PDFlib Text Extraction Toolkit (TET) Manual
PDFlib Text Extraction Toolkit (TET) Manual
PDFlib Text Extraction Toolkit (TET) Manual
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8.3 Placed Images and Image Resources<br />
<strong>TET</strong> distinguishes between placed images and image resources.<br />
> A placed image corresponds to an image on a page. A placed image has geometric<br />
properties: it is placed at a certain location and has a size (measured in points, millimeters,<br />
or some other absolute unit). In most cases the image is visible on the page,<br />
but in some cases it may be invisible because it is obscured by other objects on the<br />
page, is placed outside the visible page area, is fully or partially clipped, etc. Placed<br />
images are represented by the PlacedImage element in <strong>TET</strong>ML.<br />
> An image resource is a resource which represents the actual pixel data, colorspace and<br />
number of components, number of bits per component, etc. Unlike placed images,<br />
image resources don’t have any intrinsic geometry. However, they do have width<br />
and height properties (measured in pixels). Each image resource has a unique ID<br />
which can be used to extract its pixel data. Image resources are represented by the<br />
Image element in <strong>TET</strong>ML.<br />
An image resource may be used as the basis for an arbitrary number of placed images in<br />
the document. Commonly each image resource will be placed exactly once, but it could<br />
also be placed repeatedly on the same page or on multiple pages. For example, consider<br />
an image for a company logo which is used repeatedly on the header of each page in the<br />
document. Each logo on a page constitutes a placed image, but all those placed images<br />
may be associated with the same image resource in an optimized PDF. On the other<br />
hand, in a non-optimized PDF each placed logo could be based on its own copy of the<br />
same image resource. This would result in the same visual appearance, but a larger PDF<br />
document. Non-optimized PDF documents may even contain image resources which<br />
are not even referenced on any page (i.e. unused resources).<br />
Table 8.1 compares various properties of placed images and image resources.<br />
Table 8.1 Comparison of placed images and image resources<br />
property placed images image resources<br />
<strong>TET</strong>ML element PlacedImage Image<br />
affected by image merging yes yes<br />
associated with a page yes –<br />
width and height in pixels yes yes<br />
width and height in points yes –<br />
position on the page yes –<br />
number of appearances 1 0, 1, or more<br />
unique ID<br />
no: the imageid member returned by <strong>TET</strong>_<br />
get_image_info( ) and the PlacedImage/<br />
@image attribute in <strong>TET</strong>ML identify only the<br />
underlying image resource<br />
yes: imageid member returned by<br />
<strong>TET</strong>_get_image_info( ) Image/@id<br />
attribute in <strong>TET</strong>ML<br />
file naming convention in<br />
the <strong>TET</strong> command-line tool<br />
_p_<br />
.[tif|jpg|jpx|jbig2]<br />
_I.<br />
[tif|jpg|jpx|jbig2]<br />
8.3 Placed Images and Image Resources 117
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