Re: why is 32 bit image setting making such a difference?

AB Sauers wrote:

With the latest Photoshop's ability to work somewhat with 32 bit images, as an experiment, I decided to convert all planetary 8 and 16 bit images I had from the past to 32 bit, apply unsharp masking and see what would happen. To my surprise, these converted images seem to be able to take, in some cases, quite a bit more sharpening at 32 bit before noise and highlights become problematic. In fact, I have been able to significantly clear up several images simply by converting from 8 bit to 32 bit and then unsharp masking carefully. My question is, why is this? I always figured that once an 8 or 16 bit image was saved that converting up to a higher bit was just a waste of space and of no benefit, but yet I am seeing results.

Simply converting the images to 32 bits per channel isn't producing the
improvement. You're evidently seeing improved processing.

I haven't looked at Photoshop's implementation in detail (it's coming
to the party a bit late, since a number of high-end CG programs have
been producing and accepting images in these bit depths for a while),
but in principle there's no mystery in what you're seeing.

32-bit gives you both higher precision (smaller steps between intensity
levels) and greater range (intensities higher than the default white
level). Both of these can improve the accuracy of the intermediate
calculations involved in an unsharp mask operation.

At least some of the noise you see in a sharpened image is created by
the sharpening, rather than being inherent in the image data. It's a
form of aliasing. In 8-bit, a calculation that yields a pixel level of
127.5 has to be rounded to 127 or 128, and a level greater than 255 has
to be clipped. With floating-point levels, on the other hand, you have
about 100000 shades of gray between (the equivalent of) 127 and 128, and
about a billion shades brighter than 255.

- Ernie http://home.comcast.net/~erniew
.

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