Move Manual Perspective Selection Window?

In the Rotate and Perspective module, I can use Manual structure to create a four-sided figure to define the object to bring into perspective. Sometimes I want to move the entire rectangle without modifying it. There doesn’t seem to be a way to do this. You have to move all four sides of the definition rectangle one at a time.

Is there a way to do this? I’m not finding anything in the docs. I’ve tried a number of key combinations but no-go.

thanks!

I am not sure there is any valid reason to do this. I presume that is why the developers haven’t made this action possible. Could you explain the benefit of being able to do this? I see this option purely as a way of moving the corners to what we want to make rectangular so the correction can be applied.

I’m going to preface this rather long use case with: I was just wondering if there was already a way to do it.

Sort Answer: I’d like to copy the perspective history and past it to a large number of other photos, and only have to make slight changes to align the perspective rectangle.

Long Answer

Suppose you are archiving a couple hundred physical photos, and you don’t want to use a flatbed scanner because the only ones you can afford are too slow. So you set up a camera in your studio and after a bit of mucking around, you rig up a tilted easel with a pane of non-reflective glass behind which you put a photo. This setup reduces the glare from light pretty well, and it prevents the camera lens from reflecting into the image. The setup also lets you get a nice high quality image of the photo in about 2 seconds.

The only issue is that now you have within your digital image the original photos has a trapezoidal shape. No problem. In Darktable, you duly go into Rotate and Perspective, select the manual structure, align the sides, and boom. Just like the original.

Back to the rest of your photos. It’s a simple selective copy to get the perspective history, and you paste the change into the rest of the photos. As you go along, you notice that due to the by-hand nature of putting the photos behind the glass in the tilted easel, the photos don’t all line up exactly with the perspective adjustment. The size is correct, and shifted up or across, but maybe only one corner needs a tweak. It would be great if you could drag the entire rectangle over the image, and make the very minor move of the one corner.

So that’s it. It’s definitely an edge case. Just an efficiency thing. Carrying this pipe dream a bit further, how about rotating the rectangle? Suppose among the photos you are archiving a third of them are landscape. You could paste the portrait rectangle to the landscape image, go into perspective, and rotate the selection rectangle, position it, and make any minor corner adjustments.

But like I said. I’m not asking for the feature. Just wondering if it is there.

Thanks!

That shouldn’t change the correction needed if the rest of your setup is stable.

The trapezoid you draw is just used to calculate the corrections. Those calculated corrections are the actual module parameters, so (with a bit of practice), you might be able to just tweak the parameters between images.

Btw, if changing originals causes the required correction to change, I’d say your setup isn’t rigid enough. Perhaps weighing down the easel might help with this (so at least the feet stay where you’ve put them)?

If you shooting setup is stable enough to allow the same perspective correction between shots then the simple answer would be to make a preset and save it to apply the same correction across a number of images.

If the shooting setup is not stable then I don’t see any advantage in being able to move the selection window. Maybe use the line analysis option and then apply the correction.

Either way DT is doing this correction very easily. I sometimes have to photograph images behind glass and I can not shoot straight on and this really helps. You also may have to use the lens correction module is the straight lines are bent in the capture.

Good luck