Why can smart crop sometimes increase the aspect error?

[loni_blues]

Hi,
When cropping, I always do a pixel crop to get rid of black bars and then do a smart crop all to minimize the aspect error. But I noticed that sometimes smart crop all paradoxically increases the aspect error (though slightly, I must say) from, for instance, 0.0%-0.2% to 0.3%. How can this be? Is this a bug? Should I, in those cases, stick to the lower error values I get with pixel crop or do smart crop after all?
Many thanks for any help,
Loni Blues

[orion44]

Crop manually, without smart crop.

[yetanotherid]

It's probably not smart crop as such.
The way I understand AutoGKs resizing, you pick a width and then extra cropping (if applicable) and AutoGK resizes to the appropriate height, not width. Then after resizing it crops the sides to give you the exact dimensions. Both crops have to be rounded to a certain extent which can distort the aspect ratio a tiny bit. An example of what I mean....

You ask for a width of 704 which would normally result in a perfect aspect ratio of (just an example I'm making up) 704x394. However the mod16 dimensions need to be 704x400. Every other conversion program I've even used will simply resize to 704x400, distorting the aspect ratio far more than AutoGK ever will. Instead of that, AutoGK resizes to (I'm taking a guess) 708x400, then it crops a second time to give you 704x400 which means you lose a couple of pixels each side, but the aspect ratio will remain far closer to the original.
You can sometimes notice the effects of the double crop and it's rounding as forcing a crop of an extra 2 pixels at the top (for example) might still give you the same result as no forced cropping at all, so then you have to crop a few more. Regardless, AutoGK will still give you something much closer to the original aspect ratio than any other program I've used.

Another way to see the auto cropping at work....
When you convert a DVD, pick a width of 704 and preview it. Leave the preview open, then change the width to 720 and preview it again. Now compare the two previews. Generally you'll have (with 16:9 content and no cropping for example) 720x 416 or 704x400. Neither are perfect 16:9 but the second is closer. However when you compare the two, you'll probably notice the aspect ratio is exactly the same, only AutoGK has cropped more from the sides of the 720x416 output than it did from the 704x400 output in order not to distort the aspect ratio.

Whether you simply use autocrop, autocrop with additional cropping, or purely manual cropping, if the end result will be the same amount of cropping the output should be identical and any tiny aspect ratio distortions will then also be the same.

[yetanotherid]

Actually, were we talking about AutoGK or Gordian Knot? And is the original post really 7 years old or has something one wrong there?