Saturday, May 14, 2011

PDFs - are they all created equal?

Adobe's PDF (Portable Document Format) is ubiquitous. There are many libraries available to create output in PDF format. But do they all produce equal results?

A peek under the hood tells an interesting story.

There are two types of image files: raster and vector.

Raster files are like a TV picture: lines upon lines, divided into dots (pixels). If you stand back far enough, the dots blend into a smooth picture.

A vector file is like "connect the dots": lines are drawn between specific points. Vector drawings are crisp and precise.

Raster files can be very large; vector files are compact.

Raster files are good for photographs and "continuous tone" images. Vector files are better suited for line drawings.

When it comes to Genealogy charts, the lines and fonts look better in vector format. Naturally, photos are best stored as raster. A good PDF library will support both raster and vector, using the most appropriate storage for each format.

If a Genealogy chart is stored entirely in raster format, as in the example at the left (taken from a popular genealogy program), the raster lines can only approximate diagonal lines and letters. The result is a coarse, "stair case" appearance.

In addition, pure raster PDFs are much larger, require more storage space, are slower to transmit, and slower to download from a Web page.

Vector PDFs, such as the one at the right generated by Progeny's Charting Companion, are crisp and sharp at any magnification. They are often hundreds of times smaller than raster files. You can take a vector chart to a local print shop for printing on a large-scale plotter or printer.

Why bother at all with raster for line art? It is simpler for a program to output raster. The less expensive (and more limited) PDF libraries will output raster because it is easier. This economy is achieved at the expense of quality.

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