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Old Nov 27, 2009, 5:51 PM   #11
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Quote:
Originally Posted by peripatetic View Post
This thread makes no sense at all to me.

Just rotated a few images by 1,2,3,4 degrees in photoshop, viewed at 100% on screen. No loss of sharpness, no extra noise.

What on earth are you lot talking about?
I agree. I think I have a misaligned viewfinder (or a misaligned eye) as most of my pics are tilted left to right about 1-2 degrees. I realign almost every pic that has a straight line in it, and have never noticed a loss in sharpness. I can understand that it happens, i just doubt it's enough to notice at normal print sizes.
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Old Nov 27, 2009, 6:49 PM   #12
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The sharpest an image can be is in its actual recorded orientation.

When you alter the orientation, the actual recorded vales for each pixel is recalculated and averaged with the surrounding pixels, resulting in a loss of sharpness.

This is a test pattern in the public domain from Wikimedia:



The following two images are 400% crops of the same image. The first is straight from the original image. The second is after the image had been rotated 45 clockwise, and then again rotated 45 counterclockwise. The second image is visibly less sharp than the first. This is, of course, an extreme example of what happens, but it happens. It affects the edges and corners the most, which is where images captured with digital cameras will already be soft to a certain extent, so for very small rotations, one or two degrees, the effect is hidden by the image's own softness in these areas, but it gets more and more noticeable as the magnitude of the rotation increases.
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Old Nov 27, 2009, 9:07 PM   #13
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Interesting. Again, I understand how rotating the image can make it softer, and TC's post proves it. However, if you have to look at an image at 400% to clearly see the damage, I wouldn't worry much about small rotations.
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Old Nov 27, 2009, 9:31 PM   #14
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As I said, the effect is most pronounced in the edges and corners. Though the rotation was significant, the 400% crops I posted are from near the center of the image.

The OP's complaint was that it was happening when he aligned his images. We don't know how much he was rotating them or how he perceived the loss of sharpness. What we do know is that he did, and he did.
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Old Nov 28, 2009, 4:59 AM   #15
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Okay, point taken.

However, rendered test patterns (that isn't a photograph) which demonstrate aliasing when rotated are one thing, real-world images are another. Playing with some old images of walls rotated 45% in CS4 and back again, viewed at 500%, then sharpening both images at the end I cannot tell which is which. 50% is usually a pretty good approximation of what a print looks like and I can't tell at 500%.

But it is true that sharpening draws lines (loosely speaking) along where it detects edges in an image, so where you have an image that has already been sharpened then rotating will reduce that sharpness.

The same thing is true incidentally of a lot of other operations too. Which is why sharpening is the LAST thing you want to do before finishing off an image.

Another good reason to shoot RAW and not bake-in sharpening in the JPG when you capture the image.

Even so I think there are probably substantial differences in how well different applications deal with image rotation. My guess is that we would have to be talking about highly sharpened in-camera images processed in sub-par software to get any visible kind of effect.
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Old Nov 28, 2009, 5:44 AM   #16
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Without a doubt.
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Old Nov 28, 2009, 7:00 PM   #17
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This has intrigued me, so I went a little further.

That test pattern is available on Wikipedia as an SVG vector graphics file, so I downloaded it, opened it in Microsoft Visio, and saved it as a 4288x2848 JPEG file, the same size as my Nikon D90 makes, and with minimum compression. I opened that file in Adobe PSE6, rotated it 3 to the left, saved it with minimum compression, opened that file, rotated it 3 to the right, and saved it with minimum compression.

The following two images are 100% crops from the upper left corner of the two images. The first is the original, and the second is the image that had been rotated. Even after having been rotated just 3, twice, the loss of sharpness is evident.
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Old Nov 28, 2009, 7:51 PM   #18
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Could some of the loss of sharpness be due to (or at least made worse by) re-saving the image file?? Regardless, this is an interesting discussion, and I think I'll have to work harder at making sure my shots are aligned.
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Old Nov 28, 2009, 8:25 PM   #19
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Quote:
Originally Posted by rjseeney View Post
Could some of the loss of sharpness be due to (or at least made worse by) re-saving the image file??
Good question.

I saved the orininal JPG again, with minimum compression, then opened that file and saved it again with minimum compression. It had then been saved as often as the one that had been rotated.

Of the following three 100% crops, the first is straight from Visio, the second had only been saved twice more in PSE6, and the third had been rotated 3, saved, rotated back, and saved.
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Old Nov 28, 2009, 8:26 PM   #20
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Good test! I was trying the same with one of my picks but this one is much better. Thanks.
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