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I'm trying to create a tool to rasterise vector images—stored in PDF files—on macOS, but the resulting images contain artifacts around the edges of some shapes. Preview.app, on the other hand, always renders the PDF flawlessly, as shown in this example:

Example results

I've tried:

  • Loading the PDF document using PDFKit, and rendering the page using both draw(with:to:) and thumbnail(of:for:)
  • Loading the PDF document into an NSImage (which creates an NSPDFImageRep), and using cgImage(forProposedRect:context:hints:)

In both cases I get these aliasing-like artifacts as seen on the left-hand-side of the image above. The PDF file is out of my control, so can't be changed to fix any issues it might have. I'm currently trying to migrate away from Cairo (which renders correctly) to Apple's PDF rendering for performance reasons (PDFKit renders it much more quickly, albeit with these artifacts).

Is there anything I've missed which would fix the output?

Robert
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  • Post your code please. It looks like Preview uses a `PDFView`. Have you tried to display the PDF file in a `PDFView` or `NSImageView`? Create a test project, add the view, set accept dragged files/editable, run, drag the file, any artifacts? – Willeke Jul 18 '21 at 08:45
  • Maybe this discussion helps: [PDF to JPG Conversion in COCOA](https://stackoverflow.com/questions/1298632/pdf-to-jpg-conversion-in-cocoa) – Willeke Jul 18 '21 at 08:49
  • @Willeke I think it might actually be a separate concurrency issue, as soon as I confirm I'll post an answer. – Robert Jul 18 '21 at 09:07

1 Answers1

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So it looks like the issue was caused due to me rasterising PDFs on multiple threads (specifically my tool rasterises PDFs in multiple resolutions, so I thought why not simultaneously).

Performing the operations sequentially on the main thread instead fixed it. I thought that I had come up with a way to use it concurrently by initialising the CGContext manually (instead of using NSImage's lockFocus()/unlockFocus() and NSGraphicsContext.current), but alas, as I soon as I add a context.scaleBy (to generate the images at different sizes), it fails again.

So for now I'm just doing it on the main thread until another solution comes along.

Robert
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