Letter to the Editor

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More information on RAW image files in forensic imaging

This letter is in response to Steve Scarborough’s article in the last issue of Evidence Technology Magazine (“Forensic Imaging and RAW Files”, Volume 8, Number 4, pages 18-21). I work as a forensic scientist and have found two main advantages of RAW:

1) I can zoom in closer before the image falls apart, and

2) I can better correct exposure, color temperature, and shadows and highlights (contrast) in order to maximize the information in the photograph.

Using JPEG images for crime-scene photography is not without merit; however, JPEG is not as readily corrected as RAW. In the real world of crime scene photography, every photograph is not taken perfectly. The RAW data allows the “undeveloped” image to be corrected prior to translating it into an image format. This can be equated to the decisions that used to be made in the darkroom.

Placing crime-scene photographs into two groups—one for general photography using JPEG and one for comparison photography using RAW or TIFF—seems like a good way to minimize the need for the larger RAW and TIFF files. However, what one individual may have considered “photo-documentation” may have been sufficient for actual comparison work if the image had been captured in RAW format instead of JPEG.

The arguments that images captured as JPEG are equal in quality to images captured in RAW often seems to revolve around the appearance of the images after printing. I usually work with digital images on the computer rather than printing the images. This means that I am not limited by the quality of my printer and am able to use all of the information in the file. Working with the computer images rather than printed images makes me appreciate how much better the images translated from RAW are compared to images shot as JPEG.

RAW also is the better file format for minimizing background patterns by using selected channels.

I am more comfortable testifying from images shot in RAW because I know that what I see is as close as possible to what the camera took. I do not know exactly how each camera is modifying the RAW data in order to produce the JPEG files, but I do know that the JPEG is a modified file. The modifications can include altering the color space, applying unsharp masking in order to better define edges, converting the 12- or 14-bit depth images to 8-bit depth images, and compressing the data. These modifications are all taking place inside of the camera based on whatever conversion program the camera uses and on the settings that the photographer uses. Most of this information is not available to me in a JPEG image.

I agree with Steve Scarborough’s statement that RAW files are not universally viewable. Rather than recommend using JPEG or TIFF to make the data more universal, I would recommend that non-DNG RAW files be saved as DNG files. DNG files (Adobe Digital Negatives) are a raw file format viewable on publicly available software and they have checksums for verification of integrity. DNG is also being considered for interoperability-profile 2 (IP 2) for raw image data in the revised ISO 12234-2 standard.

Maintaining the data in either RAW or DNG file format means that the data is available for use with any future conversion software that works more effectively than the current JPEG and TIFF converters. This is very clear right now with the comparison between JPEG and JPEG 2000 files. A RAW file could be converted to either format. By saving the photograph as a JPEG, the photographer has committed to using whatever converter software is available on the camera.

Helen Griffin
Forensic Scientist
Ventura, California

 

Great response. I agree with all the main statements made by the respected Forensic Scientist and Trace Examiner; however, my main point was that all the advantages of RAW mentioned (in the response) take time and effort and man-hours to deal with, and there has to be a trade off and a decision made regarding when to use those advantages. Many agencies have huge case backlogs and need to make these decisions.

I do want to respond to these paragraphs:

“The arguments that images captured as JPEG are equal in quality to images captured in RAW often seems to revolve around the appearance of the images after printing. I usually work with digital images on the computer rather than printing the images. This means that I am not limited by the quality of my printer and am able to use all of the information in the file.

“Working with the computer images rather than printed images makes me appreciate how much better the images translated from RAW are over images shot as JPEG.”

As a Forensic Scientist, Fingerprint Examiner (retired), and current digital imaging consultant, I also do all my analysis on-screen and also recommend that process over printing. Reviewing the tests and the research, JPEG vs RAW, when viewed on-screen, are “equal in quality.” The issue with JPEG is NOT quality, it is the compression process. It seems that people are taking at face value a collective negative view about JPEG files. I always say, “Do the tests, do the research and you will see it too.”

I do want to mention the image on the first page of the article, though not specifically annotated, is an overlay of a JPEG image and a RAW image, wherein no appreciable difference can be determined. I defy anyone to tell which is which.

That said, I will continue to promote the use of RAW files and TIFF images for forensic use, at the same time suggesting an efficient workflow. And to all those forensic scientists out there dealing with these issues and working cases every day: Keep up the good work!

Steve Scarborough
Digital Imaging Consultant


ORIGINALLY PUBLISHED:
"Letter to the Editor"
September-October 2010 (Volume 8, Number 5)
Evidence Technology Magazine
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