Generation of large PDF file thumbnails causing crash on iPhone/iPad

I have written code to generate thumbnails from pdf files & save them as png pictures, but the same code that runs successfully on iPad 2 would crash on iPad 1.

After loading up instruments I saw at one point the memory usage spikes up to 20mb when generating the pictures for half a dozen pdfs, then it would drop back to normal levels after the CG drawing is done, I'm pretty sure the intensive memory load is causing the crash because I replaced that portion of the code with loading normal pictures & it worked fine on both devices.

So the question is: how to generate pdf thumbnails with CG functions and use as little memory as possible? I'm thinking of running it on a background thread but I don't think that would optimize the memory usage in any way...

The code is also applying a gradient effect on the thumbnail as well, that's a functional requirement...

Here's the code:

+ (UIImage *)imageFromPDFWithDocumentRef:(NSString *)documentPath{  
    CGPDFDocumentRef documentRef = CGPDFDocumentCreateWithURL((CFURLRef)[NSURL fileURLWithPath:documentPath]);  
    CGPDFPageRef pageRef = CGPDFDocumentGetPage(documentRef, 1);
CGRect pageRect = CGPDFPageGetBoxRect(pageRef, kCGPDFCropBox);

if(pageRect.size.width >pageRect.size.height){
    pageRect.origin.x = (pageRect.size.width - pageRect.size.height)/2;
    pageRect.size.width = pageRect.size.height;

}else{
    pageRect.origin.y = (pageRect.size.height - pageRect.size.width)/2;
    pageRect.size.height = pageRect.size.width;
}   

UIGraphicsBeginImageContext(pageRect.size);
CGContextRef context = UIGraphicsGetCurrentContext();   

CGContextTranslateCTM(context, CGRectGetMinX(pageRect),CGRectGetMaxY(pageRect));
CGContextScaleCTM(context, 1, -1);  
CGContextTranslateCTM(context, -(pageRect.origin.x), -(pageRect.origin.y));
CGContextDrawPDFPage(context, pageRef);

CGGradientRef glossGradient;
CGColorSpaceRef rgbColorspace;
size_t num_locations = 2;
CGFloat locations[2] = { 0.0, 1.0 };
CGFloat components[8] = { 1.0, 1.0, 1.0, 0.1,  // Start color
    0.0, 0.0, 1.0, 0.7 }; // End color

rgbColorspace = CGColorSpaceCreateDeviceRGB();
glossGradient = CGGradientCreateWithColorComponents(rgbColorspace, components, locations, num_locations);

CGRect currentBounds =开发者_如何学Go pageRect;

CGPoint topCenter = CGPointMake(CGRectGetMidX(currentBounds), 800);
CGPoint midCenter = CGPointMake(CGRectGetMidX(currentBounds), 0);


CGContextDrawLinearGradient(context, glossGradient, topCenter, midCenter, 0);

CGGradientRelease(glossGradient);
CGColorSpaceRelease(rgbColorspace);     

CGRect thumbRect = CGRectMake(0, 0, 187, 187);

UIImage *finalImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();
CGPDFDocumentRelease(documentRef);

return [UICommon resizedImage:finalImage rect:thumbRect];
} 

Thanks!

Calling

CGContextSetInterpolationQuality(context, kCGInterpolationHigh); 
CGContextSetRenderingIntent(context, kCGRenderingIntentDefault);

before CGContextDrawPDFPage solved a similar problem of mine.

Credits goes to this answer of Johann: CGContextDrawPDFPage taking up large amounts of memory