Acrobat Tips: Graphics in PDFs
Editor’s Note: The following article was written with Acrobat 5 in mind and does not specifically discuss the recently released Acrobat 6, although it has been reviewed for accuracy and relevance to the new software.
Any discussion of graphics in PDFs is inherently closely related to the specific content, intended uses of the PDF file, and the function of the graphics within the file. Factors such as graphic format, image resolution, and color depth all depend on the specific use of the PDF. And because of the many issues involved, there are invariably trade-offs and compromises to be made.
The manner in which graphics are displayed in a PDF depends on a number of factors, including the source graphics, its graphic format (vector/bitmap), the bitmap resolution as indicated in the authoring application, the distilling job options, and the viewing preferences.
Graphic Formats
Graphics are stored as a computer file in two distinct ways: bitmap and vector.
Bitmap graphics are a collection of dots called pixels (picture elements), arranged in a matrix, each pixel with a different color value. Bitmaps are the native format of photographs, scanned images, and screen captures. The number of pixels in a given area is known as resolution, commonly expressed as dpi (dots per inch) or ppi (pixels per inch).
Bitmaps are resolution-dependent, which means that for optimal display in Acrobat, there must be a match between the pixel matrix stored in the graphics and the display pixel matrix. When a given bitmap is displayed using a different number of pixels, let’s say a 100-by-100-pixel bitmap is displayed in an area of 160 by 160 pixels, or in an area of 90 by 90 pixels, some pixels are added or eliminated. This process is known as resampling, and results in degradation of image quality. Rotating bitmaps using angles other than 90 degrees also causes images to be distorted.
Common bitmap formats include BMP, GIF, TIFF, JPEG, and PNG.
Vector graphics are a list of drawing objects (such as lines, arcs, curves, circles, and rectangles), encoded as textual or binary instructions.
Vector graphics work best when real-life photographs are not necessary. In fact, they can often be clearer in detail than a photograph, enabling you to emphasize the important details and eliminate or minimize details that cause visual noise. However, when vector graphics try to imitate a photograph, the look can be synthetic or technical.
Vector graphics can also pose some difficulties when displayed in Acrobat:
- Thin lines may be displayed with inconsistent line thickness.
- Complex multi-component drawings may challenge Acrobat’s drawing functions, resulting in a painfully slow painting process that involves many thousands of segments. As the rendering result is not cached by Acrobat, the slower display is experienced every time you zoom in/out or go the next page and back.
- When you have layers or gradations implemented as multiple objects, Acrobat’s display process shows different objects temporarily displayed and then covered partially — which may sidetrack the reader’s attention.
As a vector graphic is independent of display/output resolution, it can be scaled or rotated flexibly. It will always display/print at the highest resolution of the output device used. Because of this, a PDF with vector graphics can be viewed at various magnifications (effectively different resolutions) with no degradation or loss of quality.
Common formats for vector graphics include WMF, SVG, and CGM. Authoring tools, such as Microsoft Word or Adobe FrameMaker, include their own vector graphics tools, which are directly carried to the PDF during distilling.
Some vector formats (WMF, for example) support bitmaps as a component. EPS can contain both vector graphics and bitmap graphics.
Following are general recommendations and guidelines to improve the handling of graphics in your PDFs.
Original Graphics
Make sure that your original graphic has the level of detail or color depth needed.
Graphics quality can only deteriorate during conversion to PDF, or when the PDF is displayed. If the original graphic is not of sufficient quality, nothing will help. Be aware also that saving original graphics in some graphics formats will either reduce the number of colors (e.g. GIF) or cause an irreversible loss of image data (JPEG). The Compress Text and Line Art setting in the Distiller Job Options affects only vector graphics. This setting should always be selected, as the compression of text and vector graphics is lossless.
As to bitmap compression/resampling, you must make a choice based on the current state of the graphics and the intended uses of the PDF. As a minimum, lossless compression should be used (Zip, 8-bit). But as lossless compression is often not good enough in terms of compression ratio, lossy JPEG compression may be used, depending on the image properties and after carefully inspecting the impact of the different quality settings on various images used. Resampling may be effective if your graphic has a resolution that is much higher than needed, such as 600 dpi (alternatively, you can resample specific images in a graphics program).
Close familiarity with the type of graphic information and the context of uses is essential. For example, JPEG compression, even of the highest quality, will damage delicate medical images to be analyzed by experts; on the other hand, its effect on general-purpose background images may not be noticeable, even when medium/low quality compression is used.
As a general recommendation, the Color Management Policies (Distiller Settings> Job Options> Color tab) should be set to Leave Color Unchanged. In some screen captures, downsampling may cause text to be unreadable when displayed or printed. In other instances, lossy compression results in blotchy areas and unreadable text.
Rewrite the PDF
Make sure to rewrite the PDF (Save As, overwriting the same file), as this operation may improve the internal storage of graphics significantly. For example, identical graphics included on every page will be stored only once and reused as necessary.
Acrobat/Reader Preferences
Although you can provide the PDF user with guidelines for setting local preferences for optimal viewing quality, it is likely that most Acrobat Reader users do not change the default display preferences (these are local to the specific computer, and cannot be controlled from a PDF file). It is best, therefore, if you plan the graphics so that they will be displayed reasonably with the default settings.
Smoothing Preferences. Smoothing, also known as anti-aliasing, softens the edges of graphics shapes and text in order to minimize the contrast between the background and the text or graphics.
Smoothing often improves on-screen display, but some people find smoothed graphics or text appear out of focus and prefer to turn the specific option off.
Smooth Text. Default: on. It applies to all text items, which may be present in vector graphics, or to text items serving as graphics (symbols or icons).
Smooth Line Art (Acrobat 5). Default: off. This preference improves the display of most vector graphics, but it may cause some thin lines to become gray. It also slows down the display, to some extent.
Smooth Images. Default: on. The image-smoothing option improves the display of some bitmaps (depending on the original image resolution and the current magnification).
Download this PDF file [44KB] to experiment with the impact of smoothing on different graphic types (Control-click or right-button click the link to download, or open the file in your Web browser with Acrobat Reader).
Accessibility Preferences (Acrobat 5). Some users may change the default color schemes to improve readability, and select their own colors for text and for page background. Black or “almost” black text will be displayed in the color specified for text; text in other colors will be left as is. When the page background color is specified to be non-white, the solid white fill of vector objects or white backgrounds of bitmap images (such as a bitmap displaying a triangle button) becomes noticeable and may be visually disturbing.
With vector graphics, you can use transparent or “no fill” settings to prevent the opaque white presence. This applies to graphic objects as well as to frames used to hold text or graphics in your authoring application.
Graphic Functions
It is widely accepted that well-designed graphics can improve communications, especially when dealing with technical subjects. This is even more applicable to on-screen documents than to paper-based documents, because of different reader expectations and reading patterns (readers tend to scan screens even more than pages, and have a shorter attention span for long paragraphs).
Which graphic format is better for PDFs: bitmap or vector? There are several considerations when deciding which format to use, first and foremost depending on the function of the graphics. The use of these main function categories in PDFs will be discussed in turn:
- Photographs (such as a product or person)
- Screen captures
- Illustrations (such as diagrams, flow charts, graphs)
- Interface graphics (such as buttons, icons and navigation maps)
- Company logos
This article was last modified on January 18, 2023
This article was first published on August 12, 2003
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