ImageServer G8 User manual (Version 6.0.0)  

E Raster PDF format

E.1 Photoshop PDF format

Adobe Photoshop offers the Photoshop PDF file format. Photoshop help pages state implicitly that generic PDF documents and Photoshop PDF documents are different. Adobe did not specify the differences between generic PDF and Photoshop PDF in the years before 2008, and there is no indication that Adobe intends to document these differences in the future. In contrast to the documented and open Photoshop EPSF format, the Photoshop PDF file format is currently undocumented and proprietary.

You can safely assume that Photoshop PDF documents are valid PDF documents. Most applications that can read PDF documents can also read Photoshop PDF documents. With well-known PDF analysis tools it can be observed that Photoshop PDF documents are standard PDF documents with additional private Photoshop data contained in so-called page-piece dictionaries. Photoshop PDF documents store two copies of the image they contain. The first copy is stored according to the PDF standard and uses the compression filter specified during saving, and this compression filter may be lossy such as JPEG. The second copy is stored in the additional private Photoshop data and is compressed with the lossless ZIP compression. This redundant storage of image information requires a lot of disk space and makes the Photoshop PDF format ill-suited for prepress production. While some properties of Photoshop PDF documents can be observed without official documentation, many details of the Photoshop PDF format stored within the page-piece dictionaries remain unknown outside of Adobe.

When images are saved as Photoshop PDF documents, the “Preserve Photoshop Editing Capabilities” option is important. If this option is turned off, the saved PDF documents do not contain additional private Photoshop data, hence they are not Photoshop PDF documents, but generic PDF documents. Photoshop can open generic PDF documents only by rasterizing them. The Photoshop import of generic PDF documents often results in substantial loss of quality. A PDF document created by Photoshop without preserved editing capabilities contains one raster image. If this PDF document is re-opened in Photoshop, the raster image is interpreted as vector data which is rasterized. The raster of the raster image within the PDF document and the raster of the rasterization may not fit together well. Another problem with Photoshop rasterization of generic PDF documents is that it does not feature in-RIP separation. In Photoshop rasterization spot colors are converted to process colors or are lost entirely.

E.2 HELIOS raster PDF

The PDF format can be a very efficient container for raster images. PDF viewing applications are available for a wide range of platforms, the PDF format includes support for many color spaces, for many image compressions, for ICC profiles and for spot colors. Since the Photoshop PDF format is undocumented, it cannot be supported by ImageServer. Therefore ImageServer supports a new raster image format derived from PDF called “HELIOS raster PDF”. All non-HELIOS applications view HELIOS raster PDF documents as generic PDF documents. This fact has both advantages and disadvantages. The advantages of the HELIOS raster PDF format are:

The disadvantages of HELIOS raster PDF documents result from the fact that non-HELIOS applications view HELIOS raster PDF documents as generic PDF documents which do not contain Photoshop resource block data. Hence many features that require Photoshop resource blocks are not supported in HELIOS raster PDF documents:

If HELIOS raster PDF supports a specific color space, an image with this color space can also have spot colors. The only way in the PDF versions up to 1.7 to combine arbitrary color space images with spot colors is the use of overprints. Overprints are a device-dependent feature by definition of all PDF versions up to 1.7. Note that the term “device-dependent” in prepress production should read “software-dependent” today. Applications often do not support spot colors or do not support overprints. Using “HELIOS raster PDF” documents with spot colors in these applications often results in partial or complete loss of spot color information.

E.3 Creating HELIOS raster PDF documents

Images can be converted to HELIOS raster PDF using the “layout” command line tool in convert mode:

$ layout -v -T PDF -l rabbit.tif rabbit.pdf

Photoshop resource block information such as a clipping path is lost during a conversion to HELIOS raster PDF.

E.4 Using HELIOS raster PDF as OPI layouts

The layout command line tool can generate layouts in HELIOS raster PDF format with OPI references, too:

$ layout -v -o RasterImageType="PDF " rabbit.tif

The resulting PDF layouts can be used in InDesign for OPI workflows.

E.5 Embedded and referenced ICC profiles

The PDF standard supports embedded ICC profiles only. ImageServer supports both embedded and referenced ICC profiles in HELIOS raster PDF documents. HELIOS raster PDF documents with referenced ICC profiles are an extension of the PDF standard. Consequently, non-HELIOS applications view HELIOS raster PDF documents with referenced ICC profiles as untagged PDF documents.

E.5.1 Rendering intent in PDF documents

The ICC standard defines four rendering intents for viewing or printing color images:

PDF documents can contain rendering intent information, unlike images in any other common raster image format. If PDF documents with rendering intent information are viewed in applications such as Adobe Acrobat 8, then the appearance of these documents depends on their rendering intent information. ImageServer supports the specification of rendering intents for the HELIOS raster PDF format during document creation.

The PDF standard defines that rendering intent information applies to CIE-based color spaces only. This means for HELIOS raster PDF documents that rendering intent information can be specified for images with color space CIE Lab and for images tagged with an embedded ICC profile. ICC profile references are not supported, because they are not part of the PDF standard. The rendering intent of images with a color space suitable for rendering intents can be specified using the “PDFIntent” attribute of the layout command line tool:

$ layout -o Attributes PDFIntent=<intent> -T PDF -l input.tif output.pdf

The value of <intent> can be 0, 1, 2 or 3. The ImageServer values 4, 5, and 6 for rendering intents with BPC (Black Point Compensation) are not allowed here, because they are neither part of the ICC standard nor part of the PDF standard.

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HELIOS Manuals October 14, 2021