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A Basic Guide to Resizing Digital Images eBook

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Basic Guide to Resizing Digital Images - Section B Part 8

8.0 Resizing Approaches, Programs and Examples.

8.1 Overview.

In this section, we will initially consider Resizing using some of Photoshops standard options. Examples of Up-Sizing and Down-Sizing will be given along with the Original Source Image which should be available from Imaging Associates International Website at http://www.imagingassociates.com.au/ebooks/resizingimagesguide/.

As you progress through this section, keep in mind that the different methods of upsizing and downsizing are for you, to use in order to satisfy your personal taste.

Different interpolation tools will need practical exploration by you, so you must always remember to keep a well defined objective in mind, along with a sound workflow and a dash of visual interpretive humor.

8.2 Photoshop Resizing Opportunities.

Photoshop which is available for both Windows and Mac machines is a useful starting point in appreciating the different Resizing Methods.

To resize an image in Photoshop CS2 the following WorkFlow applies.

From the Photoshop Menu Bar, select Image ---> Image Size to obtain the Dialog below:

Note the term "Resample Image".

The Resampling Image Selector at present is giving Bicubic as one of Photoshops standard Algorithms or Methods of Resizing.

Other methods are available in Photoshop as shown below.

8.3 Non-Technical Description of some Resizing Methods and Applications.

8.3.1 Nearest Neighbor.

Nearest Neighbor, which is sometime called Box is used for image Enlargements, and Reductions. When applied to Enlargements it magnifies the size of each pixel. When used In Reductions, pixel outputs are obtained by simple averaging of pixels within non-overlapping square areas.   Enlargement results are often said to be visually unappealing, but Reductions usually prove to be satisfactory. A point to note; is for the user to see whether the re-scaling method modifies the image where: upscaling by an integral scale factor, followed by downscaling to the original size, gives a result exactly identical to the original image. 

8.3.2 Bilinear.

Bilinear interpolation is a scaling technique that provides a form of weighted averaging between the nearest two input pixels to provide the value of a padded pixel to be inserted in the image. This weighted averaging is based on the linear distance of the output pixel position from the input pixel position. This type of scaling is said to give better results than nearest neighbor interpolation.

Bilinear interpolation, however, leads to significant loss of the high frequency content, such as sharp edges and text in the generated output image due to the performance of the weighted averaging. Loss of the high frequency content of the generated image leads to blurring of the image but according to some produces no halos (See Section 8.6.3 for illustration of halo artifacts). Some advise its main advantage is in image reduction, but recommend sharpening the original input image dramatically before applying the reduction process.

You can easily experiment with this resizing technique using our free online image resizer at http://imagingassociates.com.au/color/onlinetools.jspx.

 

8.3.3 Cubic.

High resolution cubic convolution and cubic spline interpolation are other forms of resampling techniques that are widely used for digital image processing. These methods, however, sometimes generate objectionable image artifacts for images having a high frequency content. For Bicubic Interpolation which is a standard method included in many photo editing tools including Photoshop, PaintShop Pro and others. The output pixel values are calculated from a weighted average of the nearest sixteen pixels in a rectangular grid (a 4x4 array). Often an image may need to be sharpened after this process.

8.4 Other Interpolation Methods.

There are often a bewildering number of Resizing Methods associated with the many excellent Application Programs and Plug-ins.

Imaging Associates International recommends a practical approach to determining the best method. Remember the "best" method is often dependent on the actual image content and the degree to which you wish to downsize or up-size along with your subjective appreciation of the end result.

Many excellent Resizing Programs and Plug-ins with specific resizing methods are available on a 30 day trial Download.

We list 12 of the best of them below, with their associated Website ...

... we hope you have enjoyed reading the first half of this sample chapter of “A Basic Guide to Resizing Digital Images”... to read the rest of this chapter, please buy the complete eBook ...

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          is breaking your complex overwhelming tasks into small manageable tasks,
 and then starting on the first one. MARK TWAIN.

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