not enabled (or disabled in the RIP settings) then rendering is always be drawn in all separation canvases that are specified by the current Colour. Figure 2 shows an example of drawing three Coloured objects on a page without overprint rendering. On the left are the drawing instructions. In the middle is shown the contents of each of the separation canvases after each set of instructions are performed. On the right shows the combined output of the canvases.
|Figure 2 – Basic rendering without overprint|
Notice that the separation canvases are drawn into even if there is a zero ink value defined for the Colour associated with the separation. This results in previous pixels being erased by objects that are drawn later by the drawing instructions.
Rendering with overprint allows for the rendering shown in Figure 2 to be slightly adjusted by following a fairly simple rule that can be stated in one of two ways:
“When rendering with overprint, only draw on a separation canvas if the separation ink amount is not zero”.
“When rendering with overprint, do not draw on a separation canvas if the separation ink amount is zero”.
By following this general rule pixels in separation planes are not erased if the ink amount for a separation Colour is zero. This results in a semi-transparent effect because planes that do have ink are drawn in. Figure 3 shows the results of rendering the same drawing instructions after enabling the overprint state.
|Figure 3 – Rendering with overprint|
Unlike transparency that has mixing models that can significantly deteriorate rendering performance, overprint has a low impact on rendering performance since overprint rendering simply involves not drawing to separation canvases.
Also, unlike transparency overprint rendering can be performed with EPS, PS, and PDF files.
Problems with overprint
Since overprint handling is performed at the final stage of rendering its use is dependent upon an intimate knowledge of Colours used in the document, and how they will be separated to each of the ink channels. Documents that use overprint are for an intended configuration of the printing/output system. If a document that uses overprint with one printing setup is re-purposed to a different printing setup, the steps involved in re-purposing the document can change the ink separation used and therefore the overprint rendering will result in significantly different output than what would be expected when printing to the targeted printing system.
There are at least two important cases where such re-purposing can have a dramatic effect on the printed output using overprint. The first is the case where ICC Colour management is used to re-target the Colour output for a different printing environment. The second case is where spot Colour inks are available on the target printer, but not available on the re-purposed printer, and overprint is used when drawing these inks.
Note: This becomes a significant issue when re-purposing output on large format printers with documents that were originally set up for printing on offset presses.
Using ICC Colour management
When re-purposing image output to large format printers it is often desirable to apply ICC profiles to all image content so that the Colour output appears as if it was printed on the original targeted output. This is set up either by defining the source profiles used, or by setting up a proofing workflow with an intermediate Colour space defined by an ICC profile.
Application of ICC Colour management is performed either as part of the interpreting process or as the first steps in the rendering process well before drawing to the canvas is performed. Colour management can be thought of as a Colour substitution that happens as part of a set Colour operation in the drawing instructions. However, when such a Colour substitution occurs, the desired output of overprint rendering can be can negatively impacted. Figure 4 shows an example of what can happen as ICC Colours management is applied in this fashion.
Figure 4 – Comparison of overprint related output
Notice that some overprints are maintained while others are changed when ICC Colour management is used. The rendering of overprint channels will be variable based upon how Colour management changes zero channel usage to non-zero ink usage. Additionally, this is more of a problem when the black generation defined in the output profile is significantly different than the black generation assumed by the document.
Overprint with Spot Colours
Overprint rendering is performed as part of drawing to the separation canvases, and there will exist one separation canvas for each ink that is output. When a print mode has spot channels available then separation canvases will be associated with each spot ink in the print mode. When this occurs element on a page that use a separation Colour space involving a print mode spot Colour it will only render to the canvas associated with the spot Colour.
One important example where overprint with a spot ink channel in the printer occurs is in white ink workflows. In this example one might wish to indicate a location where white ink should be placed drawing a graphic element only containing white ink.
With overprint on, drawing with the white ink will not erase the other Colours that are printed.
However, caution should be taken when using spot Colours with overprint. Figure 5 shows a simple example using overprint with cyan, magenta, and a print mode spot channel named “Pink”. In this case rendering to the Pink separation does not erase or effect the cyan and magenta separations.
|Figure 5 – Overprint example with spot Colour Separation|
If this same image is then printed to a print mode that does not have a spot channel named “Pink” then some form of spot Colour replacement will be performed. (Note: all separation Colour spaces must provide for an alternative Colour if the separation Colour is not available in the output device). Like Colour management this Colour replacement is performed as part of the set Colour operation. When this occurs the overprint characteristics of using the spot Colour are then transferred to the inks that are used to replace the spot Colour. Figure 6 shows the effect of using the same drawing instructions where the only difference is that a spot Colour replacement is performed.
|Figure 6 – Overprint of spot Colour with spot Colour replacement|
Because the spot Colour is now made up of cyan, magenta, and yellow inks, the overprint rendering rules are different. In general, re-purposing of jobs that use overprint of spot Colours is a formula for surprises. Figure 7 shows a side-by-side comparison of above results that occurred when spot Colours are used with overprint.
|Figure 7 – Comparing overprint results when using spot Colours|
Overprint allows for some transparency like effects to be accomplished without a significant negative impact to rendering performance (In fact performance may improve as elements are not drawn to various separation canvases). However, overprint is performed using a completely different technique than how transparency is performed with PDF files. Documents that make use of overprint make fundamental assumptions about how Colours are separated and rendered by the output device. Re-purposing of such documents can lead to problems.
The following recommendations can be made related to overprint usage:
1. Aside from using overprint for white ink workflows, avoid using overprint if at all possible. Overprint ties a document to a particular output device setup that can easily be violated if the document is re-purposed (which is common when printing to large format output devices).
2. If overprint is used by a document and you want to try to make it work:
a. The BLACKOVERPRINT and OVERPRINT settings in the PSRip configuration need to be enabled to turn on overprint processing. Other potentially additional useful PSRIP options related to overprint include:
i. CCOVERRIDETRANS set to OFF attempts to force incoming Colour channels to zero (even if Colour management is on)
ii. FIXGREYIMGOVERPRINT set to ON modifies Colour management operations related to grayscale images
iii. IMAGEOVERPRINT set to OFF forces images to always render to channels (not good for overprint)
iv. IMAGESEPOVERPRINT set to ON allows images that are defined using a single spot channel to only render as a single spot channel
b. Don’t use ICC Colour management. ICC Colour profiles can mess up the ink separation assumed by the document. Because of this don’t expect overprint and Colour management to work well together.
c. Make sure that you have spot Colours in the print mode set up to be compatible with the spot Colours overprinted by the document. (Note: if you cannot do this you are re-purposing the document and you will not be able to duplicate the output of the targeted output device).
Reprinted from the Onyx Graphics 'Thrive' newsletter and with full acknowledgements to the author, Max Derhak.