Ok, in this tutorial, I am going to discuss about the 'Measure tool' of Photoshop. It is under the Eyedropper tool. Press the left mouse button on the Eyedropper tool and you see 2 other tools under it. Or press 'i' then Hold the Shift pressed and tap 'i' two more time. You have now selected Measure tool.
Now look at the image below which was scanned poorly :
Many people use the Grid to make this type of image straight. But that is a tiring way. We have to step way to make it happen.
Select the Measure tool and determine a straight line. Here, Look at the image, i have determined a straight line.
Comix boxes are made straight. So, it is easy to determine the straight line.
Now, draw line with measure tool where i drew the red line.
And the last step. Go to 'Image > Rotate Canvas > Arbitrary'. You will see a predefined value is loaded. Actually, it is the value you have just drawn with measure tool. Press OK.
Tada! Your image is now straight as a floor.
This tutorial is originally created by me, all images and directions.