IOS, how to clear context graphics

Question

In my app I have a method that draws a pdf into a context:

 CGPDFPageRef page = CGPDFDocumentGetPage(pdf, index + 1);

 CGAffineTransform transform = aspectFit(CGPDFPageGetBoxRect(page, kCGPDFTrimBox),
                                                          CGContextGetClipBoundingBox(ctx));

 CGContextConcatCTM(ctx, transform);

 CGContextDrawPDFPage(ctx, page);

Now in drawLayer, that is called when zooming, I do the necessary transformations and call again CGContextDrawPDFPage(ctx, page);

What happens is that a zoomed pdf is drawn on top of the first pdf, the problem is that in a particular page with only text, the back and blurred pdf is shown. That is strange, I thought that pdf page had white background and if this happens it's because the zoomed pdf on top has transparent background.

Now, to solve this how can I clear the context right before the CGContextDrawPDFPage(ctx, page) of drawLayer method? I tried:

//self.view.transform = CGAffineTransformIdentity;

//CGAffineTransform transform = CGAffineTransformIdentity;
//CGContextConcatCTM(ctx, transform);

//CGContextClearRect(ctx, layer.bounds);

Nothing works...thanks in advance

Your question suggests that the first context is not the same context as you use in `drawLayer:`. How do you create the original ctx, when do you draw into it, and what are you doing with it afterwards? — Rob Napier, Mar 15 '12 at 16:42

score 13 · Answer 1 · answered Jan 07 '13 at 14:33

13

CGContextRef context = UIGraphicsGetCurrentContext();
CGContextClearRect(context, self.bounds);

answered Jan 07 '13 at 14:33

Will

1,573
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score 4 · Answer 2 · answered Mar 15 '12 at 16:33

4

Did you try to flush the context as below?

CGContextFlush(ctx);

answered Mar 15 '12 at 16:33

cocoakomali

1,346
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IOS, how to clear context graphics

2 Answers2