There are times when you take a picture, that the camera isn’t level, or the perspective of the background makes it look like the picture isn’t level. This is correctable in both PSP and GIMP. Take the picture below of my back deck. I took this to show how high the creek had gotten.
Your brain uses straight lines like the deck railing or the deck posts, that should appear as horizontal or vertical, and says this picture is leaning to the left. What you need to do is pick one of the straight lines, that is as close to the center of the image as possible, and make it horizontal or vertical. That will straighten out the picture.
In my experience, vertical lines are better than horizontal lines. Trees or posts that should stick straight up are less prone to perspective problems than horizontal lines like roof edges or deck rails. I used the deck post the thermometer is on to correct this picture. I manipulated the image until that post seemed to be straight up and down and the whole picture then looks much straighter. Keep in mind this image was taken with a wide-angle lens. The vertical lines will seem to lean away from the center even when the picture is perfectly level.
To accomplish this level adjustment in PSP you load the image into PSP in the Edit workspace and select the straighten tool. Notice I’ve circled the Edit workspace tab, the straighten tool, and pointed to the right end of the adjustment line. Click on the picture to display is full size.
You can simply drag the ends of the adjustment line so that the line is either aligned vertically or horizontally with the picture. I aligned it vertically with the deck post. Then you can click on the check mark in the tool bar and the image will adjust to level, based on the adjustment line. Note too that I have the check box for crop image checked. This will crop off the corners of the image that are rotated out of the rectangle of the image. I have all these controls indicated in the image below.
The final result is the second image from above. If I hadn’t had the crop image check box checked, the image would look like the one below, and you’d have to go back and crop off the areas that extend beyond a rectangle manually.
PSP allows you to easily correct the level of an image. Practice this and you’ll find you can make smaller adjustments until your satisfied with the result. PSP has an undo command, so you can back off changes and try again if you’re not satisfied. So does GIMP.
GIMP allows the same basic adjustment. I recommend using single-window mode in GIMP. Open the program, and in the Windows pull down menu, select Single-Window Mode. This will consolidate the separate GIMP windows into a single window, with menus at the top and tools on the left. Load the image you want to work with into GIMP.
GIMP doesn’t have a separate straighten tool. In order to align the deck post with the vertical, like we did with PSP, you need to have a vertical reference. In GIMP we’ll drag a vertical guideline from the ruler on the left side of the image, until it’s close to the deck post. Click the mouse on the left ruler and drag over into the image and you’ll see the guideline moving across. Drop it near the deck post as in the image below. I’ve added pointers to the ruler and the guideline.
Now select the rotate tool from the tools on the left. Most of the defaults are fine, but you need to select Crop with aspect from the Clipping menu. I’ve circled the rotate tool and the Clipping menu in the image below.
Now you click on the image itself and rotate it, by dragging until the deck post and the vertical guideline are parallel. At this point you click on the Rotate button in the Rotate window that popped up when you clicked, to complete the action. This will close the Rotate window and rotate the image.
However, notice the gray areas at the borders of the image. These need to be cropped off. I’ve pointed to two of the gray areas in the following image.
To crop these areas you click on the Image pull down menu and select Autocrop Image. The gray areas will disappear and you have a rotated clean rectangular image.
Saving the leveled image in GIMP is trickier than just saving the file. You have to export it as a jpg, or whatever format it was originally in. GIMP automatically reformats images to its idea of an image file when an image is opened or imported. Exporting your image is done by selecting Export… from the File pull down menu, and then following the prompts. Refer to GIMP documentation for more options on exporting.
Having to use more steps and export images are some if the reasons GIMP is less intuitive than PSP, but remember GIMP is free, and you can get the job done with a little more work.
Post any questions in comments and I’ll be glad to respond.