Bicubic Resizing vs Bilinear

Everyone uses Bicubic resizing religiously when they make DivX movies or any time they resize pictures in photo editing software. I have suggested that it is best to use Bilinear to shrink images and Bicubic to enlarge them. In fact I believe it is a rule that shouldn't be broken. But then again there is mixed opinion even among experts on this matter. Digital photo experts JASC (makers of Paint Shop Pro) comment: "Use the Bilinear Resampling method for shrinking these images and Bicubic for enlarging them". Whereas the help file of Adobe Photoshop just mentions that Bicubic is more exact method for resizing. In actual fact Bicubic is more precise, but only when it comes to enlarging. When it comes to shrinking its exactness can actually produces pixelation, because to shrink an image pixels must be discarded anyway.

The following shows examples demonstrating this effect. There are reasons you may prefer Bicubic shrinking methods though. Namely, if sharpness is absolutely vital and pixelation problems are less important.

ENLARGING IMAGES

The following is the effects of enlarging a small image using Bilinear Upsizing.

Notice how blocky the image appears

Next demonstrates the effects of enlarging using Bicubic Upsizing

Notice how smooth the image appears

SHRINKING IMAGES

The following demonstrates the effects of shrinking using Bicubic Downsizing:

Notice especially the hair where hard pixels appear.

Finally the following demonstrates Bilinear Downsizing:

Notice the hair again, the pixels have been anti-aliased to produce smooth lines.