Have you ever tipped your camera back to get the whole of a tall building in the picture, and then found that in the resulting photo the building looks as though it was falling over backwards? Or maybe you did not hold the camera quite straight and everything looks as though the earth is tilted?
This tutorial shows you how to correct that. You can work on either a photo from a digital camera, or a scan of a print. It does not matter what size it is, but if it is so big that there are scroll bars on its window in PhotoImpact, then hit the minus key on the number pad until it has zoomed out enough for the whole of the picture to be visible in the workspace, with no scroll bars.
The photo above is the one I am going to use for the tutorial. The original is very much bigger, but in order to save load time on the tutorial I have reduced it to 240x320 pixels. You can see that not only have I tipped the camera back, but I did not hold it straight either, so the window is tipping backwards and leaning to one side as well.
It has one thing going for it - there is a lot of unwanted black space around the edges. This technique loses some of the photo area at the sides, top and bottom, and if your building absolutely fills the frame, some of it will have to be cropped off. With this one that will not matter because it needs cropping anyway.
We need only two tools for this. Correcting the converging verticals and horizontals will be done with the transform tool, which I have outlined in green. When that is all done, we shall use the rectangular selection tool, outlined in red, to select the area we are going to keep for the final photo.
The first step in straightening a photo is to look at the basic shape of the image, and determine which way it is tilting on all four sides. In the picture above I have outlined the shape of the window, and drawn an arrow pointing to each side from the corner nearest to it on that side of the shape. You can see that the bottom right corner is the nearest corner to both the right hand side and the bottom of the picture.
If we already had one side that was parallel to the side of the image, we should not need to do anything about that, but this photo is so badly tilted that we are going to have to adjust all four sides.
We will begin by straightening the left hand side. In the diagram on the left I have drawn a line parallel to the sloping side, starting at the corner. At the top, the distance between the left side of the image and that line is the distance we have to move the bottom left corner across to straighten that side.
Click on the transform tool. On the attributes toolbar at the top you will see several transformation types. Select the one I have outlined in green below, the distort tool. Make sure that the photograph is the selected image if you have more than one open in the workspace, and when you select the distort tool, the photo will be outlined and four solid squares will appear, one in each corner. These are the handles with which we move the sides of the picture to correct the tilt.
Click on the bottom left handle and drag it inwards by about the same amount as we identified in the image with the sloping line. You will see the side of the window in the picture become parallel with the side of the image frame. Don't worry if it is not perfectly parallel. As we correct some of the other sides it may make a difference to this one, and it will need readjusting. While we are in distort transform mode we can move the handles in any direction to change the slant at any time.
Because the bottom right corner is the nearest corner to both the right side and the bottom, we can do two sides at once. Drag the bottom right handle in and up until the bottom and right sides are parallel with the frame. (below right)
Finally, bring the top left handle down to straighten the top. Check all the sides to make sure that they are still parallel and adjust any of the handles which need tweaking.
When you are satisfied that the image is now square on to its frame, select the rectangular selection tool. Click on Options on the Attributes toolbar, and make sure that Draw from Centre is not checked, but that Preserve Base Image and Move Selection Marquee are both checked.
Normally I would now select as big a rectangle as possible from the photograph, but I want to crop this one close to the window, so the picture below shows where I cropped. When you make the selection it will usually show clearly if any of the sides are not quite parallel with it, and if so, you should deselect and return to the Transform Tool to make the necessary adjustments. When everything is right, use Edit/Crop. The two pictures below show the selection and the finished, corrected window.
As you can see the finished image is quite a lot smaller than the original, so you should always do these corrections on the biggest copy of the photograph that you have. You always have the option of reducing the size further if the finished photo is too big, but there is no going back if you reduce the size first and then find that what is left after the corrections is too small.
Finally, below you can see a picture which needed a lot of adjustment, and so lost quite a lot of the photograph in the process. All of these pictures are very much reduced in size for this tutorial. The original is 960x1280, and what is left after the adjustment is still a big photograph.
The golden rule is to think when you are taking the photograph whether you are going to need to adjust it. Try to take it with the camera held straight and level. If that makes it impossible to get the whole building in, leave as much room as you can around the subject so that the whole building will still be in the photograph when you have straightened it. And work from the biggest copy that you have available.
The actual straightening is quite easy with PhotoImpact, and the improvement to your picture is amazing.
© Carol Brooksbank 2003