1、Designation: E2825 12E2825 18Standard Guide forForensic Digital Image Processing1This standard is issued under the fixed designation E2825; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in pare
2、ntheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide provides digital image processing guidelines to ensure the production of quality forensic imagery for use asevidence in a court of law.1.2
3、 This guide briefly describes advantages, disadvantages, and potential limitations of each major process.1.3 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of Internati
4、onal Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ISO/IEC Standard:2ISO/IEC 10918-1:1994 Information technologyDigital compression and coding of continuous-tone still images: Requirementsand guideli
5、nes (JPEG) (also published as CCITT Recommendation T.81 (1992)2.2 SWGIT Material:3SWGDE/SWGIT Glossary SWGDE and SWGIT Digital and3.2.2 The end result is presented as a processed or working copy of the image.3.3 Avoid the introduction of artifacts that add misleading information to the image or the
6、loss of image detail that could leadto an erroneous interpretation.4. Significance and Use4.1 Processed images are used for many purposes by the forensic science community. They can yield information not readilyapparent in the original image, which can assist an expert in drawing a conclusion that m
7、ight not otherwise be reached.4.2 This guide addresses image processing and related legal considerations in the following three categories:4.2.1 Image enhancement,4.2.2 Image restoration, and4.2.3 Image compression.5. Image Enhancement5.1 Image enhancement is any process intended to improve the visu
8、al appearance of an image.5.1.1 Use brightness adjustment when the image is too bright or too dark. If the image is made too bright, there is a risk of lossof detail in light areas. If the image is made too dark, there is a risk of loss of detail in the dark areas.5.1.2 Use color processing to modif
9、y the color characteristics of objects within an image. This includes color spacetransformations, pseudocoloring, and hue and saturation adjustments.5.1.2.1 Application of these techniques can compromise the color fidelity of the image.5.1.3 Use contrast adjustment when the image lacks sufficient co
10、ntrast. If the image contrast is increased too much, there is arisk of loss of detail in both light and dark areas.5.1.4 Use cropping to remove that portion of the image that is outside the area of interest.5.1.5 Use dodging and burning to adjust brightness in localized areas.5.1.6 Use linear filter
11、ing techniques (see Fig. 1) to increase the contrast of small detail in an image. These include sharpening,blur removal, edge enhancement, and deconvolution. If a low degree of enhancement is used, the image will remain an accuraterepresentation of the scene. If a high degree of enhancement is used,
12、 the image may no longer be an accurate representation ofthe overall scene, though it still may be useful as an adjunct for interpretation of small details.5.1.6.1 A high degree of enhancement can also increase the visibility of existing noise and artifacts; examples of noise includefilm grain, snow
13、 appearing on a television screen, or random color dots.5.1.7 Use nonlinear contrast adjustments to adjust the contrast in selected brightness ranges within the image. These includegamma correction, grayscale transformation, and the use of curves or look-up tables, or both.5.1.7.1 A nonlinear contra
14、st adjustment can be used to bring out details in the shadow areas of an image without affecting thehighlight areas.5.1.7.2 A severe adjustment can cause loss of detail, color reversal, or the introduction of artifacts, or a combination thereof.(See Fig. 2.)5.1.8 Use pattern noise reduction filters
15、to identify repeating patterns in an image and selectively remove them. This type offilter can be used to remove patterns such as fabric weaves, window screens, security patterns, and halftone dots.5.1.8.1 Overuse of this technique will remove material image detail.5.1.9 Use random noise reduction t
16、echniques to reduce the contrast of small detail in the image to suppress random noise. Theseinclude such filters as low-pass filtering, Gaussian blurring, median filtering, and speckle removing.5.1.9.1 Overuse of this technique will remove material image detail.5.1.10 Use warping to change the spat
17、ial relationships among the objects in an image. It is analogous to printing a photographon a rubber sheet, then stretching the sheet in different directions, and then tacking it down. Warping can be used, for example,to remove perspective from an image or to “unroll“ a poster that was wrapped aroun
18、d a pole.5.1.10.1 Used improperly, warping can distort the natural appearance of the objects in a scene.6. Image Restoration6.1 Image restoration is any process applied to an image that has been degraded by a known cause (for example, defocus ormotion blur) to remove the effects of that degradation
19、partially or totally.E2825 1836.2 Information that has been totally lost in the image during the original imaging process cannot be replaced throughrestoration. However, partial restoration can be successful even when total restoration is impossible.FIG. 1 This Example Illustrates the Effects of Lin
20、ear FilteringLeft: Original Image, Middle: Blurred Image, and Right: Sharpened ImageFIG. 2 This Example Shows Nonlinear Contrast AdjustmentsLeft: Original Image, Middle: Enhancement of Shadow and Highlight Ar-eas at the Expense of Midrange Tones, and Right: Enhancement of Midrange Tones at the Expen
21、se of Shadow and Highlight AreasE2825 1846.3 Restoration Techniques:6.3.1 Use blur removal to remove partially or completely an image blur imposed by a known cause.6.3.1.1 Blur removal differs from the image enhancement filtering processes because the blur removal filter is designedspecifically for
22、the process that blurred the particular image under examination. Examples include defocus and motion blur, sincethese phenomena can be described mathematically. Thus, a specific filter can be designed to compensate for each blur. The degreeto which a blur can be successfully removed is limited by no
23、ise in the image, the accuracy with which the actual blurring processcan be described mathematically, and the fact that information that has been totally lost cannot be replaced. Often partial blurremoval can be successful even when total blur removal is impossible.6.3.2 Use color balancing to rende
24、r the colors in the scene more accurately. Color balancing is the extension of grayscalelinearization to a color image and the adjustment of the color components of an image. For example, a color test target havingknown colors can be placed in the scene before recording the image. Then a grayscale t
25、ransformation (nonlinear contrast stretch)can be designed for each color channel (red, green, and blue) to place the different colors on the test target in their properrelationship. This should reproduce the other objects in the scene in their proper relationship.6.3.2.1 Improper color balance can r
26、ender colors inaccurately causing objects to appear to have the wrong color.6.3.3 Use grayscale linearization to render faithfully the different brightness values in the scene. This adjusts the brightnessrelationships among the objects in a scene. For example, a monochrome test target having known g
27、ray values can be placed in thescene before recording the image. Then a grayscale transformation (nonlinear contrast stretch) can be designed to place thedifferent gray values on the test target in their proper relationship. This should put the other objects in the scene in their properbrightness re
28、lationship as well.6.3.3.1 Improper grayscale linearization can render brightness values inaccurately so that objects may appear brighter or darkerthan they actually appeared when the image was recorded.6.3.4 Use geometric restoration to restore the proper spatial relationships among the objects in
29、the scene. This restorationremoves geometric distortion from an image. It can be used for the removal of geometric distortion, such as that introduced bya curved mirror or a fish-eye lens.6.3.4.1 Geometric restoration differs from image warping in that the geometric transformation is designed specif
30、ically for theprocess that distorted the particular image under examination.6.3.4.2 The degree to which geometric distortion can be successfully restored is limited by the accuracy with which the actualdistortion process can be described mathematically and the fact that information that has been tot
31、ally lost (for example, hiddenbehind another object or obscured from the camera) cannot be replaced. Often, partial geometric restoration can be successful evenwhen exact geometric restoration is impossible.7. Image Compression7.1 Digital images produce a large amount of data to be stored. Image com
32、pression techniques reduce the storage requirementsby making image data files smaller.7.2 Compression Processes:7.2.1 Lossless compression reduces file size by removing redundant information. Because the redundant information can beretrieved to display the image, lossless compression results in no l
33、oss of information. Lossless compression does not alter thecontent of an image when it is decompressed.7.2.2 Lossy compression achieves greater reduction in file size by removing both redundant information and data deemedexpendable by the compression algorithm. Because the expendable data cannot be
34、retrieved upon reconstruction of an image fordisplay, compression results in some loss of image content as well as the introduction of artifacts.7.2.2.1 Degradation occurs each time the image is compressed using a lossy process, such as saving to a compressed format.7.2.2.2 Higher compression ratios
35、 result in the loss of more information. Normally, the degree of compression can be specified.7.2.2.3 Depending upon the application, lossy compression may render an image less useful.7.2.3 The Joint Photographic Experts Group developed an image compression standard known as JPEG (ISO/IEC 10918-1:19
36、94). This compression algorithm is applied to the image in 8 by 8-pixel blocks. Normally, it is used as a lossy compressionscheme in which the degree of compression can be specified before storing the image. However, JPEG can also be used as alossless compression scheme. At high-compression ratios,
37、JPEG could remove important image detail and introduce blockingartifacts as the block boundaries become visible (see Fig. 3). JPEG is but one of many compression algorithms.7.2.3.1 Compression should be used with care to avoid material degradation of the image.7.2.3.2 The compression settings used b
38、y one camera or software program may not be the same as the compression settings usedby another camera or software program.7.3 Use of Compression:7.3.1 Many digital cameras store images using JPEG compression, so that some compression is unavoidable. Some digitalcameras are capable of storing images
39、 in an uncompressed form. The degree of compression should be set low enough that materialimage content is not lost or obscured by artifacts.7.3.2 In instances in which the primary or original image is already compressed, it should not be further compressed using lossycompression processes; addition
40、al data will be lost. Sources of compressed primary images may include electronic bookingE2825 185photographs, some types of digital camera images, and images downloaded from the internet or email. The file format is not anindicator of the compression history for an image. For example, aTagged Image
41、 File Format (TIFF3) file may have been previouslycompressed using a lossy algorithm (1).47.3.3 The material use of an image may change over time. Any compression used to save an image should be appropriate forthe intended use at that time.7.3.3.1 Images intended for laboratory analysis should not b
42、e compressed using a lossy process unless the resulting image stillretains the relevant information as determined by the laboratory personnel conducting the analysis.8. Guidelines for Digital Image Processing Standard Operating Procedures8.1 The purpose of image-processing procedures is to apply pro
43、cessing techniques intended to enhance, restore, or compressdigital images, or a combination thereof. Standard operating procedures should be developed and followed. A sample standardoperating procedure is included in the SWGIT document “Guidelines for Image Processing” (2). See also the SWGDE/SWGIT
44、document, “Recommended Guidelines for Developing Standard Operating Procedures” (3).8.2 EquipmentThe laboratory standard operating procedure (SOP) should define minimum hardware and software equipmentrequirements including, but not limited to:8.2.1 Hardware:8.2.1.1 Input/capture device,8.2.1.2 Image
45、-processing systems,8.2.1.3 Output devices, and8.2.1.4 Storage/archive.8.2.2 Software:8.2.2.1 Image management, and8.2.2.2 Image processing.8.3 ProceduresLaboratories should establish specific step-by-step procedures for image processing according to publishedguidelines. These procedures should addr
46、ess the following as a minimum:8.3.1 Capture,3 TIFF is a trademark of Adobe Systems Incorporated.4 The boldface numbers in parentheses refer to a list of references at the end of this standard.FIG. 3 Left: Original Image, Middle: the Result of JPEG Compression (Compression Ratio = 15:1), and Right:
47、the Result of Edge En-hancement after CompressionE2825 1868.3.2 Processing,8.3.3 Storage and archiving,8.3.4 Image management,8.3.5 Data security, and8.3.6 OutputOutput.8.4 CalibrationLaboratories should develop SOPs for calibrating all equipment that produces test results. These proceduresshould be
48、 consistent with the manufacturers recommendations.8.5 LimitationsLaboratories should document the limitations of their processes and equipment in their SOPs.8.6 SafetyLaboratories should develop safety procedures specific to their needs.8.7 ReferencesLaboratories should maintain their laboratory sp
49、ecific documentation, manufacturers manuals, and publishedguidelines.8.8 TrainingLaboratories should define the level of training necessary to perform the procedure. Refer to the SWGIT“Guidelines and Recommendations for Training in Imaging Technology in the Criminal Justice System” (4) and “SWGDE/SWGITGuidelines and Recommendations for Training in Digital and Multimedia Evidence” (5).9. Keywords9.1 criminal justice system; digital image processing; image processingAPPENDIX(Nonmandatory Information)X1. SAMPLE STANDARD OPERATING PROCEDURES FOR LATENT PRINT DIGITAL IMAGI
