reseting CGContextRef after drawing pdf page using CGContextDrawPDFPage - iphone

Question

I am trying to create thumb images for every pdf page in PDF document and place it in a UISCrollVIew. I have succeeded in this, but scrolling is not so smooth as I want when it's too fast.
And I want to optimize thumb images creating for Pdf page.
I want to create one CGContextRef and reset its content after CGContextDrawPDFPage, as a consequence I wouldn't have to create a context each time and perform some other calculation, which takes a lot of resources.
Is it possible to reset CGContextRef content after CGContextDrawPDFPage? CGContextRestoreGState and CGContextSaveGState seems to doesn't help in this situation.
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
GCPdfSource *pdfSource = [GCPdfSource sharedInstance];
for (int i = pageRange.location; i <= pageRange.length; i++) {
UIView *thumbPdfView = [scrollView viewWithTag:i+1];
if (thumbPdfView == nil) {
CGPDFPageRef pdfPage = [pdfSource pageAt:i + 1];
float xPosition = THUMB_H_PADDING + THUMB_H_PADDING * i + THUMB_WIDTH * i;
CGRect frame = CGRectMake(xPosition, THUMB_H_PADDING, THUMB_WIDTH, THUMB_HEIGHT);
thumbPdfView = [[UIView alloc] initWithFrame:frame];
thumbPdfView.opaque = YES;
thumbPdfView.backgroundColor = [UIColor whiteColor];
[thumbPdfView setTag:i+1];
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(NULL,
frame.size.width,
frame.size.height,
8, /* bits per component*/
frame.size.width * 4, /* bytes per row */
colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextClipToRect(context, CGRectMake(0, 0, frame.size.width,frame.size.height));
CGRect pdfPageRect = CGPDFPageGetBoxRect(pdfPage, kCGPDFMediaBox);
CGRect contextRect = CGContextGetClipBoundingBox(context);
CGAffineTransform transform = aspectFit(pdfPageRect, contextRect);
CGContextConcatCTM(context, transform);
CGContextDrawPDFPage(context, pdfPage);
CGImageRef image = CGBitmapContextCreateImage(context);
CGContextRelease(context);
UIImage *uiImage = [[UIImage alloc]initWithCGImage:image];
CGImageRelease(image);
UIImageView *imageVIew = [[UIImageView alloc]initWithImage:uiImage];
[uiImage release];
[thumbPdfView addSubview:imageVIew];
[imageVIew release];
[scrollView addSubview:thumbPdfView];
[thumbPdfView release];
}
}
[pool release];//release
and aspectFit function...
CGAffineTransform aspectFit(CGRect innerRect, CGRect outerRect) {
CGFloat scaleFactor = MIN(outerRect.size.width/innerRect.size.width, outerRect.size.height/innerRect.size.height);
CGAffineTransform scale = CGAffineTransformMakeScale(scaleFactor, scaleFactor);
CGRect scaledInnerRect = CGRectApplyAffineTransform(innerRect, scale);
CGAffineTransform translation =
CGAffineTransformMakeTranslation((outerRect.size.width - scaledInnerRect.size.width) / 2 - scaledInnerRect.origin.x,
(outerRect.size.height - scaledInnerRect.size.height) / 2 - scaledInnerRect.origin.y);
return CGAffineTransformConcat(scale, translation);
}

Answer 1

Try CGContextClearRect(context, contextBounds).
CGContextSaveGState and CGContextRestoreGState do not have any effect on the content of a context. They push and pop changes made to state aspects of the context like the current fill color.