A PDF file is a faithful reproduction of a document. Obviously it can also contain images. The PDF file format has been designed to cope with a wide range of image types and color spaces. This page provides an overview of this support.
Images in PDF files can be:
- black-and-white images (line-art) – Older versions of Acrobat (<4.0) occasionally had problems when such images were colorized in a front-end application like QuarkXPress (e.g. you give a red tint of a scanned line-art logo). Nowadays this should no longer be a problem.
- grayscale images – These can be 2, 4, 8 or 16 bit. Usually images are 8 bit, which means that there are 256 shades of grey going from white to black
- multitones – Duotones, tritones, Hexachrome files,… were not possible in PDF 1.2 files (created using Acrobat 3) but PDF 1.3 provided a new color space called DeviceN which is specifically meant for such images.
- color images – These can be 1, 2, 4, 8 or 16 bit. Usually each color plane is 8-bit (256 colors) which means that an RGB image can be contain 16 million colors. Instead of gradual tints, an image can also contain a fixed list of a limited number of colors. This is called ‘indexed color’.
The following color spaces are supported:- RGB
- LAB/ICC based
- CMYK
As you can see, just about anything that is possible in front-end applications can be maintained in a PDF file. Sometimes the creation of a PostScript file and the subsequent conversion to a PDF changes the content of images. Fairly common things that can go wrong are CMYK images that end up as RGB images in the PDF document or documents in color that get converted to black-and-white PDF files.
In most cases, images are compressed to limit the file size of the resulting PDF. More information can be found on the pages about various compression algorithms.
Images appear to use OPI
Sometimes a PDF files contain OPI-references for images, even though no OPI is used in the workflow. This curiosity is not a ‘feature’ of the PDF file format but rather a consequence of the way some front-end applications like XPress and PageMaker handle images. You can read more about it in the “When OPI is no OPI” section of this site.
Images embedded as Form XObjects
Sometimes images are embedded as an object called a Form XObject within a PDF. Internally to PDF, Form XObjects are the logical equivalent of EPS files. Some PDF tools are not capable of handling Form XObjects. For more information on Form XObjects, jump to this page.
DeviceN color space for multitones
In PDF 1.2 (files created using Acrobat 3), there was no way multitones (duotones, tritones, Hexachrome files,…) could properly be embedded. Some companies worked around this limitation by putting images in overprint on top of each other. While this approach offered proper output, the images could not be viewed correctly on-screen.
From PDF 1.3 onwards (files created using Acrobat 4 and later) a new color space called DeviceN is available which is specifically meant for such images. DeviceN can also used for other objects like spot color blends. The main drawback of DeviceN is the fact that it can only be separated properly by a PostScript 3 RIP.