Blends in PDF files

A blend is a gradual transition from one color or tint to another. Sometimes a blend is also referred to as a gradient or fountains.

A lot of applications can be used to create such blends. There are also quite a number of ways in which this effect can be achieved. We can make a distinction between the techniques used prior to PostScript 3 and Acrobat 4 and those used after those releases.

Blends in a PostScript (level 2) workflow

Prior to the release of PostScript 3 and Acrobat 4, a PDF would contain a blend (or vignette) in a similar fashion to the way the originating application had encoded the blend in its PostScript print file.
Most applications encode a blend in PostScript by creating a multitude of objects (lines, rectangles or ellipses in case of a circular blend) that are offset slightly or change in size and that all have a slightly different color. On screen, and on the output, you see a blend but in PostScript code, you may see 256 boxes that all overlap each other and are colored slightly different.
Some applications like PowerPoint create such code in a very inefficient way and Acrobat Distiller or PDFwriter will include the same inefficient algorithm in the resulting PDF file. Have a look at the PowerPoint page to see what I mean.

PostScript 3 smooth shading

All of this changed when ‘smooth shading’ and ‘idiom recognition’ made an appearance in Adobe products. Smooth shading is a technique that Adobe introduced in PostScript 3 and PDF 1.3. It is a very fast and compact way of defining blends that look great and output fantastic, even at fairly low resolutions or high screen rulings.

Unfortunately there are still a lot of applications on the market that cannot generate smooth shadings themselves, either because the programmers are too lazy to implement the algorithm or because the programmers want to guarantee compatibility with older non-PostScript 3 RIPs.

Idiom recognition

The above problem explains why Adobe introduced an mechanism called ‘idiom recognition’ in PostScript 3. This algorithm scans a PostScript file and if it finds a blend created in one of the popular prepress applications like QuarkXPress, Illustrator or FreeHand, it replaces that blend with the superior PostScript 3 smooth shadings. A similar technique was already used for years by companies like Scitex in their proprietary RIP-technology.

  • Acrobat 3 and idiom recognition – Acrobat 3 does not perform idiom recognition. Even worse, idiom recognition will not work when a RIP tries to process PDF files created using Acrobat 3.
  • Acrobat 4 and idiom recognition – Adobe integrated the same idiom recognition algorithm used in its RIPs in Acrobat 4 Distiller as well. You can switch off this mechanism if needed. In the help subdirectory of Acrobat 4 there is a document called “distparam.pdf” which describes how this can be done.
  • Acrobat 5 & later and idiom recognition – Acrobat 5 contains the same algorithm as version 4, but now it can be activated or deactivated using the “Advanced” tab in the “Job options” of Acrobat Distiller. The option is called “convert gradients to smooth shades”.

Displaying blends in Adobe Acrobat or Adobe Reader

Blends should look great on screen. If this is not the case, it is usually caused by the settings of the display. If your graphics card is set to 256 colors or (in case of PCs) to high color (also called ’16-bit color’), the video card does not have enough colors at its disposal to display blends perfectly.

Printing blends from Adobe Acrobat or Adobe Reader

When a PDF document containing smooth shadings is printed from Acrobat or Acrobat Reader, the handling of the blend depends on the type of output device:

  • On PostScript 3 printers, Acrobat simply forwards the smooth shading code, so the output is both fast and of excellent quality.
  • On older non-PostScript 3 devices or printers that do not use PostScript at all, Acrobat will convert the blend to a bitmap image at a resolution that is optimised for the output device. The output will also look fine but it will take a lot longer to print.