PDFlib 8 Windows COM/.NET Tutorial

channels which are identified by name. These channels may be used to convey transparency
or other information. When unassociated channels are found in a TIFF image
PDFlib will by default use the first channel as alpha channel. However, you can
explicitly select an unassociated alpha channel by supplying its name:
image = p.load_image("tiff", filename, "alphachannelname={apple}");
> PNG images may contain an associated alpha channel which will automatically be
used by PDFlib.
> As an alternative to a full alpha channel, PNG images may contain single transparent
color values which will be honored by PDFlib. If multiple color values with an attached
alpha value are given, only the first one with an alpha value below 50 percent
is used.
Note In addition to a full alpha channel Photoshop can create transparent backgrounds in a proprietary
format which is not understood by PDFlib. In order to use such transparent images with
PDFlib you must save them in Photoshop in the TIFF file format and select Save Transparency
in the TIFF options dialog box.
Sometimes it is desirable to ignore any implicit transparency which may be contained
in an image file. PDFlib’s transparency support can be disabled with the ignoremask option
when loading the image:
image = p.load_image("tiff", filename, "ignoremask");
Explicit transparency. The explicit case requires two steps, both of which involve image
operations. First, a grayscale image must be prepared for later use as a mask. This is
accomplished by loading the mask image. The following kinds of images can be used for
constructing a mask:
> PNG images
> TIFF images: the nopassthrough option for load_image( ) is recommended to avoid
multi-strip images.
> raw image data
Pixel values of 0 (zero) in the mask will result in the corresponding area of the masked
image being painted, while high pixel values result in the background shining through.
If the pixel has more than 1 bit per pixel, intermediate values will blend the foreground
image against the background, providing a transparency effect.
In the second step the mask is applied to another image with the masked option:
mask = p.load_image("png", maskfilename, "");
if (mask == -1)
throw new Exception("Error: " + p.get_errmsg());
String optlist = "masked=" + mask;
image = p.load_image(type, filename, optlist)
if (image == -1)
throw new Exception("Error: " + p.get_errmsg());
p.fit_image(image, x, y, "");
The image and the mask may have different pixel dimensions; the mask will automatically
be scaled to the image’s size.
7.1 Importing Raster Images 185