Although I rarely use it, every now and then I want to do a bit of perspective correction (or perspective control) with my images. In Lightroom I’d gotten used to using the perspective correction feature, but I hadn’t yet seen anything like this in any photo apps that I’d liked*.
If you aren’t familiar with perspective correction, it basically means adjusting the image so that lines that are parallel in the real world stay parallel in the image. Your eye and mind are very good at “perceiving” parallel lines, but cameras can’t do that, so anytime you are not pointed directly at the main plane of an object (such as directly above a piece of paper), the perspective won’t be ideal. These images give you a quick idea of what this means.
In the left image the sides of the building seem to slant towards the upper middle portion of the picture. When viewing the actual scenery your mind knows that the sides of the building go straight up and it perceives it that way. To make the snapshot show what your mind perceives you need to do a bit of correction. You’ll then end up with the image on the right.
It almost looks as if the camera was moved up in to the sky so it is sitting right in the middle of the building. This is a correction to the perspective (or view).
Can you do that on the iPhone/iPad? In an email exchange with the developer of the Image Straightener and Image Blender apps (both superb apps) he pointed me towards JotNot Scanner. This app is for the iPhone (runs fine on my iPad 2) in the business/productivity category, but it turns out it is useful in photography.
When JotNot starts up you have some basic options. Since this will be your first image you can just click on the camera or album icon in the lower right corner of the initial screen. I then chose to import the picture that was taken with my iPhone 4S.
After you import the image, JotNot will try to guess the edges of your picture as if it was trying to fix the perspective of a piece of paper, so the initial settings are often far from what you want, but no need to worry.
First, before continuing, click on the Settings icon near the lower right corner (the icon that looks like a gear). Then, be sure to select the “No enhancement” option, and also turn the contrast all the way down (this seemed to work for me, rather than leaving it in the middle). If you scroll down a bit further there is a file size and JPEG quality option. You can slide those to the maximum value to make sure you keep as many pixels as you can.
You will now want to drag the corners around so that horizontal and vertical lines within the image line up with the edges of the shape you are making. The best way to do that is try to get the blue grid lines to line up with things that should be parallel in your photo.
In the left photo above I primarily made use of the vertical grid lines. The line on the furthest right lines up pretty well with the street sign which I know goes straight up from the ground. The blue vertical line in the middle seemed well aligned, since it runs along with the tall gas station sign. And although it is hard to see, there was another blue vertical line which runs runs through the phone pole on the left side of the photo.
Here are the initial and final images for comparison. In this particular case you don’t immediately see a large difference (such as you can with the buildings at the start of this post), so I posted an overlay comparison below these. I lined up the base of the gas station sign as a reference point.
While using JotNot I noticed that you can drag the corners beyond the edge of your picture. If you do this, you’ll end up with white being filled in around the image. I also learned that you don’t have to be very accurate when trying to “grab” a corner point. The app will just grab the nearest corner point and you will then be dragging that one.
JotNot Scanner is free (ad-based) and can be downloaded on iTunes. That is the version I used. There are several other versions of JotNot which cost a few bucks. There is a YouTube video from the developer showing some ways it can be used.
* There are two other apps I can think of that I’ve used for perspective correction. PhotoFactory allows you to do correction, but it tends to crash a lot, and has a few GUI issues. FrontView is another one geared towards perspective correction, but has significant GUI issues which cause you to lose large parts of your image. Neither has the grid overlay which is very helpful when doing correction.