Crash when I convert PDF pages to PNG images

Question

I need to convert each page of a PDF document to PNG images. These images are then displayed in a scrollView. I need to create two images per page: one at the screen dimension, and one 2.5 times bigger (it is used when the user zoom in the scrollView).

My problem is that I have sometimes memory warnings and crashes when I create big images. The way I do it is well-known:

    CGRect pageRect = CGPDFPageGetBoxRect(pdfPage, kCGPDFMediaBox);
    float pdfScale = 2.5*self.view.frame.size.height/pageRect.size.height;
    pageRect.size = CGSizeMake(pageRect.size.width*pdfScale, pageRect.size.height*pdfScale);    

    UIGraphicsBeginImageContext(pageRect.size); 
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetRGBFillColor(context, 1.0,1.0,1.0,1.0);
    CGContextFillRect(context,pageRect);    
    CGContextSaveGState(context);
    CGContextTranslateCTM(context, 0.0, pageRect.size.height);
    CGContextScaleCTM(context, pdfScale,-pdfScale); 
    CGContextDrawPDFPage(context, page);
    CGContextRestoreGState(context);

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

The problem occurs on iPhone 3G/3GS and iPod Touch. How can I limit the memory consumption while still having a zoom scale of 2.5 ?

Thanks !

Answer 1

You could consider using a webview to display the PDF, that takes care of the zooming and memory issues. Basically rendering the vector format PDF to a bitmap PNG at a large size is possibly not the most appropriate design decision. Without knowing what your requirements for the PNG are though, it's hard to comment.