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Image scaling

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Image scaling

Subject: Image scaling
From: Adam Knight <email@hidden>
Date: Fri, 4 Jan 2008 16:25:53 -0600

I think I'm missing something here.

I need to take an image and scale it (very cleanly) down while keeping the metadata. The source is a JPEG file on-disk (typically from a digital camera). The destination is an NSData object in memory that is JPEG-encoded with the same metadata.

I tried this using CIImage and a CILanczosScaleTransform filter then using NSBitmapImageRep's initWithCIImage method. This worked very well and made a great image, but it looks like Apple's initWithCIImage code leaks a whole copy of the image outside of my reach. I'm working with too many images for this to be good. Also, this method drops the file metadata entirely (EXIF, etc.) and doesn't appear to give me the chance to add it back in.

So then I tried using a CGImageRef with CGImageSourceCreateThumbnailAtIndex and copying the metadata with CGImageSourceCopyPropertiesAtIndex. This works well except that the scaled image is blurry and not at all professional. I tried fiddling with CGContextSetInterpolationQuality to no avail. Almost there.

It appears that NSImage does not scale the backing image, but only the display one. Tricks involving drawing onto a new context are not thread-safe. Thus, I cannot use that.


The only solution that's coming to mind is one I really dread:

* Get the JPEG data with a CGImageSourceRef * Store the metadata * Make a CIImage (from the original file or the CGImage) * Do the transform that works * Make the CIImage a CGImage without using NSBitmapImageRep (Can I even do this? I haven't found this path.) * Copy the metadata to a CGImageDestinationRef item along with the CGImage * Generate the JPEG data object using CGImageDestinationCreateWithData

This seems overly complicated just to scale an image properly. Everything appears highly optimized for drawing on-screen rather than to a data structure in a threaded app.

Can anyone tell me how I can easily scale an image with a good/ excellent quality in a thread-safe manner in Mac OS X?


Adam Knight
Co-Founder; CTO
Barton Springs Software

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