Searches a PDF for objects to which custom halftones are applied.
Halftoning is a method of printing shades using a single color ink. By varying the size or density of dots, the eye can see a shade somewhere between the solid color and the color of the background paper. Custom halftones are mostly used for screening frequency and angles.
Associated with every output device is a default halftone definition that is appropriate for that device. Only relatively sophisticated documents need to define their own halftones to achieve special effects. In general, when a PDF document provides its own halftone specifications, it sacrifices portability: they may lead to unpredictable printed output when the same document is sent to different printers. This is why they are generally undesirable.