Transform Image

A lot of old images are photocopies with all sorts of distortions, eg



(Maybe someone has a nice simple plan as a better example.)

How do people handle this issue?

1. A good start is to import the image, scale it and then rotate it.

This is problematic because the rotate tool only displays the image outline while the image is rotated but it is likely the image needs to be rotated to align part of the content. A line can be drawn over the image and that line rotated to the required alignment.

Can the image under the line be rotated at the same time as the line on another layer?

2. Scale and rotation is only part of transforming images. I'm familiar with the term warping ( https://www.tutorialspoint.com/computer_graphics/2d_transformation.htm mentions: Translation, Scaling, Shearing, Rotation, and Reflection). Are there any suggestions for a more comprehensive transformation?

I'm interested in any comments.

My preferred tool for such kind of jobs is GIMP.
It has all the mentioned warping and distortion tools.

The only image manipulation I do in LibreCAD is scaling, to bring the image to the correct scale.
All other manipulations, including rotation, I do in GIMP.

It has been a long time since I used Gimp but it showed up in my search and it is powerful with all the transform tools:



Linked is lot-demo.png. It is distorted linearly in all directions with uneven x and y axis distortion.

For the linked image Perspective is the only tool needed.
With 4 guiding lines the result is a perfect rectangle.
And a 90° rotation can be applied by Image/Transform menu.
Guiding lines also can have any angle, e.g. if an alignment with the content is needed.

Other images, from greater distance, may also have kind of a polar/barrel distortion, depending on the camera lens.
This can probably be fixed too, with other tools and a bit more efforts. But the question is, how exact the image has to be.
Usually a couple of remarkable points are enough to create a true to scale drawing from an image.