What technique(s) can I use to enlarge small digital (raster) image in order to best avoid pixelation? I can print the image and scan at high resolution, but I also want to preserve as close to exact color as I can. I'm happy to hear any technique using any tool or toolset.
I'm sure there are other (better?) alternatives, but S-Spline interpolation creates pretty good-looking images. A quick Google search revealed this tool: PhotoZoom Pro.
Also, here's a lengthy comparison of different interpolation techniques and tools: Digital Photo Interpolation - what is the best image resizing / resampling method?
Bicubic resize in photoshop works well for me. 110% at a time until it's to the size you require.
The Liquid Rescale plugin for GIMP resizes (smaller, larger, different aspect ratios) images using seam carving which is able to find the least significant lines of a photo and add or remove those lines. Since I don't understand the algorithm very well, I can't explain it, but it essentially lets you resize and/or rescale images without loosing quality.
When scaling in Photoshop, make sure to select "Bicubic Smoother" (best for enlargements). Then you can always run the Unsharp Mask filter over the image to sharpen it up (it will still come out a little blurry).
But honestly if you want to get the best enlargements you can, try the Genuine Fractals plugin (apparently now called "Perfect Resize") from OnOne software. If you are a NAPP member, you get a discount, too.
I've personally had good, usable results over the years using Stairstep Image Size plugin, which AFAIK works with any version of Photoshop.
Taken from the description:
Stairstep Image Size is a Photoshop plug-in which uses a "stairstep" bicubic interpolation to resize (enlarge and reduce) images. Stairstepping is a method by which you take small steps until you arrive at your goal. In the case of resizing images, this method enlarges (or reduces) an image by small amounts multiple times until the desired size has been reached. Bottom line is that Stairstep Image Size is another image enlarging (scaling) program.
It depends on what you want the image to look like in the end. Do you prefer blurring over "pixelation"? Do you want a balance between the two? Here's a great article at Wikipedia on Image scaling.
What you don't want is to use Nearest Neigbor, because that's the "pixelation" you're talking about.
Paint.NET has Bilinear/Bicubic (Blurred)/Nearest-Neighbor, but if you want other scaling methods, you'll have to look at the other answers.