Tutorial: Rotating Skewed Images Accurately


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Darren

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#1
This tutorial is for the benefit of members who routinely ask how to correct skewed images. Enjoy!

Acknowledgement: The image used here was taken from ClubSNAP member hong from this thread for lack of any suitable image in my library. If hong has any objection, I will remove this image and try to get another one.

This is the original image as posted:-
 

Darren

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#2
We will use the Measure tool to accurately determine the angle for correction. The Measure tool is found here:-


Using the Measure tool, we then measure a known good horizontal or vertical line in the image. In this example, I measured the vertical side of the HDB block as shown:-


Next we use the Arbitrary Rotate menu command to do the actual rotation. The menu can be found here:-


which will bring up this dialog box with the angle and direction of rotation already entered, and all you have to do is to hit ENTER:-
 

Darren

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#3
The result of the rotation will be a canvas which shows an image with corrected verticals, but with white spaces where the canvas has been "expanded" in the rotation process:-


We then do a crop to get the final image as shown here -
 

StreetShooter

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#4
To confirm that the image is indeed correctly aligned, I would suggest turning on the grid ie View > Show Grid.

This is especially so if you want to correct converging or diverging verticals as well, using Distort.
 

Fella

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Dec 14, 2002
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#6
This is great!
Thus far, I've always been using "agar-ation"!
Very much thanks!

How about a tutorial on correcting converging or diverging verticals? I'm quite sure many users (myself inclusive) will benefit from it.

Have another question which I hope someone can help.
After doing a rotation, let's say I wanna crop the picture to a particular size of exactly x times y pixels, how do I actually go about doing it? At the moment, what I do is "agar-ation" again, just select, resize down and hope for the best... :dunno:

Thank you very much!
 

A

Allen

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#7
Turn on rulers, Ctrl-' and place a ruler along all vertical lines and then Ctrl-A, Ctrl-T and drag corner nodes.
 

Darren

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Jan 16, 2002
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#8
Originally posted by Fella
Have another question which I hope someone can help.
After doing a rotation, let's say I wanna crop the picture to a particular size of exactly x times y pixels, how do I actually go about doing it? At the moment, what I do is "agar-ation" again, just select, resize down and hope for the best...
This one already done ... *sigh* we all die-die write review no one refer to ... very, very sad ... :cry:

Guide to Resizing and Cropping by Darren ;)

shameless self-promotion ;p
 

Jed

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Jan 19, 2002
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#10
Actually, this way works but I don't think it's the most efficient. There is another way ;)
 

Java_Guru

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#11
u can actually ROTATE and CROP at the same time. But u need a steady hand/optical mouse and keyboard to do well.
 

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