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.
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:
HELIOS raster PDF documents are as portable as generic PDF documents; applications that can read generic PDF documents can read HELIOS raster PDF documents, too
All major color spaces such as Bilevel, Grayscale, Indexed, RGB, CMYK, and CIE Lab are supported
All major image compressions such as CCITT G4, ZIP, JPEG, and JPEG 2000 are supported
ICC profiles are supported
Spot colors are supported
XMP is supported
OPI is supported
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:
Spot colors can be identified by their name string only, identification of a spot color as a Photoshop spot color table entry is not available
No clipping path support
No alpha and mask channels
No transparency channels
No multiple layers
IPTC is not supported
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.
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.
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.
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.
The ICC standard defines four rendering intents for viewing or printing color images:
Perceptual (0)
Relative Colorimetric (1)
Saturation (2)
Absolute Colorimetric (3)
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.