ImageServer UB+ Addendum

Raster image transparency

Categories of image documents with transparency

In recent years the use of transparency has become more and more common in media production. There are three different categories of image documents with transparency:

1. Raster image files (e.g. TIFF)
2. Native page layout application documents (e.g. Adobe InDesign)
3. PDF 1.4 and later documents

PDF 1.4 and later documents are supported by PDF HandShake. The support of ImageServer for native page layout application documents consists of the InDesign plug-in to generate previews of native InDesign documents (see 5.8.1 "The HELIOS Preview plug-in" in the ImageServer UB+ manual) and of the QuarkXPress XTension to generate previews of native QuarkXPress documents (see 5.7.3 "The HELIOS Preview XT" in the ImageServer UB+ manual). The following text explains transparency in raster images only.

Transparency in raster images

There are several different techniques to create transparency in raster images:

• Clipping paths
• Alpha and mask channels
• Transparency channels
• Multiple layers

Clipping paths

The most common way of creating transparency in prepress production uses clipping paths. Clipping paths isolate foreground objects and make their background transparent. Clipping paths are vector-based line structures which create hard edges between foreground objects and transparent background.

Alpha and mask channels

Some image formats support alpha or mask channels.

In all raster images each pixel is represented by a set of numerical values for all colorants of the image, depending on its color space. If an image contains an alpha channel, each image pixel is represented additionally by a numerical value which specifies the opacity of that pixel. In alpha channels, 100% specifies an opaque pixel and 0% specifies a transparent pixel. Mask channels are exactly like alpha channels except for their polarity. In mask channels, 100% specifies a transparent pixel and 0% specifies an opaque pixel. Alpha and mask channels offer multilevel transparency for drop shadows and soft edges of objects, also known as feather effects. If an image format supports alpha and mask channels, any image of that format may contain zero, one or multiple alpha or mask channels. The extra channels can be identified by their unique names. If an image contains one or more alpha or mask channels, the user has the option to apply none of the extra channels or exactly one of them.

Transparency channels

Transparency channels are much like alpha channels. Raster image formats supporting transparency channels can contain exactly one transparency channel or no transparency channel. The user usually does not have the choice whether to apply the transparency channel or not. The application of a transparency channel is mandatory. Each image pixel is represented by a set of numerical values for the visible colorants and one additional numerical value containing the opacity information. 100% specifies an opaque pixel and 0% specifies a transparent pixel. Transparency channels offer multilevel transparency, too. A well-known example of an image format with transparency channels is PNG with RGBA color.

Multiple layers

The most complex way of creating transparency currently is to combine multiple layers in Adobe Photoshop. Only TIFF and Photoshop native documents support multiple layers. TIFF images always contain a main image which is a combination of all Photoshop layers. But Photoshop native images contain a main image which is not necessarily a combination of all Photoshop layers. If ImageServer reads an image with multiple Photoshop layers, it uses the main image exclusively.

The following table lists the transparency capabilities of the most important image file formats

* Image format standard or Adobe Photoshop CS3 supports this feature, but ImageServer does not.

Raster image format	Clipping paths	Alpha/Mask channels	Transparency channels	Multple layers
BMP	-	-	•*	-
DCS1	•	-	-	-
DCS2	•	-	-	-
HELIOS Raster PDF	-	-	-	-
JBIG2	-	-	-	-
JPEG	•	-	-	-
JPEG 2000	•*	-	•*	-
Photoshop native	•	•	•	•
PICT	-	-	•*	-
PNG	-	-	•	-
Scitex CT	-	-	-	-
TIFF	•	•	•	•
Raster EPSF	•	-	-	-

How transparency properties of images are displayed in Adobe Photoshop

• If an image contains a clipping path, the clipping path is displayed in the Photoshop "Paths" window with its name displayed in bold font.
• If an image contains an alpha or mask channel, the extra channel is displayed in the "Channels" window. Every channel in the "Channels" window has an icon of the current image. If you double-click this icon, nothing happens for process colors, but a "Channel Options" window is opened for masked, alpha, and spot channels. This "Channel Options" window displays properties of extra channels.
• If an image contains a transparency channel, it is NOT displayed in the "Channels" window. Instead, the "Layers" window contains exactly one layer entry (displayed in non-italic font), which is not the background layer entry (displayed in italic font).
• If an image contains multiple layers, the "Layers" window contains more than one layer entry.

Raster image transparency support in ImageServer

Adobe InDesign and QuarkXPress support OPI workflows using high-resolution images with clipping paths, but they do not support OPI workflows using high-resolution images with alpha, mask or transparency channels or with multiple layers. More precisely, if ImageServer generated layouts with alpha, mask or transparency channels or with multiple layers, and these layouts are placed in Adobe InDesign, then InDesign does not generate OPI information in PostScript print output or in exported PDF documents. The same applies to QuarkXPress. Therefore ImageServer does not generate layouts with alpha, mask or transparency channels or with multiple layers.

ImageServer supports transparency via alpha, mask and transparency channels, and via multiple layers for the file formats TIFF, Photoshop native and PNG. Support for multiple layers in ImageServer is limited. If ImageServer reads an image with multiple layers, it uses the main image only, see "Multiple layers". The main image of TIFF images is always the expected combined image of all layers, the main image of Photoshop native images may not include all visible layers. ImageServer cannot write images with multiple layers. If an image with multiple layers is converted to another image, the multiple layer information is converted to an alpha or transparency channel. Another limitation is that transparency channels can be read from Photoshop native images, but only alpha channels can be written to Photoshop native images.

ImageServer tries to preserve transparency information in image conversions if the IgnoreMasks preference with default FALSE and the UseAlpha preference with default TRUE remain unaltered. If the IgnoreMasks preference is changed to TRUE, clipping paths in input images are ignored. If the UseAlpha preference is changed to FALSE, alpha, mask, and transparency channels in input images are ignored. If an input image contains transparency information that the output image format does not support, ImageServer tries to convert the transparency information to a type which the output image format does support. ImageServer does currently not support conversions between transparency information types that are not induced by output image format limitations. Here are some examples of raster image transparency support in ImageServer:

• If an input image with a clipping path is converted to the PNG image format, the clipping path is converted to a transparency channel since PNG does not support clipping paths.
• If you want to convert a TIFF image with a clipping path to a TIFF image with a transparency channel, you can first convert the input TIFF image to an intermediate PNG image, and then convert the PNG image to a TIFF image.
• If a PNG image with transparency channel is converted to a Photoshop Native image, the transparency channel is converted to an alpha channel.
• If a TIFF or Photoshop image with an alpha channel is converted to a PNG image, the resulting PNG image does not contain transparency information. ImageServer does not convert an optional alpha channel to a mandatory transparency channel. If the input image has more than one alpha channel, the resulting PNG image does not contain transparency information because only one transparency channel can be created, and there is no way currently to select one alpha channel to convert.

Additional “layout” options

-o LayoutAlpha <boolean:FALSE>

Most layout programs do not support OPI for transparent images, so transparency channels are stripped from layout images by default.
If Alpha support is active (UseAlpha = TRUE) and LayoutAlpha =TRUE, layout images will not be stripped of transparency channels. Using this option can make sense in a PDF-native OPI workflow.

Additional ImageServer Preferences

Key: Programs/opisrv/<preference>

LayoutAlpha

bool FALSE

The “layout” option described above can also be set as a server preference, either for the “opisrv” service or globally, with the key: 'Global/Opi/LayoutAlpha'.