Acrobat Tips: Graphics in PDFs
Photographs
The display quality of photographs depends on the quality of the original photograph (resolution, color depth) and on the display zoom.
Acrobat’s display resolution at 100% zoom is 72 dpi. When you view an image present on a PDF page at a 100% zoom, the higher resolution an image may have will not be of value. As the display resolution cannot be changed, hardware downsampling effectively takes place — where different pixels are omitted.
However, the higher resolution will cause a slower display and a larger file size. Higher resolution images will be beneficial when you print the page or when you zoom in — a 400% zoom will mean an effective resolution of 288 dpi, for example.
Bitmaps in a PDF file are displayed with optimal quality only at a specific effective resolution. Changing the magnification (the same document may open with different magnifications in different computers based on view settings and preferences) will have an immediate effect on bitmaps. (This differs from display of bitmaps in HTML pages in Web browsers, where the same 100% magnification is used at all times, so that image distortion is not possible.) Depending on the page size and text layout, it may be better to display optimized graphics in a separate window. To do this, store the graphics in a separate, dedicated PDF with a default 100% zoom, and link references from the textual PDF to the graphics. This way, the zoom level of the text document can be specified differently from the graphics.
Reference to graphics can be linked to figures placed in a multi-page PDF. Alternatively, if each figure is displayed in a separate PDF, it can have a unique title (displayed in the title bar).
The target PDF can be displayed in a new window (overriding preferences). When this PDF is closed, the text document is displayed in its previous location.
Meaningful Resolution. When you integrate images in your authoring application, make sure to use a resolution that will match your PDF needs. If the PDF is primarily designed for on-screen display, use resolutions that will make the image display well in 100-200% zoom. If print is intended as well, higher resolutions will be needed. (In addition to actual printing, you can also have an idea as to how an image may print by inspecting the PDF at 300% zoom.)
Need both display and print? You may have a compromise approach where the resolution is a bit higher than needed for display purposes, and not too low for printing purposes (e.g. 150 dpi). Alternatively, in PDFs with lots of images, you may have two different PDFs: one optimized for print, and the other optimized for screen. (The same source file may be distilled with different downsampling job options to achieve the appropriate image resolutions in the PDF.)
Dynamic Image Interpolation for Photographic Images. Dynamic Image Interpolation improves the visual result and is recommended for photographic images, especially when these will be used at higher magnifications. When the resolution of an image is significantly lower than the effective display resolution, image interpolation attempts to produce a smooth transition between adjacent pixels, reducing jaggy or blocky visual artifacts.
Dynamic image interpolation can be turned on for a specific bitmap through the EPS export in Photoshop, or globally — for all bitmaps in the PDF being distilled — through a special command in Acrobat Distiller’s prologue.ps file. This setting is best applied individually, providing you with a higher level of control. For example, you can have all background graphics or photographs created with interpolation turned on, and screen captures without image interpolation. Note that the dynamic image interpolation somewhat slows the display. Image interpolation may cause text to appear “foggy” when the PDF is displayed at standard magnifications. Download an example here [364KB] (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader).
Callouts. If your photograph includes callouts, consider implementing these through text in your authoring program, rather than as part of the bitmap image, for these reasons:
- Bitmap text is adversely affected by downsampling and display distortions, and is often rendered unintelligible.
- Bitmap text cannot be searched with Acrobat’s Find or Search functions.
- If you have lines pointing to specific items in a bitmap, consider adding a white border for lines crossing dark areas (where the line merges with the background).
- Avoid diagonal lines/oblique angles as much as possible, because of the staircase effect. Alternatively, use diagonals with 45 degrees (or 30 or 60 degrees as a second choice).
Screen Captures
Although screen captures are a kind of photograph, they are used — and misused — so often that they warrant discussion as a separate category.
Screen captures are so common in technical documentation that we tend to overlook the fact that they are not always necessary. Your first consideration should be whether the screen capture is necessary at all, and if so, whether a schematic drawing can do a better job.
When you do decide to use screen captures, follow these guidelines:
Capturing the screen. Turn off text anti-aliasing smoothing in your operating system (or the ClearType rendering option in Windows XP) before taking the screen capture, so that the captured image is as clear as possible.
Color schemes. Consider using a different set of colors in screen captures to identify these as static.
Cropping. If not necessary, don’t use the entire screen. Unless the entire screen is necessary, consider showing only a cut-out of the relevant section. This helps to focus the user and to remove excess noise.
Resolution. For a screen capture to look best at a zoom level of 100%, it has to be incorporated into the page (at the authoring stage) with a resolution of 72 dpi.
The screen capture will then be displayed as intended at a zoom of 100%, and with the least distortion at zoom multiples of 50%.
When screen captures are viewed at arbitrary magnifications, the display results can vary significantly, and this is particularly evident in the text. This is the case when the zoom level is set to Fit Page, or Fit Width. If you wish to display the screen captures optimally, you may want, at least initially, to set the default zoom of the PDF to 100% (or any other specific value matching the resolution of your images).
Example: Using a picture viewer to display optimal screen captures. When displayed in Acrobat/Reader (not through a Web browser), screen captures are displayed in a separate window (overriding the Open Cross-Document Links in Same Window local preference). Download the text example here [56KB] and download the graphics example here [48KB] (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader).
In any case, it is important to inspect the text quality at various magnifications, as well as in print (if print is intended).
Dynamic Image Interpolation. Acrobat’s Dynamic Image interpolation is not recommended for screen captures or other bitmaps that include text. When used with common zoom levels, interpolation makes the text fuzzy or greyish — when displayed and possibly also when printed.
This article was last modified on January 18, 2023
This article was first published on August 12, 2003
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