Quarter Life Crisis

The world according to Sven-S. Porst

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Gradients revisited

478 words

When taking the screenshots for my recent Core Image gradient experiment, I ran into a really strange situation: At some stage my gradient looked ‘broken’ as well – not unlike the one I used as an example for a ‘bad’ gradient. And a bit later things looked just fine again and I couldn’t reproduce or otherwise analyse the problem. Really puzzling. But I started bugging people to send me their screenshots of the same Quartz Composer file (thanks Dan, G and Tom) and , hoping that I could perhaps track this down to some difference in hardware or software versions.

First I suspected that cheapo Intel graphics chip set, then I suspected CoreDuo MacBooks versus Core2Duo MacBooks, then I suspected subtle differences in system versions. But without any conclusive result – I got counterexamples for each of these suspicions. In particular, I got both a good and a broken gradient on my own machine at different times. Puzzling, very puzzling. Running out of options to blame this problem on and hoping that the Mac’s graphics system doesn’t magically change the way it works in the middle of the night, I looked at the only remaining component which is the screencapture tool that OS X uses to take screenshots.

And indeed that tool – or at least that tool in combination with Quartz Composer windows seems to be what caused the problem. Enlarging the image by 500%, and enhancing the contrast you can see the difference. When the image is cropped from a rectangular screenshot (taken with Command-Shift-3 or Command-Shift-4 and the selection tool) everything looks how it should look – with vertical bars:

Enlarged gradient

However, when using Command-Shift-4 and the space bar to take a screenshot of the Quartz Composer display window only and cropping the gradient from there, I got this result with not exactly vertical bars:

Enlarged gradient

The good thing about this is that it probably doesn’t indicate a weirdo problem with OS X’s graphics system – that is, the actual on-screen display should be fine – but it may just be a quirk of the screencapture tool. Admittedly a quirk which seems very strange to me and which I fail to understand, but just a quirk of that tool – possibly just on Quartz Composer windows.

I am very tempted to write something about conspiracy theories here – after all these distortions could be data which the screenshot tool embeds in your images. I’m pretty sure that if you play your cards well you could get a story like that on the Mac hysteria sites. Particularly as pretty much all of the X.5 screenshots they run have distortion in them that doesn’t look like the JPEG artifacts I get when JPEG compressing images. One certainly could try to get such a story out and see who starts to panic. That certainly would be fun.

June 5, 2007, 0:32

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