Straighten Texture

Steps:

1. Drag the "barrel_scan.png" into Krita.
2. Use two horizontal and two vertical guides by dragging with the cursor from the rulers onto the canvas. If you can't see the rulers, go to View > Show Rulers.
3. Duplicate the current layer by using Ctrl + J en hide the bottom layer.
4. Lock the guides by going to View > Lock Guides, to avoid accidentally moving them while adjusting the texture.
5. Hit Ctrl + T to go into Transform Mode and open the Tool Options (Usually at the top right) and choose the Liguify (droplet icon).
6. Start matching the edges of the scan with the guides, increase/decrease the size of the brush if necessary.
7. Scale the layer down a bit so we have more room to work with.