Angular Correction of Forensic Photographs
Written by Arthur H. Borchers   

IT HAS LONG BEEN STRESSED that forensic photographers take close-up evidence photos with a scale perpendicular to the object or surface. The reason for that emphasis is that you may need to make precise measurements either of the object itself or the object in relation to something nearby. You rarely remember to take every necessary measurement while processing an active crime scene. The ability to take critical measurements from your photographs after the fact is highly valuable insurance. When the camera-to-subject angle is not perpendicular, the effects of perspective come into play. Edges are no longer parallel. Squares do not have four 90° corners, they become trapezoidal in shape. The following information is presented to allow you to confirm your photographs are perpendicular to the image plane. If the photographs are not at right angles, we explain here how to modify them.

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There are many different designs of scales, from small to large, but the most useful for evidence photography purposes is the ABFO No. 2 Photomacrographic Scale—or similar variants that provide scales in two directions at right angles to each other. The ABFO No. 2 provides millimeter scales demarcated into five one-centimeter sections also extending to the ends of the legs; a photo-gray reference; and three circles also included are useful in confirming perpendicularity. ABFO-style scales are produced by multiple manufacturers in both metric and imperial gradations; in varying colors; on metal, plastic, magnetic, and paper substrates; and in a variety of sizes, including some capable of footwear documentation or larger. A 50mm macro lens or longer on a full-frame image sensor is necessary to capture images without circular distortion found with wide-angle lenses. The macro feature is important to enable filling the image with as much detail as possible for future enlargement.

An immediate task is confirming your photographs were in fact taken perpendicular to the subject. This is handled simply by importing your image into a CAD or photo-editing program where constrained circles are drawn over the circles on the scale (Figure 1). The consistent alignment of the red circles with the scale’s circles confirms perpendicularity. An image confirming this fact is saved with your investigative file.


Figure 1 – Perpendicularity confirmed with circles drawn over image circles.

The image below (Figure 2) was taken at a 10° off-angle and later imported into CAD and checked for perpendicularity. The green circle aligns with the top and bottom of the scale’s circle, but the gaps at the left and right sides of the scale’s black circle show the captured image is an ellipse and not round. This makes the image a candidate for perspective correction, also called rectification, procedure detailed below.


Figure 2 – Circle comparison displays elliptical image shape.

You will need a photo-editing program with a “perspective warp” or “transform” function to be able to modify your images to accurately correct for perpendicularity. The following dicsussion focuses on the use of Adobe Photoshop and the tools contained within to perform the required editing steps. Other editors including the free program GIMP (GNU Image Manipulation Program) are capable of performing similar modifications. As with all forensic image manipulation, careful notes of the changes should be maintained and kept on file.

This discusison will be a step-by-step journey from an unknown-angle photograph to a perpendicular photograph suitable for further examination and analysis.

The image in Figure 3 is our starting photograph. The first step is to open Photoshop and import a copy of the original image. Important: If you open the image and then save your project as a .PSD file, the original image file will remain intact and unmodified.


Figure 3 – Starting image.

When you have a photo imported, turn on Rulers (Menu Selections: View > Rulers, or Ctrl-R) and then right click on the displayed ruler and change the units to Pixels. Once this is done, you should never have to do it again.

Next, you will add a visual grid to the image to help you align edges (View > New Guide Layout). The dialog box allows you to have as many guides and patterns as you like. You want to roughly duplicate the shape of the object you are correcting the perspective of. In this case, the ABFO scale is close to a square in shape. The first information that you need is the size of your original photo in pixels. This can be found at the bottom of the Windows Explorer view when you click on the file, or by right-clicking on the file and viewing the properties. For example, the original size of Figure 3 is 5720 x 3840 px, which reduces to a ratio of exactly 3:2; this carries over from the 35mm film size. Other common image ratios are 4:3, frequently refered to as four thirds; 16:9 is a wide-screen format; and finally, you may be working with square images which would have an aspect ratio of 1:1. The number you choose to create your grid should be multiples of your aspect ratio. As shown, the 20 columns wide by 15 rows tall grid (Figure 4) will create medium-size squares.


Figure 4 – New Guide dialog and view.

Now you should align one edge of your image’s scale to the grid. To do that, you will need to rotate your entire image (Image > Image Rotation Arbitrary…). For Figure 5 shown, the amount of rotation is about 8° counter-clockwise.


Figure 5 – Rotate Image dialog.


Figure 6 – Rotate to align one edge.

With one edge aligned to the grid, as you can see in Figure 6, the right angle of the bottom left corner is visually incorrect. Your next step is to choose the Perspective Warp tool (Edit > Perspective Warp) and the cursor will visually change to a crosshair-like appearance. Click on the scale’s upper left corner, (Figure 7, #1). A warp grid layout will now appear with its toolbar at the top of the window. Drag the lower corners of the warp grid to the remaining corners of the scale (Figure 7, #2 & 3). When postioning the final corner of the warp grid, attention must be paid to aligning the edges of the warp grid to the ends of the scale legs so the lines are parallel to the projected edges (Figure 7 ¬– Arrows). As you can see in Figure 8, the remaining corner of the grid generally matches the projected lines in Figure 7. Once you have the warp grid positioned to your satisfaction, simply press the Enter key. The corners of the grid will change from light to dark.


Figure 7 – Perspective Warp to Alignment.


Figure 8 – Place Perspective Warp Grid.

The Warp toolbar (Figure 9) will change the highlight from “Layout” to “Warp”. With this change, you can manually drag the corners to adjust the warp, but the easiest way is to click on the hashtag-shaped section of toolbar (#). That tool will align your grid to vertical and horizontal axes—which is your goal, as you can see in Figure 10. If you need to move any of the corners of the warp layout, you will find that the Guide Alignment Grid will help you by snapping your movements to the corners of the alignment grid. Be sure that you maintain the squareness of the ABFO scale by ensuring the number of vertical and horizontal boxes are equal.

 


Figure 9 – Perspective Warp Toolbar.


Figure 10 – Perspective Warp Vertical and Horizontal Adjustment.

Your next step is to confirm the horizontal and vertical scales are equal. This is accomplished with the Measure Tool, found by right-clicking on the Eyedropper Tool along the left-side toolbar. When you select the Measure Tool, its toolbar appears above the image window. The length of your measurement is displayed in pixels as the “L1” value (“D1” in older versions of Photoshop). It is recommended for you to measure a longer distance, such as the 5 cm line, rather than a shorter one. The values for the two distances should agree within a pixel or two over several measurement attempts. Using the longer distance instead of a shorter one should account for any differences in starting and ending locations (Figure 12).


Figure 11 – Measure toolbar with length displayed in pixels.


Figure 12 – Measure and compare the horizontal and vertical scales.

If the measured values are not equal, the image scale or calibration can be easily adjusted. If, for example, your values are 952 pixels for the vertical distance and 970 pixels for the horizontal distance, your next step is to divide one value by the second value. If divided in the order given, the result is 0.981443. Should you divide 970 by 952, your result is 1.018907.


Figure 13 – Scale Transform toolbar.

To fix your image scale, select “Image Size” (Edit > Transform > Scale) and the Scale Adjustment toolbar appears above your work window (Figure 13). Click on the link icon between the Width (W) and Height (H) percentage values so any highlight is turned off. This will allow image modification in only one axis. Please note, the Photoshop terms for horizontal and vertical dimension are Width and Height.


Figure 14 – Scale Transform adjustment.

Using the values from above, convert the ratio between the horizontal and vertical axes (0.981443) to a percentage by shifting the decimal point two places to the right, giving you a value of 98.1443%. If you were to do the same conversion with the second value (1.018907), your result would be 101.8907%. In our example, we want to reduce the horizontal width to 98.1443% of its original value without affecting the vertical size. Enter 98.1443% in the “W” box and press tab. This should move your cursor to the “H” box and not change the value. If the value does change, click on the link icon to turn off the highlight and enter 100% in the “H” box and press tab again. Verify your values are still what you want, and then click the checkmark at the end of the toolbar to confirm your change. This will adjust the width only of your image to be approximately 1.8% smaller. Remeasure the pixel values of the scales to verify they are now reading the same or closely similar values.

Your next step will be to adjust the image size (Image > Image Size) to adjust your ability to print to scale.


Figure 15 – Image Size dialog.

Your first changes to the dialog box will be to uncheck the “Resample” box, then click on the link icon to the left of the Width and Height values and change the resolution scale to reflect the actual scale of your photographed measuring tool to either “Pixels/Inch” or “Pixels/Centimeter”. Next, divide the pixel measurement you took earlier by the number of inch or centimeter segments and enter the value into the Resolution box. The Width and Height values will update as you type, but visually you will not see any change to your image when you click OK. When you print your file, however, it will print to the actual size of your pictured scale, allowing you to take direct measurements from objects.


Figure 16 – Image Size adjustment.

Your final step is the same as the first. You will need to verify the circles in your scale are in fact circular. This can be done in Photoshop using the Ellipse Tool near the bottom of the left toolbar. When you select the Ellipse, click on the gear icon in the toolbar (Figure 17) and change the setting to match those below (Figure 18). When you draw your Ellipse—now, more accurately, a circle—click on the intersecting lines in the center of the reference circle (Figure 19) and draw out to match the diameter. If the drawn circles accurately match the reference circles, you are finished. Print a file copy and continue your case work. With practice, this process takes little time to accomplish and goes a long way toward showing that your work is accurate.


Figure 17 – Elipse toolbar.


Figure 18 – Elipse Tool settings.


Figure 19 – Perpendicularity confirmed by circles.

The images in Figures 20 – 23 depict actual image edits as examples.


Figure 20 – Big Ben file photo, before.



Figure 21 – Big Ben file photo, corrected.


Figure 22 – Bullet hole entry to wall, as photographed.


Figure 23 – Perspective-corrected bullet hole, to scale.

Note: Tammy Koelling of Words Matter Publishing obtained Figures 20 & 21 for publication through her account at depositphotos.com. All other photos are the property of Art Borchers.


About the Author

Arthur Borchers is currently an adjunct instructor for the Suburban Law Enforcement Academy at the College of DuPage and a Forensic Consultant with Larsen Forensics & Associates, both in Glen Ellyn, Illinois. He has advanced training and experience in photography, photogrammetry, firearms, shooting incident, and crime scene and traffic crash reconstruction. Borchers retired from the Oak Park (Illinois) Police Department in 2013.


Note: This article is an excerpt of the forthcoming book by Sanford Weiss, Forensic Photography for the Preservation of Evidence. An update to Weiss’s 2008 Forensic Photography: The Importance of Accuracy, the new book includes work from a selection of the foremost forensic experts from around the world. It is projected to be released toward the end of this year. Please watch for it, or write This e-mail address is being protected from spam bots, you need JavaScript enabled to view it to be put on a mailing list for current details.

References

Bowers C.M., Johansen RJ, "Photographic Evidence Protocol: the Use of Digital Imaging Methods to Rectify Angular Distortion and Create Life Size Reproductions of Bite Mark Evidence", Journal of Forensic Science 2002;47(1): 178-185

Pierce, Guy. “Resizing Digital Images to Actual Size (1:1) Using Adobe® Photoshop®.” Journal of the Association for Crime Scene Reconstruction, Volume 15, Issue 1, Spring 2009: 13-16.

 

 

This article appeared in the Spring 2019 issue of Evidence Technology Magazine.
Click here to read the full issue.

 
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