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I'm willing to print a black and white picture with quite big dimension (more or less, 3x2 feet), but the image has pretty low resolution and appears quite blury. I was wondering which kind of post processing operation could help me sharpening the image, with softwares as GIMP or photoshop.

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    What resolution is the source image, what media are you printing onto (and with what printer), and from how far away will it be seen? And, does the image have a lot of delicate gray contrast, or is it "hard" black and white? – mattdm Jun 2 '14 at 11:14
  • Please post a sample picture if possible. – TFuto Jun 3 '14 at 11:35
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The apparent sharpness of an image can be improved by adding some background noise, texture, or "grain". This can be done digitally or by selecting an output medium such as matte paper or canvas. This works by triggering the brain's built-in pattern-seeking mechanisms.

Though covered elsewhere, I will go over some resizing and sharpening techniques for completeness.

Resizing

  • Some people have success by resizing in small increments, such as by 10%, until the image has reached its final size.
  • Lanczos works well and is commonly available across different programs and platforms.
  • Photoshop has a new Preserve Details method.
  • The examples using A Sharper Scaling look impressive. The program is available for Windows only.
  • I have had good results with waifu2x.

Sharpening

  • Most use unsharp mask, perhaps with a layer mask to limit sharpening to edges.
  • Some like to use a layer derived from wavelet decompose.
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Three techniques worth looking at, both using open source tools. 1. Using gimp try oversampling, scaling up, using the Lanczos algorithm. 2. Intentionally reduce resolution but provide texture using a halftone filter. This is what magazines used for printing. 3. Use Inkscape bitmap importing using potrace. This gives a vector representation that can be manipulated and gives a lithographic or engraved look. This is very effective for scans of photos from old books or newspapers.

  • care to explain a little about " Intentionally reduce resolution but provide texture using a halftone filter.". any good pointer would be great ... – kmonsoor Jun 4 '14 at 17:39
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Contrary to all those Sci-Fi and police shows on TV it is not possible to create detail where there is non in an image. So if your image is low resolution, blowing it up will only add to the pain. If there is any detail in the image and the image has sufficient bit depth and has not been compressed by creating a jpeg, then you may have a chance.

Use PhotoZoom Pro 5 for the enlargement and you won't lose details. Then sharpen with Photoshop's Smart Sharpen tool to remove as much blur as possible in your image. Play around with the settings to see what lens blur or motion blur, or gaussian blur will do to help bring back details in your image.

If your image is real bad you may be better off sharpening it a bit first to remove the blur but not actually adding sharpness. Then increase the sharpness after the enlargement.

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