Interpolation - making your images bigger

Cheryl Surry looks at the various options available that you can use to make bigger enlargements from your digital photographs.

Whether you have a 2 or a 6 megapixel camera, making your images bigger is something you’ll want to do from time to time, whether to print a large poster or to submit to an image library. The problem is, just like a 35mm negative is fixed in size at 24x36mm, a digital image is fixed by its pixel dimensions. Negatives are enlarged by projecting their image onto light sensitive paper, and the bigger you go, the lower the quality becomes. Digital images are no different, and whatever you do, you can’t make them bigger without suffering some degradation in quality.

When we need to make our images bigger we have a number of options. We can reduce the resolution, although lowering image quality, when viewed from appropriate distances, results are acceptable, examples of this include billboards where close inspection reveals the individual pixels.

Alternatively we can use software based interpolation methods, but what is interpolation? According to one dictionary definition it is to insert between or among others.

Whenever we work with digital images interpolation is used although we may not realise. When we scan an image at a resolution higher than the scanners optical resolution, the scanner’s software uses interpolation to meet our requirements.

Equally, the image sensor in a digital camera has a fixed resolution, if we choose to save our images at a different resolution then the camera will employ software based interpolation to save the data at our required resolution.

Printers also have a fixed resolution, and if the resolution of the image to be printed is different then the printers’ driver software uses interpolation to alter the image resolution to match that expected by the printer.

Image editing software uses interpolation whenever we rotate, resample or resize an image, but it is only when using these programs that we have some control over the type of interpolation being used.

I took a photograph of a bird of prey and enlarged it by 400% to see what four software options produced. The results below are a magnified section of the bird's eye to show the difference each program made.

Photoshop and PS Elements These image editors offer us various choices within the ‘Image Size’ dialog, which can be a powerful tool when used with care, nearest neighbour, bilinear and bicubic.   Nearest Neighbour doesn’t use interpolation, it simply takes the value of neighbouring pixels and adds new pixels without averaging, and this can result in pixelation, where individual pixels become visible. Bilinear ‘scans’ the surrounding pixels and interpolates an average value, it is faster than bicubic but offers lesser quality. Bicubic is the slowest of the three methods as it averages a wider area of pixels but produces the best estimation of new values, and is generally the recommended choice for photos. If the image is to be resized by a small amount, say up to a doubling of the image’s original size it is possible to do this in a single call to ‘Image Size’ by entering the new required value directly. Many people believe that beyond this degree of resizing it is better to use what is commonly called stair interpolation. Here small increments of about 5 to 10 percent are applied to the image consecutively until the desired size is reached; to save time these increments can be saved as actions and replayed as required. Key features Works on 16-bit images Stair interpolation acceptable to 200-400% Actions can be recorded for stair interpolation Image can be adjusted to exact final size Can alter true colour of original Always defaults to Bicubic

Stair Bicubic 400% using PS Bicubic

Paint Shop Pro In addition to offering similar resizing options to Photoshop, Paint Shop Pro has a trick of its own, SmartSize. Bilinear and Bicubic are similar to the PhotoShop offerings of the same name, whilst Pixel Resize gives similar results to PhotoShops Nearest Neighbour. The Weighted Average option calculates a weighted-average colour value of neighbouring pixels to determine how the newly created pixels will appear and seems to lie somewhere between Bilinear and Bicubic in terms of the quality and speed of the algorithm. Smart Size lets Paint Shop Pro choose one of the four algorithms listed above based on the new pixel dimensions that we set. It is unclear which algorithm has been chosen and this may not suit all users as again we are sacrificing control. We can also use stair interpolation in Paint Shop Pro to increase the perceived image quality. Like Photoshop we can save time by recording our actions and replaying them. Key features Works on 16-bit images Stair interpolation acceptable to 200-250%  Stair interpolation produces strange results  Extra SmartSize option Easy to use interface Retains last used algorithm Bicubic algorithm performed marginally worse than PS version

400% using PSP Bicubic 10% stair 400% using PSP Bicubic

Genuine Fractals LizardTech’s Genuine Fractals works as a file format option rather than a straight filter type plug-in. This means that before we can resize an image we must first ‘save as’ selecting the Genuine Fractals option in the file type drop-down menu. This saves our file with a .STN extension, we have the option on saving of using lossless or virtually lossless compression. The file size created is significantly smaller than if we had saved the same file in TIFF or Photoshop PSD format, but it does not work with 16-bit images which may be a limitation for some. When we open any file with a .STN extension the Genuine Fractals interface is automatically launched giving us the option to choose how we wish to resize the image. The top third of the dialog shows us the original image settings, including the compressed file size. The middle part allows us to crop the image if we need only to enlarge a portion of it. The bottom third allows us to choose the size we want to enlarge the file to, and to choose the quality from high to low, always choose high, although this takes longer it is worthwhile waiting.

400% using Genuine Fractals

Key features

• Two-stage operation [.STN file has to be opened for each resize]
• Own lossless file format
• Good for 400-600% increases
• Retains true colour of original
• Preview size is limited
• Exact size selectable
• Three versions available to minimise cost
  

Extensis pxl SmartScale The software from Extensis works differently from Genuine Fractals as it is a true plug-in filter. SmartScale offers two options, we can either scale the current image or scale an image from file. When we select the ‘Scale Current Image’ option we are presented with an interface that in addition to offering us the option to rescale also allows us to control sharpening of the image. The large preview window and ability to zoom into the image to check the effect of changes in sharpening is a plus over the smaller preview offered by Genuine Fractals. We can choose to rescale in a number of different ways, as a percentage, by absolute size or by using a slider. The sharpening options need to be tailored for each image being resized and the extreme edges option should not be used with photos, instead it should be restricted to use with graphics. In fact it is probably better to sharpen afte resizing using USM options.

400% using Pxl Smartscale

Key features

• Own lossless file format
• Can increase by up to 1600% [actually limited by 30000 pixel limit in PS]
• Large preview window with zoom
• In-built sharpening options
• Retains true colour of original
• Exact size selectable
• Limited to 8-bit images

Conclusion
By far the best way to create high quality enlarged prints is to use originals with a suitable resolution for the purpose intended, however the advanced interpolation techniques offered by the specialist software do provide an acceptable alternative for scaling beyond 300%. The dedicated resizing plug-ins produce an image that is much closer to the original than either Photoshop’s or Paint Shop Pro’s standard bicubic method of interpolation. Both produce significantly less noise in shadow or dark image areas and retain a better overall colour integrity to the original.

If you only need to resize by increasing the original by 100% then any of the products tested perform adequately, and beyond that the stair bicubic interpolation method offers improved results over a one-step enlargement.

When it is not possible to capture at a appropriate resolution due to the limitations in the capture device, then capture at the highest available native resolution and keep interpolation to a minimum. If you need to scale an image beyond triple the size of the original then the custom packages do offer a significant advantage in terms of quality and convenience.

Both of the custom products offer free trial downloads, and given that variations occur in results depending on the nature of the original image this is worth considering before purchasing.

Review by Cheryl Surry