To continue on the recent image resizing theme (probably of interest to Python scripters only), I made some changes as a result of upgrading to Panther last week. I wanted to use the new built-in Mac OS X version of Python 2.3 (plus the MacPython Extras from Jack Jansen—thanks, Jack!). But a problem with the initial Package Manger distribution of the Python Imaging Library made me look at a new Panther feature that let Python scripts use the native Quartz graphics library directly. (The hitch with PIL was that it was built to require a Fink install of libjpeg for full JPEG support. A quick compile of libjpeg and placement of it and its headers into Fink’s preferred locations didn’t work, and either installing Fink or compiling PIL from source would have taken a while.)

That was as good a reason as any to explore Panther’s new Quartz scripting feature. So I read what I could find on Quartz, and modified my photo album code to use Quartz if available. It still uses PIL to gather EXIF and size information, which works even without libjpeg, but then it uses Quartz to manipulate the actual image content.

The results were terrific, mostly. In real-world testing on an 800 MHz PowerBook G4, the PIL-only version spat out 8 JPEGs per minute, and the Quartz version spat out 65 JPEGs per minute. That’s a welcome improvement, especially when you multiply my typical batch of 100 photos by 3 sizes apiece.

The one problem is that I don’t yet know how to set the quality level. There’s a parameter that should contain this number, but as far as I can tell it isn’t documented anywhere. All of the supplied examples save as PNG or PDF, rather than JPEG, and the function isn’t documented along with the rest of Quartz because it’s not a real Quartz function—the release notes say that image export is actually handled through QuickTime. (This will be the first public mention in the history of the world, as far as Google is concerned, of the Core Graphics function that the API summary says it calls: CGBitmapContextWriteToFile. The last parameter, vaguely named “params” and defaulting to a zero-length string, is where a data structure including the quality level would obviously go.)

So for now it’s using a default JPEG quality level, which, whatever it is, is noticeably worse than the quality=90 setting I used with PIL, especially on thumbnails. Though I haven’t done a controlled side-by-side test, it seemed that lower quality levels resulted in some low-frequency blurriness, which looked much less objectionable than the high-frequency ringing (making macroblock boundaries visible) that PIL tended to show. It looked bad enough that I couldn’t really run PIL with anything below quality=90. And because of the lower quality setting, the file sizes on the Quartz side were half that of the PIL versions.

Here’s all the code the deals with Quartz in the new photo album. newImagesInfo holds a list of destination file paths and pre-calculated pixel dimensions.

def resizeImagesQuartz(origFilename, newImagesInfo):
    # newImagesInfo is a list of 
    # (newFilename, newWidth, newHeight) tuples
    if not newImagesInfo: return
    import CoreGraphics
    origImage = CoreGraphics.CGImageCreateWithJPEGDataProvider(
        [0,1,0,1,0,1], 1, CoreGraphics.kCGRenderingIntentDefault) 
    for newFilename, newWidth, newHeight in newImagesInfo:
        print "Resizing image with Quartz: ", newFilename, \
            newWidth, newHeight
        cs = CoreGraphics.CGColorSpaceCreateDeviceRGB()
        c = CoreGraphics.CGBitmapContextCreateWithColor(
            newWidth, newHeight, cs, (0,0,0,0))
        newRect = CoreGraphics.CGRectMake(0, 0, newWidth, newHeight)
        c.drawImage(newRect, origImage)
        c.writeToFile(newFilename, CoreGraphics.kCGImageFormatJPEG)
            # final params parameter?

If you’re on a Panther machine with the Developer Tools installed, you can find the examples I started with in:


Seems obvious where they would be in retrospect. Thanks to the folks on the MacPython channel in iChat for pointing me to them.

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