Interpolation: Magical or Mythical?

Background

Years ago, when most of us were taking photos using cameras with 1-3 MP (megapixel) resolution, interpolation or "upsampling" was a hot topic. To get decent photos at larger sizes of 8x10 and beyond, the ability to upsample photos seemed more of a necessity than an option. Don't do it and you might end up with jagged edges. Do it and it would smooth over the jaggies to make the photo a bit softer but without the pixelization artifacts that made the photo look more like a bad video capture than a good photo. Fast forward to present time. With cameras approaching and soon surpassing the 8-10 MP mark, is there really much call for interpolation? How important is it and what does the best job? It seems that specialized interpolation software and plugins have lost little steam and people are still spending $200 on packages that claim to do the best job adding pixels. Do you really need these expensive solutions? How much better do they do than your average photo editor? Let's take a look.

The problem

Interpolation attempts to hide a problem that can be described simply as not having enough pixels for the amount of space where they are displayed. The effect is similar to walking too close to your TV. Get too close and you start to be able to see the individual pixels and these are distracting when you are trying to see the overall picture. The same occurs when you take a limited number of pixels and try to "stretch them out" over a large area.

Let's look at a crop from a larger picture:

This tiny crop looks pretty good. We can tell that it is the wheel of a car and we don't notice anything strange about it. Take the exact same image, however, and display it larger (4x) and we get:

Now we can see that we simply don't have enough pixels for this larger size: the spokes on the wheel look more like saws than straight lines and the outline of the chrome part of the wheel looks jagged and not smooth.

The solution

To get rid of the visually distracting pixelization in the above larger image, we can use interpolation methods to add pixels to the image. The pixels in the original (smaller photo) describe the data that we have to work with, so interpolation cannot add any true data to the image, but it can smooth over some of the rough edges and can add "apparent detail" by predicting what should appear between pixels in the original image. Look at interpolation like making a prediction. If I showed you the sequence A C E G, you could make a logical assumption and fill in the missing letters to get ABCDEFG. Are B, D, and F really the missing letters though? You were thinking of the alphabet, when the missing letters could have really been from a person's name: A CHENG. This just goes to show that you can only "guess" so much information when you are missing a significant portion of that information.

What does interpolation do to the above large/pixelated image?

Without interpolation
PhotoShop "bicubic smoother" interpolation
Qimage "pyramid sharper" interpolation

The top image shows what the photo would look like at the 4x expanded size without interpolation. By using interpolation, we are able to smooth out the distracting jagged look (center and bottom photo) and improve the overall appearance of the photo. Note that in doing so, we've reduced or eliminated the coarse look of the image but the image now looks a bit soft (blurry). This is a necessary tradeoff, since there is simply not enough data from the original to determine which edges should be sharp and which edges might be slightly out of focus due to depth of field, lens distortions, etc. Older methods such as the bicubic methods used in PhotoShop tend to do a good job while more advanced methods like fractal resampling, edge directed resampling, or the pyramid resampling method available in Qimage (bottom image above) tend to do even better by further reducing jagged edges to produce an even smoother result.

Understanding the tradeoffs

The above is a 4x upsample which is considered fairly "radical". The truth is that if you have a recent model digital camera, you will probably never need to resample to the degree shown above. When you print your photos, a slight upsample or downsample may be needed, but you'll rarely ever need a drastic change in resolution to get a good print unless you do extreme crops or billboard size printing. The most important thing is to use a good interpolation algorithm to interpolate to the PPI (pixels per inch) used by your printer, or an integer multiple thereof. Some print drivers don't do such a great job of interpolation so if you send them an "oddball" size by just printing the original, you may end up with prints that have jagged edges. This can be true even if you send the printer too many pixels, as some drivers don't even handle downsampling well! One example showing the problem can be seen in Imaging Resource's review of the Olympus P400 dye sub printer. Notice near the end of the page how the 400 PPI image looks much worse (more jagged) than the image that was downsampled to 314 PPI (the PPI of the printer) first. This illustrates the importance of being able to resample to the PPI used by the printer prior to sending images to the print driver.

We can see some of the benefits and tradeoffs of upsampling in the above samples, but downsampling is just as important. When we take an image consisting of concentric circles of increasing frequency and downsample that image with an appropriate amount of antialiasing, we get the following result showing a single set of rings emanating from the center:

Qimage default downsampling (includes antialiasing step)

If we take the same image and downsample using a standard downsampling routine without first performing the antialiasing step, we get:

PhotoShop bicubic sharper downsampling (does not include antialiasing)

As can be seen in this second example, lack of antialiasing has caused extra patterns to appear that were not in the original image. While at first it may look like the second example has more "detail", in fact the extra detail is nothing more than artifacts caused by the resampling algorithm trying to consider data beyond the frequency limit.

A balanced approach

A well balanced interpolator will be able to downsample without aliasing artifacts while also being able to upsample without jaggies or over-softening the image. Old tried and true methods such as bicubic or lanczos are usually good enough for most upsampling needs. More advanced methods can increase visual quality for very large prints or special jobs, but be aware that there is only so much detail you can "add" to an image. Some of the newer interpolation methods try to make all edges as sharp as possible and while these methods can make upsampled results appear sharper, they often tend to break the correlation between sharpness and depth of field and can make results look a bit like fingerpaintings. Methods that produce smooth (jaggy free) images but don't try to sharpen, on the other hand, can appear a bit too blurry. As with many things, there are tradeoffs to each method.

The key to the best results with any interpolation method is often to pick the appropriate amount of sharpening. Interpolation methods that produce softer results can often handle much more sharpening before showing any artifacts, so a touch of extra sharpening can correct that soft look. Similarly, a slight edge blur can remove that "painterly" feel of some of the sharper interpolation methods if needed. Generally the more you stretch an image (the more interpolation you use), the more sharpening will be needed to compensate. "Smart printing" tools like Qimage and some PhotoShop print sharpening plugins take all this into account and can automatically apply the proper amount of final sharpening based on the resolution of the original, the size of the final print, the resolution of your printer, and other factors to allow the print to be the most visually consistent at any size.

Probably the most important thing to realize in this entire article is that there is only so much you can do to "invent" data that is not there and when displaying or printing photos, you have to go to extremes in most cases to be able to see the difference between interpolation methods. If you are captivated by some software or plugin that claims to do a much better job at interpolation, my suggestion would be to download a trial or do a search for reviews of the product before you buy. I've seen some ridiculous samples posted on interpolation software websites showing a vast difference between their method and others, only to download the software and find out that it really does no better than the old bicubic method. When you consider what the image "should" look like versus what we get out of various interpolators on the market, there really is very little difference between the better ones. If you find yourself about to plunk down $200 for an interpolation program or plugin, you might want to think twice. There are interpolation programs out there that offer a wide variety of methods for less than $50 that do as good or better than the high priced software.

Just for a sense of "calibration", one of the better methods available produced this result for the car wheel:

Some interpolation algorithms may render this image a little sharper, with a little more/less jagged edges, but the result will always be pretty similar to the above as far as the overall amount of detail that can be seen. Now consider what this image would look like if we had taken it with enough resolution to begin with and didn't need to interpolate:

This final image clearly shows the limitations of interpolation. Interpolation can reduce the appearance of artifacts like jagged edges but it simply cannot retrieve detail that is not there. The 4x reduced image simply has only 1/16th the amount of data which means that 94% of the data in the interpolated result had to be "guessed" in the above samples. Visually, the interpolated result is miserable in comparison to this last sample above regardless of the method used, but technically the result isn't too bad considering the fact that you started with only about 6% of the data you needed and you guessed at the other 94%!

Bottom line

The bottom line here is that interpolation can and does help improve the visual quality of images. That said, don't expect magical results and beware of some of the mythical claims out there. If you work for a magazine that normally starts with extreme crops and blows them up to 8x10 photos for print, you might be in the market for specialized interpolation software that allows you to pick the best method for each image/situation. Just be aware that many of the "miraculous" results displayed on the web sites for some of these interpolation programs and plugins are quite exaggerated. Better to try them first if they have a trial than to spend a significant amount of money and find out later that they really don't do much better than what you already have. Finally, keep in mind that if you have a 5+ megapixel camera and you normally don't do much cropping nor printing above 8x10 size, interpolation method may never be a concern for you. More important will be to find software that gives you the most benefit as far as the time it saves you and the quality of the final result.

-- Mike Chaney