Imagener Image Enlarger
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Enlarge photos from digital cameras
Resharp Function Sharpness Control

All digital images can benefit from being sharpened at some stage in their lives. This is especially true for enlarged images, but enlarged or not, all captured images - digital or analog - suffer from blurring or softening of detail in some way. This is precisely the motivation behind Kneson Software’s addition of the Resharp function in Imagener Professional and Imagener Unlimited. However, our customers have been asking for more information about this function as use of it does require a certain level of skill and knowledge to master.

Blurring occurs when the representation of an object is at a lower contrast or retains less detail than is present in the original. Sharpening compensates for this loss by improving the visibility of the information in the image. It works by applying a matrix of numbers over an array of the pixels in the image with the matrix centered over one pixel at a time. Sharpening can add image data that improve the appearance or impression of sharpness.

This function is formally known as "Unsharp Mask." We named it "Resharp" to avoid any confusion, but the technique is actually a darkroom technique for improving the sharpness of paper prints that was used before the age of digital pictures.

There are three variables to the function, Amount, Radius, and Threshold. Each variable interacts with the other so it is possible to obtain nearly identical effects with very different settings. Keep the following characteristics in mind:

Amount
Radius
Threshold
  • A measure of the strength of sharpening, roughly as a percentage of the increase in edge contrast.

  • Works best within the 50-200% range (enter 50 up to 200 in the amount box in Imagener).
  • A measures of the number of pixels over which the function operates.

  • Needs to be set with care as radius has the greatest effect.

  • A radius of three may in fact cover seven or more pixels.

  • In general, low figures give crisp edges; larger figures produce broader edges, and increase overall contrast.
  • Measures the minimum difference between two boundaries that the function will operate on.

  • Is based on 255 levels and therefore will only accept a numerical value of 0 to 255.

  • Zero threshold tells the function to operate over the entire image.

  • A value of 127 (about half of the 255 levels) will cause sharpening to occur only where pixels are next to other pixels that are 50% lighter or darker.
For example, the images below were enlarged using Imagener Unlimited. The image used was of somewhat low resolution, hence the slight blur that appears in the image to the left. Using the Resharp settings shown at the right, the image below right is made to look much more visually appealing. Edges contrast better, and the overall appearance is improved.  
Image Resized
Image Resized and Resharpened

These photo examples are unusually small ... most image enlargement uses start with images much larger with higher resolution. These images are for this discussion as examples only.

General Guidelines
Start your resharp operations with the following settings as a guide.

Image consists mostly of fine details
 
Image has large areas of even tone
Set a strong amount of sharpening, with very small radius and a low threshold, such as Amount: 200, Radius: 1-2, Threshold 1-2. This improves all boundaries in the image over a small distance. While these settings will also increase grain or noise, they will be hidden by the detail.   Set a modest amount of sharpening, and a large radius with a relatively high threshold, such as Amount: 50, Radius: 20, Threshold: 10. The high threshold ignores noise while the high radius compensates for the moderate amount.

You may discover that the wrong settings can create bright, bare patches. Make sure you remember your enlargement settings as you will need to click the Reset button if this occurs. Keep experimenting. With practice, you will increase the visual clarity and appeal of your enlarged images.

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