When plagued with original photos, images, or graphics which contain jagged pixels (jaggies), you can quite often clean-up the appearance of that file if you simply reduce it’s overall size without resampling it at the same time. Granted, most garden-variety image editors provide no control over this resampling step. But if your editor has a field to specify resolution, which is a number indicating pixels per inch (ppi) or dots per inch (dpi), then you’re one step closer to improving image quality.
The concept behind this issue is called effective resolution. An excellent description of what this signifies is located at Digital Expert.
Raster images have a specific resolution (i.e., a specific number of pixels per inch), scaling a raster image involves the distribution of available pixels across the designated space.
Image resolution subsequent to scaling is referred to as effective resolution. If an image is enlarged, unless additional pixels have been added by means of interpolation (resampling), then accordingly the size of each pixel must be increased?consequently, the enlarged image will have fewer pixels per inch (lower resolution). Conversely, if an image is reduced, unless existing pixels have been discarded (downsampling), the size of each pixel must be decreased (higher resolution). Although scaling reduction generally is less problematic regarding visible defects, unnecessary resolution can contribute to excessive physical file size.
Cropping the scanned image to the size intended for print also will benefit toward the reduction of physical file size. If an image will require scaling, scanning resolution should be adjusted accordingly. When placing and scaling images, you are changing what is known as the image effective resolution.
Effective resolution is a calculation of the actual resolution factored for the scaling performed in the page layout application. The math works out so that if you reduce the scale of an image in the page layout, you increase the effective resolution. As you increase the amount of scaling, you will decrease the effective resolution. The equation is:
(Actual Image Resolution) / (scale) = Effective Resolution
So if you have an image that is 640×480 @ 72 dpi and looks jagged, you can improve it’s look by reducing it’s size by 50% which yields a greater resolution of 144 dpi. Try it on some of your own images and let me know what you think!