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Problem Landscape

Resolution, or resolving power, has been an issue for photographers since the invention of the medium. The higher the resolution (think of this as the amount of real information or detail in an image) the higher quality or larger size a print could be produced. Before digital imaging, photographers tackled the issue of resolution by using larger film formats. Instead of using 35mm film which captured a relatively small amount of detail, photographers would move up to medium format or large format film. These larger films captured a great deal more information. You could think of it as going from a three megapixel camera to a 10 or 20 megapixel camera. Photographers would also choose different film types, developers and even optics to optimize their ability to capture detail, or resolution.

With digital imaging, photographers still seek higher detail and quality. We have all seen the megapixel march over the last decade. From simple one megapixel cameras that barely look good on a computer screen to huge 40 megapixel digital backs that capture almost 250MB in a single shot. However every digital camera captures a finite amount of information, limiting its ability to create prints larger than the camera's native resolution. For example a 10 megapixel camera captures enough information to create about an 8.5x13 print, just larger than a letter size piece of paper. If a photographer wants to make a larger print they have two choices, either purchase a camera that captures more information or use a process called interpolation.

Interpolation is a software technique which attempts to add more information in the form of pixels. There are several popular methods available in Photoshop.

Nearest Neighbor: A fast but less precise method that replicates the pixels in an image. This method is for use with illustrations
containing edges that are not anti-aliased, to preserve hard edges and produce a smaller file. However, this method
can produce jagged effects that become apparent when you distort or scale an image or perform multiple
manipulations on a selection.
Bilinear: A method that adds pixels by averaging the color values of the surrounding 2x2 block of pixels for each pixel in the
source image. It produces medium-quality results.
Bicubic: A method based on the examination of the values of the surrounding 4x4 block of pixels for each pixel in the source
image. Bicubic produces smoother tonal gradations than Nearest Neighbor or Bilinear.
Bicubic Smoother: A good method for enlarging images based on Bicubic interpolation but designed to produce smoother results.
Bicubic Sharper: A good method for reducing the size of an image based on Bicubic interpolation with enhanced sharpening. This
method maintains the detail in a resampled image. Bicubic Sharper can have a tendency to over-sharpen some areas
of an image.

Of the methods offered by Photoshop, the Bicubic method is probably the one most often used. The Bicubic method takes the target pixel and then looks at the surrounding pixels to determine the color value of the new pixels. This works well for continuious tone areas but falls short when it comes to maintaining detail. For example, if an edge in the original image is one or two pixels wide when the image is interpolated, the edge may become four or more pixels wide which has the effect of softening the image.

Interpolating an image over 200% will begin to cause a visual loss of detail and sharpness. This limits the user's ability to create a print of the size they want while maintaining quality.