Photoclubalpha

This is a question I recently asked myself: Take a picture and rotate it by 1 degree to straighten a horizon. Or apply some lens distortion correction. What happens to all those pixels that have previously been optimized to make a nice picture? Obviously, exposure, histogram, colours, overall contrast etc will not be affected. But what about sharpness?

Now, of course sharpening is your last step in your workflow. But if I understand correctly, some sharpening is always applied during raw conversion (doesn't matter if you shoot raw or jpg). Now all these carefully interpolated pixels suddenly find themselves 34% overlapping with the left pixel and 71% or so overlapping with the upper pixel. Not a single pixel remains in place and each pixel is interpolated and merged with surrounding pixels to form a new picture.

Isn't this a serious degradation of quality? Could converters rotate the rawfile before interpolation? Am I just creating a problem that doesn't exist??

I don't see why there would be any overlapping. if you notice when you do straightening on a picture you generally have to crop to make the frame square or rectangular again which indicates that the pixels are all just moved to new locations rather that squashed into a certain size.. can pixels even overlap? I always imagined them being layed out in a grid and the only time you would lose quality is if the computer had to guess what an adjacent pixel is supposed to look like for example if you resized a photo to be larger the computer would have to make up information to fill out the grid.

Please run the experiment and report back.

I often tilt my images a little using Picasa, and sometimes I tilt them and alter the perspective using PTLens, which is a much more complex kind of image transformation and remapping. I wanted to know how much image detailed resolution was lost in whatever internal pixel adjustments went on behind the scenes. I was very pleasantly surprised to find that very little indeed was lost, so little that it took quite a lot of careful pixel peeping to find special nasty cases where some minor degradation could just be discerned. And I can't find any other kinds of quality loss. I am, by the way, a fan of highly detailed architectural shots and very fussy about fine detail resolution.

So for all practical purposes I now consider that these operations involve no loss of image quality. Of course, there may be some editors which so them badly. For example, I've heard some people sneer at Photoshop's failure to use Lanczos interpolation when doing these kinds of things. I don't now if that's true, or if it matters.

bossel wrote:But if I understand correctly, some sharpening is always applied during raw conversion (doesn't matter if you shoot raw or jpg).

You don't understand correctly. Sharpening can be applied at raw conversion time but it's not mandatory. It's your choice if and when to sharpen.

bossel wrote:Now all these carefully interpolated pixels suddenly find themselves 34 % overlapping with the left pixel and 71 % or so overlapping with the upper pixel. Not a single pixel remains in place and each pixel is interpolated and merged with surrounding pixels to form a new picture. Isn't this a serious degradation of quality?

Usually not. Of course it would be a good idea to postpone your final sharpening until after the rotation. Don't think of the original pixels as the absolute truth and interpolated pixels as cheated ones. In fact, even the original pixels were nothing but the result of an interpolation process. Another won't hurt ... if it helps to get the horizon straight ...

bossel wrote:Could converters rotate the raw file before interpolation?

In theory, yes ... umm, maybe they do exactly that. Or maybe not. I don't know.

bossel wrote:Am I just creating a problem that doesn't exist??

I think so. If your image requires getting rotated then by all means go ahead. Do the rotation early in your workflow, and final sharpening later.

-- Olaf

01af wrote:

bossel wrote:Am I just creating a problem that doesn't exist??

I think so. If your image requires getting rotated then by all means go ahead. Do the rotation early in your workflow, and final sharpening later.

Thanks Olaf, I shall then continue to rotate my pics without any worries