Can I warp this vintage illustration into a proper rectangle?

[GregL65]

Hi, I haven't used Gimp but I'm looking for a tool that can help me with this. 

I make Kindle books featuring vintage illustrations from children's books. Many of the illustrations were printed not quite square. They look okay in the books because the margins are very large and they are not very far off from square.

But in my Kindle books I want to make the illustrations full screen. When I do so, the slight slants of some of the borders become obvious. 

In the attached example, see how the bottom border (and the text too), are horizontal, but the other three borders are not square. I don't think the artist made it that way; I think the distortion happened during the book production in 1918. If Gimp can help me warp it back into square, I think that would be closer to the artist's original work.

Any suggestions?

[Ofnuts]

You use the Perpective tool in "Corrective" mode: You start the tool, set it to "Corrective mode" (this can be made a default), drag the four corners to points that should make a rectangle (I used the corners of the black frame) and hit "Transform".

But:

• The contents of the shape in the perspective tool is mapped to the canvas, so part of the image are outside the current canvas, to get the full image you have to use Image>Fit canvas ti layers
  
  
     
  
  
  
• But this also often stretches the image in one direction, so you have to use the Scale tool to scale the image in the appropriate direction to restore the aspect ratio.
  
  
     

[rich2005]

Much the same as Ofnuts, using the perspective tool. I think the borders are worth keeping.

   

Add alpha channel
Use guides (positioning to preserve border)
Apply perspective transform
Fit canvas to layers
Fill in the border

Video demo https://youtu.be/UD8Zg_43FqE  3 minutes.

[GregL65]

Thanks, Ofnuts and rich2005! Looks like Gimp is exactly what I need. I'll get started on this soon.