Saturday, May 25, 2013

How to fix PDFs that appear with colors too dark or too bright when viewed on iPad or iPhone.

In some of my iOS projects I had to display PDFs of documents originally intended for print format. Some of those PDFs would appear horribly on iOS devices, with colors messed up. It was a pretty surprising behaviour, which I initially attributed to bugs in Apple's PDF rendering code.

My first approach to getting around such problems was to use a server-side component powered by Ghostscript to convert each PDF page into an image, that would show up in the right colors, and display that. The advantage of this approach is that it can be fully automated. The disadvantage is that images may show up poorly (pixelated) when zoomed in, depending on their pixel density.

Upon investigating this problem further I discovered a way to fix this problem at its root, that is the source PDF document. Unfortunately it can't be automated easily because it involves running Adobe's Acrobat tool. In the absence of a public API to run that conversion programatically one has the option of using workflow-recording tools like Apple's Automator to drive the Acrobat automatically. While that approach may work, it would be hard to scale in a cost-effective manner.

Another option to handle this may be to use iText to automatically convert CYMK profiles to RGB, or if that's not possible, to at least detect the presence of CYMK profiles and alert the user about the need to convert the PDF manually using Acrobat.

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