Acrobat Tips: Graphics in PDFs
Illustrations
For the most part, illustrations are displayed better as vector graphics than as bitmaps.
However, as much as possible, try to refrain from using diagonal lines/oblique angles, because of the staircase effect (Smooth Line Art will minimize this effect). Shapes made of vertical and horizontal lines will be displayed better. If diagonals are essential, use diagonals with 45 degrees (or 30 or 60 degrees as a second choice).
If you use complex vector illustrations (such as engineering drawings originating from CAD programs), you may experience extremely slow display (related to numerous fragments) or undesired display-by-layer effects.” Simplify such vector graphics as much as possible; if this is not possible (for example if the original was created in a 3-D program and then converted to DXF), consider converting specific illustrations to bitmaps when the primary use is on-screen display (experiment with scaling the vector graphics before conversion to bitmaps). Be aware that after conversion, print quality may be affected and text objects will not be searchable (although often the text in engineering drawings is composed of strokes and is not real text).
Text in Illustrations. If your illustrations or charts include text that users may want to search for, or if you want to make sure that the text is displayed as best as possible at various magnifications, avoid the Convert Text to Outlines function in your graphic program. This damages text display and will also prevent access to the text when using Acrobat’s Find/Search functions.
Patterns & Fills. Most patterns and hatches (such as those available in FrameMaker, Word or Excel) are substituted with shades of gray, or disappear altogether, when converted to PDF.
There are variations, related to applications, printer drivers used and specific Acrobat releases. (In some cases, Type 3 bitmap fonts are used to simulate patterns — with gray stripes at normal magnifications, reasonable display at large magnifications, and slower rendering.) Even PostScript patterns are not displayed correctly at lower magnifications, sometimes looking like a solid black or white box.
Semi-transparent fills from Microsoft Office documents are not displayed correctly (opaque if the PDF is created with Acrobat Distiller).
Gradient fills often end up in PDFs as hundreds or thousands of vector items working together to achieve the specific gradient fill effect. If applicable to your drawing or authoring application, check whether Distiller’s Convert Gradients to Smooth Shades is selected (Settings> Job Options> Advanced).
Inconsistent display of line thickness. Acrobat frequently displays lines of the same weight at varying widths. This distortion only applies to display, and varies with magnification (thin lines in one magnification may appear thicker in another magnification, and vise versa). Using the thinnest lines supported by your graphics/authoring program may reduce the problem, but does not solve it. Setting the default zoom of the PDF to a multiple of 50% may also help.
Acrobat supports the PostScript setstrokeadjust setting, which was introduced to improve line thickness consistency in lower resolutions. But this has to be supported at the authoring application level as well as carried through the distilling process.
While display results with setstrokeadjust enabled are generally much better, line width consistency is still not guaranteed at all zoom levels. In addition, as a result of display bugs, some lines may gain extra length when this option is activated. Download an example here [20KB] (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader).
Interface items (such as buttons, icons, etc.)
For optimized display of smaller, repeating items, consider using characters from symbol fonts or custom fonts. Display of these will generally be better than vectors or bitmaps, especially as text smoothing is turned on by default and as it is generally implemented more efficiently than smoothing of bitmaps or vector graphics (if enabled).
Note however that recent Adobe PostScript drivers for Windows 2000/XP may fail to process custom fonts (omitting these characters altogether) so check this with your specific combination of tools.
Company Logos
As smoothing of vector graphics is turned off by default, logos implemented as vector graphics may exhibit lower-quality display, especially when the graphics are in small scale.
For optimized display, consider having a company logo implemented through a font (integrated as a character in your authoring application) or as a bitmap.
Interactivity Aspects Relating to Graphics
Entries in the List of Figures (together with other key navigational items, such as Table of Contents, Index, List of Tables), as well as cross-references to figures, should be linked to the corresponding figures, so that items are easily accessible when reading the document on screen.
A common practice is that links point to the figure title (typically the result of a cross-reference to a paragraph holding the title). When such a link is clicked, the figure itself may not be displayed in full. For optimal performance, it is best if the link has the figure itself (graphics plus title) as a target (a non-printing Back button may be implemented as a special icon in the graphics area). Download an example here [172KB] (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader).
In terms of bookmarks, which provide an on-going and easy access to all key headings and items, the List of Figures itself should be bookmarked, as a minimum.
Specific bookmarks pointing to the figures directly may be useful in publications with many titled figures, intended to be used frequently. But, to be effective, such bookmarks to figures must not be “mixed” with other bookmarks according to the location of the figures, often collapsed under lower-level headings. Instead, a separate list of figure bookmarks under a corresponding heading is much more efficient — simulating the List of Figures in printed books. Download an example here [120KB] (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader).
Mostly applicable to complex graphics, it is also possible to have bookmarks that display specific areas in a figure. Download an example here [64KB] (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader).
Within the graphic itself, references to other document parts can be linked as needed. In the case of troubleshooting diagrams or flowcharts, for example, each box may be linked to the corresponding procedure (and marked as a link). In the case of a drawing of a machine, for example, clicking each part may take the reader to more information on that item, with tool tips or rollover text providing additional information on the link target or highlighted part. The display of specific label only when the cursor points to a specific part improves usability and helps reduce the clutter. Such a hyperlinked diagram can serve as a visual table of contents, in additional to a text-based listing.
Display: Interaction through thumbnails . Acrobat’s thumbnails may be very useful with larger graphics, as they identify the viewed area within the graphics in the form of a red rectangle in the thumbnail. The red rectangle can be dragged by its border, with an immediate change of the displayed part and continuous indication on the viewed part in the larger diagram. Dragging the box at the bottom-right corner of the red rectangle in the thumbnail will adjust the zoom level.
Starting with Acrobat 5, thumbnails are generated dynamically when opening the PDF in Acrobat or in Reader (but not when the PDF is displayed through a Web browser).
Thumbnails can be embedded in the PDF for the benefit of users who view PDFs through a Web browser or use previous Acrobat/Reader versions, or for a slightly faster display. Embedding the thumbnails increases the file size, typically in the range of 6-12K per page. Acrobat Distiller can create thumbnails and embed them in the PDF being distilled, but these are of lower quality compared to the thumbnails displayed/embedded in Acrobat itself.
Thumbnails can also be useful when viewers are interested in locating a specific illustration in graphics-intensive documents. In such documents, a default opening mode showing thumbnails may be a better choice than bookmark display.
Zoom-in/sequence using Acrobat articles. Acrobat articles can be used to zoom-in on a specific figure, if that figure is defined as a single-rectangle article. Clicking the graphic area will zoom in to show as much as possible in terms of magnification. Clicking the article area again will show additional parts of the graphics if not currently displayed (depending on the maximum zoom specified and graphics size) or go back to the regular view if the article area was viewed in full. Download an example here [40KB] (Control-click or right-button click the link to download the .zip file).
Related graphics can be linked to form a single article thread (each graphics being a separate rectangle), enabling easy navigation between the different items.
Tool tips. Tool tips can be used to display brief information about graphics when there is no immediate information above/below the graphics, to display additional information (e.g. effective dates), or to provide brief information on the purpose of buttons/icons. Download an example here [344KB] (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader, note that version 5 is required for this one).
Movies. Movies open up additional graphics display and interaction capabilities. QuickTime movies with multiple still images (slide shows) can show sequences of images, such as before/after or steps in a process. QuickTime virtual reality movies enable the reader to inspect an item from different view points, zoom in/out on items of interest.
Shlomo Perets is founder of Microtype, which specializes in FrameMaker and PDF training and resources.
Copyright © 2002-2003 BinaryThing Pty. Ltd. All Rights Reserved. Used with Permission.
This article was last modified on January 18, 2023
This article was first published on August 12, 2003
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