SECTION 4.8
349
Images
In PDF 1.5, optional content (see Section 4.10) can be used to facilitate selection
between alternate images. If an image XObject contains both an
Alternates
entry
and an
OC
entry, the choice of which image to use is determined as follows:
1. If the image’s
OC
entry specifies that the base image is visible, that image is dis-
played.
2. Otherwise, the list of alternates specified by the
Alternates
entry is examined,
and the first alternate containing an
OC
entry specifying that its content
should be visible is shown. (Alternate images that have no
OC
entry are not
shown.)
4.8.5 Masked Images
Ordinarily, in the opaque imaging model, images mark all areas they occupy on
the page as if with opaque paint. All portions of the image, whether black, white,
gray, or color, completely obscure any marks that may previously have existed in
the same place on the page. In the graphic arts industry and page layout appli-
cations, however, it is common to crop or mask out the background of an image
and then place the masked image on a different background so that the existing
background shows through the masked areas. A number of PDF features are
available for achieving such masking effects (see implementation note 54 in Ap-
•
The
ImageMask
entry in the image dictionary, available in all versions of PDF,
specifies that the image data is to be used as a
stencil mask
for painting in the
current color.
•
The
Mask
entry in the image dictionary
(PDF 1.3)
may specify a separate image
XObject to be used as an
explicit mask
specifying which areas of the image to
paint and which to mask out.
•
Alternatively, the
Mask
entry
(PDF 1.3)
may specify a range of colors to be
masked out wherever they occur within the image. This technique is known as
color key masking.
Note:
Although the
Mask
entry is a PDF 1.3 feature, its effects are commonly simu-
lated in earlier versions of PDF by defining a clipping path enclosing only those of an
image’s samples that are to be painted. However, implementation limits can cause
errors if the clipping path is very complex (or if there is more than one clipping
path). An alternative way to achieve the effect of an explicit mask in PDF 1.2 is to