If not careful, when reducing the dimensions of a digital image, the resulting image may suffer from aliasing, a type of distortion seen in all types of digital media. A few tips can help prevent aliasing.
Aliasing occurs when breaking a continuous signal into discrete pieces such as when a digital camera assembles an array of individual points or pixels from a scene. Once in a digital format, one must be cautious when altering the size.
There are various techniques that graphic design applications may use to reduce image dimensions, although most involve an average of the original pictures. Imagine a simple image, 100 pixels wide and high. If this image is reduced in size by half to 50×50 pixels, each resulting pixel represents 2×2 or 4 pixels of the original image. This reduction is optimal and will yield a true representation of the original image.
If the original 100×100 pixel image is reduced 10% to a 90×90 pixel image, then each resulting pixel is not a simple average of neighboring pixels. Many applications will simply throw away pixels which leads to the uneven, aliased appearance.
The best approach to resizing images is to reduce by an even fraction – 1/2, 1/3, 1/4, etc. Keep in mind the reduction to be perfomed when taking the original images and attempt to size the relevant region of the image appropriately. Crop the image first to a size that is an integer multiple of the final image (400×400 pixels, for example, if the target image size is 100×100 pixels). If any modifications are to be made, perform them on the larger image. When the image is reduced, minor defects may be hidden that would be more apparent if modifcations were made to the smaller image.