To begin with, various aspects about image resizing will be explained: what the problems are, how it works, who should be interested. To continue, a comparison will be made between existing methods. Finally, the conclusion will mention the best method for photo enlargement to use.
Problems to be solved by upsizing algorithms
One of them is edge pixel aliasing in the original images. This becomes even more apparent when enlarging. The edges in the enlarged image appear very jagged. With upsizing as with other algorithms there are many trade-offs. In one extreme you have edges that are sharp but jagged in another smooth but blurry. A compromise between smoothing an edge and keeping it sharp is searched for. But, over-smoothing and maintaining sharpness creates an unrealistic look to images. After enlargement photos start looking like paintings, and that makes it unacceptable sometimes. Another problem is loss of detailed texture in the upsized image. The only method that even considers this is reshading.
How enlarging works
Upsizing images increases their resolution (the size in pixels/points). Information from the original image is used to create an enlarged version. How to do this? Imagine a black and white image as a grid of pixels. They are spread out in a plane each having their own values. These values represent the brightness levels. In the original image the pixels are all side by side. Enlarging the image means expanding this grid. The more you stretch it, the more separated the pixels become. Those pixel places in-between need to be filled with their own values. This is called interpolation. There can be other processing steps after/before interpolation.
When to upsize an image
Increasing the resolution by 200% means transforming a 3 megapixel (MP) image into a 12 MP one. Doing this without losing quality is like transforming an average camera into a professional one. Cropping and enlarging a part of an picture is like having a better zoom lens. Also, starting with a professional camera and using upsizing would make very large prints easy.
Methods for enlarging pictures
Some well known approaches are commercially available: Photoshop, Genuine fractals, Photo zoom. There’s also a newer method that is free: Reshade.
Although everybody takes Photoshop as the tool for any image processing, its features cover only basic needs. It is designed to be a framework for image manipulation by allowing the inclusion of plug-ins that can easily interoperate. Even experienced Photoshop users have a hard time when enlarging pictures. The default methods for upsizing are made to be fast but not very powerful.
The other methods presented next do a better job at upsizing pictures. They are designed to allow greater enlargement factors without serious degradation in image quality.
This is the newest among the ones mentioned. It can increase the size up to 200% without losing quality. Beyond this zoom, the pictures look a little “painted”. Even at higher zooms reshade works better on most images than any of the other techniques. In most cases a 200% upsize is the limit, because after that the sparseness of the details becomes obvious. Larger zooms can be useful in other situation. Large posters that are generally viewed from afar don’t need perfect quality when you look up-close. Try the free online image resizer implementing this method here.
This method tries not to overdo the sharpening and deblurring like photo zoom. For zoom factors of about 500% it maintains the quality of the image pretty well. For smaller zooms the image quality is very similar to what you can get in Photoshop. So, there still are blurry edges.
Even though the clarity of the edges is better than for Genuine fractals sometimes (at large zoom factors, 400%), there is an obvious smoothing effect that creates the impression of over-painting. For photography, it adds an unrealistic look to the resized images. Although small details when enlarged remain smooth, reshading works better, also maintaining their aesthetics.
It is not suggested to use upsizing for every photo. The original can always look better even if an enlarged version has minor quality loss.
It often happens that you need to crop a part of a picture and it becomes too small. Or maybe you don’t have your best camera nearby and use a lower megapixel one. In these situations enlarging becomes the best alternative. I recommend reshading your images for best quality. You can try the online image resizing engine or get the desktop photo enlarger.