PDF Highlighting above image / below text

Question

i'm trying to highlight text in a pdf, and have the highlighted rectangle to be drawn under the text.

It works fine on most PDF's, but I jumped into a problem when the text I'm trying to highlight has an image/background under it. The problem is that the highlight rectangle is drawn under the image as well, so it is not visible.

The drawing order I have is this:

• draw a blank rectangle with the page size
• draw the highlight
• draw the pdf using CGContextDrawPDFPage(context, page);

Is there a way to draw the PDF images and text separately? so that I could go

• blank rectangle
• pdf images/background
• highlight
• pdf text

Do I have to do something to the pdf / context so that it draws it automatically the way I want it to? I've tried messing with the context but nothing worked so far, it's all drawn entirely under or entirely above the full pdf

Every reader I've seen does this (PDFExpert, GoodReader, iAnnotate to name a few), so it can't be impossible, I just haven't found the solution yet :) Any help will help, thanks in advance!!

Cheers

Answer 1

My understanding is that these other apps are reading and rendering the PDF themselves (they support selecting text, or adding annotations, for example), so they would be able to much more easily layer things in the way you're mentioning.

The CGPDFDocument you're starting with is an opaque object (in the OO sense, not transparency) that can draw itself, but I don't know of any way to break out and render various sublayers of the document.

As a way forward, you could look at using Core Image (iOS5+) or some other method to blend the highlights layer with the PDF. If you used the right filter (Multiply, maybe), the darker text would still come through and a .3 alpha highlight would blend with any background.