Resizing Images In A Layout Program
You have probably heard a few times that you are not supposed to resize an image within the layout application, and that it is better to do it in a graphics editing program such as Photoshop. This article will explain why.
Nothing happens to the image itself when you rescale it into a layout program (such as InDesign, QuarkXPress, Pagemaker and so on). As an example we’ll take a sponge… Not very DTP related but… Anyway, you take a sponge in your hand whch has, let’s say, 30 holes in one inch. If you now squeeze that sponge it will have more than 30 holes in one inch. If you stretch the sponge, you will have less that 30 holes in one inch. The sponge is the same though, it doesn’t get resampled, i.e., holes are not added or subtracted from the sponge itself. You’d have to remake the sponge to do that: you would have to go into Photoshop or another graphics software program to resample your image).
That’s what happens with layout applications. You have an image that is 300 ppi (pixels per inch). If you make it smaller in the layout program nothing happens to the original. However you have now “squeezed” the image down and you have more pixels per inch (in the document of the layout program). InDesign calls this Effective ppi while it calls the actual resolution of the image… well… Actual ppi.
For the same reason it is always better to scale your images in the graphics software program than in the layout application. Especially if you enlarge the graphics. If you enlarge them, while nothing happens to the original picture, you will have less pixels per inch, the same way you had less holes per inch when you stretched the sponge. This will reduce the quality of the final print.
Some printers might tell you that if you resize an image just 10%, it will still be acceptable. If you do that, this is what actually happens:
- If your original resolution (actual ppi) is 300 ppi, when you enlarge 10% you will get an image that is 270 ppi. If you enlarge it a lot in the layout application, you can see that the resolution will start going down and you don’t want to do that.
- If you shrink the same image 10% then you get an image that is 330 ppi. If you shrink an image a lot, if you have many images, it will take more time for the service bureau of your printer to handle your files, while you don’t really need to. You could just resize your image in Photoshop et al, so the size of your files will be smaller, thus you will speed up the RIP process.
Didn’t quite grasp this tutorial or feel confused about it? You might have encountered one of the barriers to study.
August 22, 2005