The reason for the degradation is because bitmapped or raster image types are limited by their pixel resolution. When you attempt to resize these types of images, your software either has to increase the size of each pixel (resulting in a jagged image) or it has to “guess” at the best way to add pixels to the image to make it larger. Not long ago, there weren’t many options for increasing resolution other than using your editing software’s built-in resampling methods. Today, we are faced with more possibilities than ever. Of course, it’s always best to capture the resolution you need right from the beginning. If you have the option to rescan an image at a higher resolution, by all means, you should do that before resorting to software solutions. And if you have the money to put into a camera capable of higher resolutions, you might find that money is better spent than if you were to put it into a software solution. Having said that, there are times when you may have no other choice than to resort to software.
Resizing vs. Resampling
Most software only has one command for both resizing and resampling. Resizing an image involves changing the print dimensions without changing the total pixel dimensions. As the resolution is increased, the print size becomes smaller, and vice versa. When you increase resolution without changing pixel dimensions, there is no loss in quality, but you must sacrifice print size. Resizing an image using resampling, however, involves changing the pixel dimensions and will always introduce a loss in quality. That’s because resampling uses a process called interpolation for increasing the size of an image. The interpolation process estimates the values of the pixels the software needs to create based on the existing pixels in the image. Resampling via interpolation results in serious blurring of the resized image, especially in areas where there are sharp lines and distinct changes in color. Another aspect of this issue is the rise of the smartphone, the tablet, and the corresponding focus on the device pixel. These devices contain two to three pixels in the same space occupied by one pixel on your computer screen. Moving an image from your computer to a device requires you to create multiple versions of the same image (e.g. 1X, 2X and 3X) to ensure they display correctly on the device. Does one increase the size of the image or increase the number of pixels?
Common Interpolation Methods
Photo editing software generally offers a few different interpolation methods for calculating new pixels when an image us upsampled. Here are descriptions of the three methods available in Photoshop. If you don’t use Photoshop, your software probably offers similar options although they may use slightly different terminology.
Bicubic is the slowest but produces the best estimation of new pixel values.Bilinear is faster than bicubic but does a poorer job. Both bicubic and bilinear interpolation result in a blurred image, especially when upsampling.Nearest Neighbor doesn’t use interpolation. It simply takes the value of the neighboring pixels and adds new pixels without averaging them. This is when you get the jaggies or stair-step effect.
Note that there are more than just these three methods of interpolation and even using the same method in different software may produce different results. Photoshop offers the best bicubic interpolation of any other software that we have compared.
Other Interpolation Methods
A few other image enhancement programs offer other resampling algorithms that claim to do a better job even than Photoshop’s bicubic method. Some of these are Lanczos, B-spline, and Mitchell. A few programs that offer these alternate resampling methods are Qimage Pro, IrfanView (a free image browser), and Photo Cleaner. If your software offers one of these resampling algorithms or another one not mentioned here, you should certainly experiment with them to see which one gives you the best results. You may even find that different interpolation methods produce better results depending on the image used.
Stair Interpolation
Some folks have discovered that you can get better results when upsampling by increasing the image size in several small increments rather than one extreme step. This technique is referred to as stair interpolation. One advantage to using stair interpolation is that it will work on 16-bit mode images and it requires no additional software other than a standard photo editor, such as Photoshop. The concept of stair interpolation is simple: Rather than using the image size command to go directly from 100% to 400%, you would use the image size command and only increase, say, 110%. Then you would repeat the command as many times as it takes to get to the size you need. This can be tedious if your software does not have some automation capability. If you use Photoshop 5.0 or higher, you can purchase Fred Miranda’s stair interpolation action for $15 US. You’ll also find more information and image comparisons. Since this article was originally written, new resampling algorithms and software technologies have been developed which make stair interpolation essentially obsolete.
Genuine Fractals
LizardTech’s Genuine Fractals software (formerly from Altamira Group) attempts to break through image resolution limitations with its award-winning resolution-on-demand technology. Genuine Fractals is available for Windows and Macintosh. It operates as a plug-in to Photoshop and other Photoshop plug-in compatible image editors. With it, you can encode low to medium resolution files to a scalable, resolution-free format called STiNG (*.stn). These STN files can then be opened at any resolution you choose. Until recently, this technology was your best bet for increasing resolution. Today, cameras and scanners have gotten better and come down in price, and the investment in Genuine Fractals is not as easily justified as it once was. If you have the option of putting your money into better hardware rather than software solutions, it’s usually the better way to go. Still, for extreme upsampling, Genuine Fractals is pretty amazing. It also offers other benefits such as smaller encoded files for archival and storage.
Alien Skin Blow Up
Although Genuine Fractals was the early leader in upscaling technology, Alien Skin’s Blow Up plugin for Photoshop is worth a look if extreme enlargements are something you require. Blow Up supports most image modes, including high bit-depth images. It can resize layered images without flattening, and options to resize in place, or as a new image. Blow Up uses a specialized sharpening method and simulated film grain to improve the appearance of extreme enlargements.
The Bottom Line
When evaluating these methods for increasing resolution on your own, try to avoid getting caught up with how the images look on-screen. Your printer capabilities are going to play a big factor in the final results. Some comparisons may appear distinctly different on screen, but barely discernible when printed. Always make your final judgment based on the printed results.
