PDFlib 8 Windows COM/.NET Tutorial

Fig. 7.1
Using a clipping path to separate
foreground and background
7.1.3 Clipping Paths
PDFlib supports clipping paths in TIFF and JPEG images created with Adobe Photoshop.
An image file may contain multiple named paths. Using the clippingpathname option of
load_image( ) one of the named paths can be selected and will be used as a clipping path:
only those parts of the image inside the clipping path will be visible, other parts will remain
invisible. This is useful to separate background and foreground, eliminate unwanted
portions of an image, etc.
Alternatively, an image file may specify a default clipping path. If PDFlib finds a default
clipping path in an image file it will automatically apply it to an image (see Figure
7.1). In order to prevent the default clipping path from being applied set the honorclippingpath
option in load_image( ) to false. If you have several instances of the same
image and only some instances shall have the clipping path applied, you can supply the
ignoreclippingpath option in fit_image( ) in order to disable the clipping path. When a
clipping path is applied, the bounding box of the clipped image will be used as the basis
for all calculations related to placing or fitting the image.
Cookbook A full code sample can be found in the Cookbook topic images/integrated_clipping_path.
7.1.4 Image Masks and Transparency
PDFlib supports three kinds of transparency information in images: implicit transparency
with alpha channels, explicit transparency, and image masks.
Implicit transparency with alpha channels. Raster images may be partially transparent,
i.e. the background shines through the image. This is useful, for example, to ignore
the background of an image and display only the person or object in the foreground.
Transparency information can be stored in a separate alpha channel or (in palettebased
images) as a transparent palette entry. Transparent images are not allowed in
PDF/A-1, PDF/X-1 and PDF/X-3. PDFlib processes transparency information in the following
image formats:
> GIF image files may contain a single transparent color value (palette entry) which is
respected by PDFlib.
> TIFF images may contain a single associated alpha channel which will be honored by
PDFlib. Alternatively, a TIFF image may contain an arbitrary number of unassociated
184 Chapter 7: Importing Images and PDF Pages (Edition for COM, .NET, and REALbasic)