Sobotka: Why GIMP is inadequate
Posted Jan 12, 2011 9:11 UTC (Wed) by farnz
In reply to: Sobotka: Why GIMP is inadequate
Parent article: Sobotka: Why GIMP is inadequate
Where it becomes important is not when painting afresh, but when editing an image. Take, for example, a bad photograph taken with inadequate lighting, and shadow on an amateur DSLR like the Canon Rebel series (D550 in my part of the world). The sensor has 12 bits of accuracy, or, put differently, 4 to 5 bits I can throw away when trying to fix my mistakes before putting this photo on the web. If I'm working in 16-bit, I can lighten the bits that were hidden in shadow by a small amount (say the equivalent of 2 bits), and then enhance contrast image-wide, costing me the equivalent of 3 bits of accuracy. Because I started with 12 bits, my resulting, visually improved image has between 7 and 9 bits of accuracy - when I downconvert to 8-bit for final output, I get 7 to 8 bits of accuracy; some people may notice this in the areas that were shadowy, but it's no longer that bad.
Now do the same process with an 8-bit image. I throw away 2 bits in the shadowy areas, leaving 6. I throw away up to 3 bits improving contrast, making the bits in shadow have just 3 bits of accuracy in the range 0x00 to 0xff. That makes the image look bad.
This is part of why photographers disagree on why it's needed; a better photographer would have got a decent exposure in the first place, one that didn't need quite so much fiddling to make it look good. I'm no expert, so I get a bad exposure of a well-composed scene, and have to play to make it look good (fortunately, my current tool of choice, digiKam, works in 16-bit, and gives me the adjustments I need to fix things).
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