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Forensics – Recovering the Most Detail from Your Image

In recovering the most detail from an image, the most important part of the image restoration process is what you do before running Focus Magic. It is important to start with the best image you can. Digital cameras tend to suffer from insufficient resolution, while conventional (film) cameras tend to suffer from insufficient accuracy (of each pixel value). Security (video) camera's tend to suffer from both. The principles explained in this tutorial apply to the sharpening of any image, not just forensic images. This tutorial uses the Photoshop plugin.

1. Get the Image into the "Forensic Recovery Range"

For Focus Magic the "Forensic Recovery Range" is the range of blur widths (or blur distances for motion blur) between 10 and 20. When Focus Magic focuses or corrects motion blur in this range it can recover lost detail quite well, but as the blur width drops below 10 the amount of detail that can be recovered progressively drops. This is illustrated with the example of a cars licence plate below.


Original image focused using a Blur Width of 19. The license plate is quite clear to read. This image is within the "Forensic Recovery Range".

 

Original image reduced to half its size and focused using a Blur Width of 10. The license plate is still quite clear to read. This image is also within the "Forensic Recovery Range".

 

Original image reduced to one quarter of its size and focused using a Blur Width of 5. The license plate although a little clearer, can no longer be read. This image is no longer within the "Forensic Recovery Range".

 

Original image reduced to one quarter of its size, saved, opened again, increased back to it's original size and focused using a Blur Width of 19. After scaling the image down and up again to its original size, the license plate is surprisingly clear. By simply scaling the image size up, this image has been brought back within the "Forensic Recovery Range".

For scanned photos, scan them in using a resolution which is high enough to have a blur width between 10 and 20. If for example the photo was scanned in at 1200 bpi (bits per inch), and the blur width is 6, then scan it in again using 2400 bpi so that you can use a blur width of 12. If the image was scanned in at its highest resolution then increase the image size as for a digital camera (next). Not all scanners produce sharp scans. If you are scanning in an image with quite a low blur width, it is vital to use a good scanner which doesn't contribute to the blur of the image. After scanning the image in, compare the real image with what you see on the screen and make sure that it hasn't contributed any blur to the image. If you have the negative of the photo, then you could make a large print of the photo before scanning it in. Make sure that the photo lab understands the importance of getting a sharp image.

If the photo is from a digital camera, then increase the image size so that the blur width is in the 10 to 20 range. It is probably best to increase the file size by a factor of 2 or 4 so that the interpolation is kept as simple and hence as accurate as possible.

If the blur width is greater than 20 then scale the image down so that the blur width is in the 10 to 20 range. Focus Magic can process a maximum blur width of 20. If it could go above 20 then you wouldn't gain much (or anything) anyway.

2. Minimize or Eliminate JPG Compression

The following portion of the license plate shown twice full size shows how high JPG compression produces tiles which are 8 pixels by 8 pixels.

Image saved with little or no JPG compression.

 

Image was saved with high JPG compression (Quality of 1 in Photoshop). If you look carefully, you'll see that the left hand image looks a bit blocky (ie. has lots of 8 X 8 squares of pixels). The After image is not as clear as above.

 

This magified part of the top left hand corner of the above (before) image shows the squares more clearly.

When scanning images in, either save them as a TIF file or some other lossless format, or else as a JPG only if you can set the Quality to Maximum. For digital cameras you can usually adjust the image quality. If you can, then set the quality to Maximum, or the compression to Minimum.

3. The Effect of Noise

A grainy image won't focus as well as a smooth clean image.

Image no noise (grain).

 

Noise (10 % uniform noise using Photoshop) was added to simulate a grainy image. The re-focusing is not nearly as good.

If you already have an image ther may not be a lot you can do to remove noise or grain. If you have a negative, you might be able to reduce the noise by developing the print again using better equipment. If the image has dust on it, then you can gently wipe the dust off.

4. Focusing Strength

Focus Magic has different focusing strengths which are determined automatically from the Image Source setting. Digital camera images can be restored using a higher strength than conventional (film) cameras. This is because digital camera pixel values are more accurate. By the time a conventional camera image has been captured on a negative, developed into a print and then scanned in, the pixel value's accuracy is greatly degraded. Where a digital camera goes straight from an image to a CCD, a scanned conventional camera image goes through two extra chemical processes which introduce both grain and inaccuracy.

The Forensic Strengths of the different Image Source settings are as follows :-

Image Source Focusing Strength
Grainy Image Low
Conventional (Film) Camera, Analog Video Camera, Magazine / Newspaper, General Medium
Digital Camera, Digital Video Camera High
Forensic, Software Very High

You can experiment with the different focusing strengths so as to get the best result. As a general rule, select a Focusing Strength which is one step stronger than the strength that would be selected from the Image Source. For example for a Digital Camera, which has a High setting you would choose Very High, and for a Conventional Camera which has a Medium setting you would choose a High setting.

Note that the Image Source also sets typical values for Random Noise (grain) and Shot Noise (dust and scratch removal) which Focus Magic uses to correct the image optimally for that type of image.

Low Forensic Strength


Image Source = Grainy Image

 

Medium Forensic Strength


Image Source = Conventional Camera

 

High Forensic Strength


Image Source = Digital Camera

 

Very High Forensic Strength


Image Source = Forensic

You can see in the above example which is from a digital camera, that the clarity increases as the Focusing Strength increases. This only works however for high quality input images (with accurately defined pixels).

Low Forensic Strength


Image Source = Grainy Image

 

Medium Forensic Strength


Image Source = Conventional Camera

 

High Forensic Strength


Image Source = Digital Camera

 

Very High Forensic Strength


Image Source = Forensic

In this image which is a scanned image from a print, we are trying to tell the time at which the photo was taken by zooming into the ladies watch (shown upside down). In this case due to the pixel values lower accuracy the image gets quite distorted when the Focusing Strength is set to Very High. It looks as though the hands of the watch can just be seen. If the negative is available, then a better result could possibly be obtained by enlarging the print and scanning again with a better scanner (this image was scanned using a cheap scanner).

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