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computing: image editing: basic

Overview

This page is for tutorials and explanations of basic image editing techniques.

Tutorials

Putting two images on the same page

This example uses PaintShop Pro, though this procedure can also be done in GIMP.

Let's say you have two images, which we'll call A and B, which you want to stack vertically on a page so that A is above and B is below. Note that if the images are not to the proper scale with respect to each other, you will want to make the smaller image larger first; see #Correcting for different scan resolutions.

1. Find B's width and height in pixels. There are several ways to do this; the obvious one is to load the image, make sure its window is in the foreground, and look at the right side of the bottom status bar, where it should give the image's dimensions as width x height x colors.
2. Optional: Save image A under a new name. This will prevent accidentally overwriting the original image if you save your work during the following steps (which is a good idea). The new name should probably be a PSPImage file type, as this will also preserve layering.
3. Resize A's canvas. With image A's window in the foreground, select Image -> Canvas Size... from the menu.
   • The width needs to be as large as the wider of the two images. If A is larger than B, it can remain the same, i.e. for "Width:" under "New Dimensions", type in the width shown under "Original Dimensions". If A is smaller than B, then use B's width.
   • The height needs to be expanded by at least as many pixels as the height of B. It's generally a good idea to leave yourself some margin, however; 100 pixels extra will probably do, but you can refine this figure to meet your needs as you gain experience with the particular types of images you are working with. In short, however, Take the number for "Height:" under "Original Dimensions", add in the height of image B, add 100, and enter the result in "Height:" under "New Dimensions".
   • In the "Placement" section, select the arrow that points directly up (so the new canvas space will be below the original image A).
   • Finally, click [ Ok ] to make the change.
   • Note that resizing A's canvas may cause the image to scroll off the bottom of the screen a little, which can make you think you don't have enough room when you are pasting in B (next step). It's probably a good idea to zoom out a little so you can see the whole image, including the new blank space.
4. In some image editors such as GIMP, the new (unoccupied) space may be configured by default so that nothing placed there is visible. I'm not sure why this happens or what the best way is to fix it, but "flattening" the image (to remove any transparency layers) seems to work.
5. Copy image B into the new space:
   • Go to image B's window, make sure there is no selection (or that the entire image is selected), and select Edit -> Copy from the menu.
   • Go to image A's window, and select Edit -> Paste -> Paste as New Layer. ("Paste as New Selection" would also work, but I haven't tested this to see if any different steps are needed afterwards.)
6. Move the pasted image around so it is where you want it. To move the pasted image, select the "Move Tool", which is the cross-shaped tool with the arrows at the end of each point, in the toolbar (it's the 4th tool showing on my toolbar, but YMMV).
7. Crop the results to remove any unwanted extra space
8. Save the new, combined image. You're done.

Correcting for different scan resolutions

Let's say the two images you have need to be "the same size" (or, more importantly, objects within the images – such as text – need to have a consistent size from one image to the other), but they've been scanned at different resolutions. You want to correct one of the images so that the two will be on the same scale when you put them together.

Assuming the two images have not been re-scaled already, i.e. you are working more or less directly from the original scanned image, there is an easy way to do this.

1. For each image, find the scanning resolution: select Image -> Image Information... (or type SHIFT-I). Look in the "Image" box, after "pixels per inch" (or possibly "pixels per cm", depending on your settings). Write this number down for both images. We'll call this the "DPI" (dots per inch) for each image, and we'll call the image with the smaller DPI "image S", and the other one "image L".
2. Re-scale the "smaller" image": For image S, do the following:
   • Image -> Resize...
   • Take image S's "Original Dimensions" width, divide by image S's DPI, and multiply by image L's DPI. Put that number in image S's "New Dimensions" width.
   • Do the same for image S's height (note: if the aspect ratio is "locked", PaintShop will do the calculation for you automatically).
   • Press the [ Ok ] button to make the change.
3. Both images should now be on the same scale.