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Computers have long been useful in rock art site recordingand database management. More recently, techniques forstitching overlapping photographs to create panoramas or

mosaics of panels or entire sites have been developed for sitedocumentation. This was discussed in detail by Mark and Billo(1999). Current techniques have advanced due to new softwarepackages that add spherical and cubic projections (e.g. REALVISStitcher). Digital images can be linked to the site locations withGIS software such as ArcView (Harry et al. 2001:154–156). Inorder to improve the quality of imagery we have developeddigital image enhancement techniques using Photoshop.™ Otherresearchers have reported on multivariate manipulation of multi-band images using Idris software (Buchner et al. 2000)

The emphasis of this paper will be on describing those digitaltechniques used to enhance digital or digitized photographs. Werely on the use of Adobe Photoshop, and assume that the reader isgenerally familiar with this software. Significant informationabout rock art such as details of elements or pigments can some-times be obtained from the color information in photographs(Mark and Billo 2001). Photoshop allows for alternate colorspaces, CMYK, and Lab in addition to RGB (Red, Green, Blue).

Application of Digital ImageEnhancement in Rock Art

Recording

Robert Mark and Evelyn Billo

Robert Mark

and

Evelyn Billo

Rupestrian CyberServices,

Flagstaff, [email protected]

http://www.infomagic.net/~rockart

Faded pictographs and almost invisible petroglyphs were photographed, digitized, and then enhanced usingAdobe Photoshop.™ Use of alternate color spaces, CMYK (cyan, magenta, yellow, black), and Lab (Lightness,and two orthogonal color channels) may permit discrimination of subtle color differences. Each color channelis examined and enhanced separately (sometimes inverted). One or more channels are selected as gray-scale images, or are recombined into enhanced or false-color images. Some enhancements require multiplelayers with various blending modes such as soft light or overlay. Depending on color and variability of therock image as compared with background substrate, results can be astounding. Algorithmically enhancedimages should be so identified, and distinguished from any method using interpretive artwork (pen, brush, orselection tools).

[paper presented at

ARARA 2000, Phoenix, AZ]

American Indian Rock Art, Volume 28. Alanah Woody, Editor. American Rock Art Research Association, 2002, pp. 121–128.

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Each color channel can be examined andenhanced. One or more channels are selectedas grayscale images, or can be recombinedinto false-color images.

In some cases, elements that were almostinvisible in the field can be made visible(Figure 1). In other cases, different pigmentscan be made to stand out clearly and sepa-rately from each other, helping to analyzecomplex panels (Figure 2). We use grayscaleimages to illustrate a subset of the AdobePhotoshop techniques useful for imageenhancement. Color illustrations are requiredto show the advantages of enhanced color andfalse color images.

IMAGES: THE STARTING POINT

We use 35-mm SLR cameras, and variousFuji slide film (ISO 50, 100, and 400) depend-ing on the lighting conditions. In general, it isimportant to have uniform lighting for picto-graphs. Noteworthy is the observation thatsome pictographs are seen better with head-on flash rather than a natural light that illumi-nates a rough surface from an oblique angle.In one instance, where the pictograph was indeep shadow with only diffuse light fromabove, the faint painting did not show up in

Figure 1. Two images on the left are as photographed(original in color). Right images are digitally enhancedimages: upper right is LAB-red and lower right isCMYK yellow channel. The spray paint initials arein blue and therefore can be separated from the origi-nal yellow painted figure.

Figure 2. Grayscale renditionof color image, and threeenhanced CMYK colorchannels. Each channelemphasizes different pig-ments (and elements).

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a film time exposure but was visible in thedigital flash image. Oblique lighting, whethernatural or flash, is usually best for petro-glyphs. Sometimes a flash held to the side canbring out fine-line imagery not visible withdirect lighting.

We obtain most of our digital images byscanning color slides. We also use digitalcameras in the field (Nikon Coolpix 950 and995). Images can be acquired by scanningphotographic prints; however, there is a lossof information when prints are scanned. Nega-tives can provide high quality scans, but theyare more difficult to handle in our automatedsystem (Nikon Coolscan 4000 ED with stackloader).

Significant compression of an imageparticularly degrades color information.This can be a problem, producing serious arti-facts during enhancement (Figure 3). While

best-quality JPEG compression in Photoshopis generally not a problem, best-quality JPEGcompression from a digital camera can be.By “best-quality” we mean selecting thehighest number available in the version ofsoftware being used (for Photoshop 6 thisnumber is 12).

FIRST STEPS

After opening an image in Photoshop,the first step is usually to examine the Levelshistograms (note: use of bold indicates Photo-shop entities). Histograms display the rangeof color information available to work with.They graph the number of pixels (picture ele-ments) at each color intensity level. Adjust-ments are usually needed in Levels (orCurves). Eyedroppers in the Levels windowcan sometimes be used, in conjunction with a

Figure 3. Grayscale image of highly-compressed (compression ratio of 15) color image, with anthropomorph(?) infaded red. The second panel shows the JPEG artifacts made visible by the enhancement process of the first image.The third panel shows a LAB-a enhancement of a different image that was compressed using a JPEG compressionratio of 3.


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white or gray scale (if one was in the originalimage), to adjust color balance.

After minimal adjustments, and beforeimage enhancements, a copy should be saved.We recommend that enhanced images be soidentified, and that unenhanced images also bepreserved. This will enable future researchersto see how the original site looked as photo-graphed, and have the original images towork from as additional techniques and meth-ods of enhancement are developed.

Applying the Autolevels command some-times brings out much detail, but typicallyproduces a “burned-out” image. Fade Levels(Edit menu in Photoshop 6) can then be usedto reduce the effect or change its mode tocolor or luminosity. Also useful are RGB tech-niques to adjust Color Balance, Hue/Satura-tion, and Curves. For some images thesesimple steps may be the only ones needed.

OTHER ENHANCEMENTS

Other image enhancements utilize alter-nate color spaces: Lab (Luminosity or light-ness, and two orthogonal color channels, aand b) and CMYK (cyan, magenta, yellow,and black) (Adobe Systems Inc. 2000:111–112).Both of these color spaces tend to separatecolor information from luminosity, and thusare useful for amplifying subtle differencesin color. We define enhancements in threegeneral classes:

1. Grayscale images: the only class wefully illustrate in this paper, although wediscuss the other classes and techniques andillustrate grayscale versions of them. Theseare single color channels or mathematicalcombinations of color channels.2. False color images: the output colorsare not similar to the original, but are usedwhen rock art elements are difficult to dis-criminate in the original color hues. Manyfalse-color enhancements are created in Lab,but occasionally simply taking the Inverse(negative) of an RGB image and adjusting

Levels produces a useful false-color image.This is particularly helpful to bring out theimagery in some low contrast petroglyphs(Figure 4).3. Enhanced color images: in the case ofpictographs, the final product has moresaturated colors; these colors are in thegeneral range of hues of the paler pigmentsfound in the original. Most of these are cre-ated in CMYK.

Lab Color Space

Adobe defines L as lightness with a rangeof 0 to 100. L contains no color information.The a component (green-red axis) and the bcomponent (blue-yellow axis) range from+120 to –120. While green-red and blue-yellow are not quite accurate descriptionsof the color axes, they are close enough for

Figure 4. a) Some difficult-to-see petroglyphs. b) ImageInverted and Levels adjusted. Enhancement is betterin color.

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discussion. After conversion to Lab colorspace (Mode Lab), the individual channelsare examined (Levels) using the ChannelsPalette. Both color channels start with verylow contrast (Figure 5a). Their contrast is sub-stantially increased (Levels) in the enhance-ment process (Figure 5b). Any of the enhancedchannels (L, a, and/or b) can be saved as gray-scale images (Mode Gray or by Split Chan-nels, Figure 6). The Lab image, after increasingthe contrast of the a and/or b channels, is afalse-color image. It can be converted back toRGB (Mode RGB) and saved. Alternatively,an individual channel (R, G, or B) can then besaved as a grayscale image.

CMYK Color Space

CMYK color space (usually used for print-ing) is used by us for color enhancement. Afterconverting to CMYK (Mode CMYK), the indi-vidual channels are examined (Channels

Figure 5. Photoshop screen images of: a) LAB-b channel before adjustment. b) after Levels adjust-ment to increase contrast.

Figure 6. Enhanced LAB-b channel, converted toMode grayscale, and Inverted.


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Palette, Levels). Contrast is usually increasedin each of the three color channels (Figure 7),and appropriate channels saved as grayscaleimages (Mode Gray or Split Channels). Theenhanced CMYK image usually shows vividbut generally realistic colors. The image is con-verted back to RGB (Mode RGB) and saved asan enhanced image. It should be noted that thechoice of CMYK working color space (ColorPreferences) will affect the results of thismethod. In Photoshop 6, we use SWOP v2.

COMPLEX METHODS

There are many possible variations onthis process that may prove useful in photo

enhancement. These include Inverting one ormore channels, using the High Pass filter,combining grayscale images (Apply Image orCalculate), or blending a grayscale image overa color image (Layers Palette, BlendingMode). Particularly useful is blending anenhanced color channel over the original orCMYK-enhanced color image, using multiply,overlay, soft light or hard light blendingmodes (Figure 8). Sometimes the color chan-nel image needs to be inverted. Other complexmethods include using adjustment layerswith color-channel images as layer masks. Theimages are usually flattened (layers merged)before saving. If they are not flattened, theymust be saved in Photoshop format.

Figure 7. Grayscale rendition of color image with red and green pigments, and three CMYK enhanced colorchannels.

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NOMENCLATURE

We save files in RGB or gray modes, andtry to name them in such a way as to describethe enhancement used. Images are usuallysaved in best-quality JPEG, TIFF, or Photo-shop formats. For an original file namedsubject.jpg, an enhanced color image would benamed subjectLAB.jpg when Lab color spacewas used or subjectCMYK.jpg when the resultsof CMYK work are saved. Individual color-channel (grayscale) images for example,would be named:

subjectCyan.jpgsubjectMagenta.jpgsubjectYellow.jpgsubjectLABl.jpgsubjectLABa.jpgsubjectLABb.jpgsubjectLABred.jpg.etc.

A complex image with overlays might benamed:

subject_ovYellow.jpg (overlay mode)subjectCMYKslLABb.jpg (soft light mode).

In all cases enhanced images are identifiedas enhanced, and unenhanced images aresaved for reference, comparison, and futureenhancements. We do not use paint or erasertools, or drawn selections to alter images. Itmay be desirable to place enhancement-information text into the image to preventfuture misinterpretations.

CONCLUSIONS

Algorithmic digital image enhancement isa powerful tool in rock art recording andanalysis. It in effect extends our senses andmakes additional data available for study.Adobe Photoshop™ can be used to implement

Figure 8. Grayscale renditions of two color images. The second image is generated by pasting the LABa enhanced(grayscale) image over the original color image, with a Soft Light blending mode in the Layers palette.


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many different methods of manipulatingimages in different color spaces, which areapplicable in various situations. Color illustra-tions would further demonstrate enhance-ment techniques available in this program.

Acknowledgments. We sincerely thank theTexas Parks and Wildlife Department for sup-porting the Hueco Tanks State Historical Parkrock art mapping and baseline study wheremany of the original techniques were devel-oped, and the National Park Service at Canyonde Chelly National Monument (Rock ImageCondition Assessment and DocumentationProject) where some of the advanced tech-niques have been utilized.

REFERENCES CITED

Adobe Systems Inc.2000 Adobe Photoshop 6.0 Users Guide, pp. 109–114.

San Jose, California.

Buchner, A. P., S. Hathout, and B. Russell2000 Digital Enhancement Of A Prehistoric Rock

Painting From Hazlet, Saskatchewan. In 1999International Rock Art Congress Proceedings, vol. 1,edited by P. Whitehead and L. Loendorf, pp. 19–24.American Rock Art Research Association, Tucson,Arizona.

Harry, K. G., E. Billo, and R. Mark2001 The Challenge of Long-Term Preservation:

Managing Impacts to Rock Art at Hueco Tanks StateHistorical Park. In American Indian Rock Art, vol. 27,edited by S. M. Freers and A. Woody, pp. 151–159.American Rock Art Research Association, Tucson,Arizona.

Mark, R., and E. Billo1999 A Stitch in Time: Digital Panoramas and

Mosaics. In American Indian Rock Art, vol. 25, editedby S. M. Freers, pp. 155–168. American Rock ArtResearch Association, Tucson, Arizona.

2001 Digital Image Enhancement Techniques inRock Art Recording. Paper presented at the 66thannual Meeting of the Society for American Archae-ology, New Orleans, Louisiana.

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