Tests using CMC Method for Optical Image 

Correlations of Cartridge Cases Fired from 

Consecutively Manufactured Pistol Slides

Mingsi Tong, John Song, Wei Chu, Johannes Soons, and 

Robert M. Thompson 

National Institute of Standards and Technology

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Disclaimer

Certain commercial equipment, 

instruments, or materials are identified 

in this report in order to specify the 

experimental procedure adequately. 

Such identification is not intended to 

imply recommendation or endorsement 

by the National Institute of Standards 

and Technology, nor is it intended to 

imply that the materials or equipment 

identified are necessarily the best 

available for the purpose.

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1. Test Samples

The test samples are from a study by the Miami 

Dade Crime Laboratory. 40 cartridge cases fired 

from handguns with the 10 consecutively 

manufactured slides were correlated with one 

another. That includes 20 known cartridge cases 

(2 per slide) for references and 20 unknown 

cartridge cases for the validation test. There are 

a total of 780 pair combinations; comprising 63 

known-matching (KM) and 717 known-nonmatching (KNM). The topography images of 

these samples had been tested by NIST. 

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Test Samples (continued)

The optical intensity images of 40 cases were 

captured by Leica comparison microscope using 

top-ring light. In this experiment, we only 

measured and compared the breech face mark. 

The other areas on the images were trimmed. 

The final image size is (700 × 700) pixel with 

estimated pixel spacing 4.95 μm. A low-pass 

Gaussian filter (short cut-off is 26 μm) was 

applied to remove the high frequency 

components which is mainly noise.

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Test Samples (continued)

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(a) Breech Face Raw Data Image 

(b) Trimmed Image (c) The Image after the Gaussian Filter

Fig. 1 Image Processing
2. Implement of CMC Method

CMC Method

To reduce the effect of correlation from the invalid 

correlation area, the CMC method divides the image data 

into correlation cells. In this experiment, the cell size is 

set as (100 × 100) pixel or (0.49 × 0.49) mm2. The 

resulted number of cells is (7 × 7). 

For a pair of images A and B, every valid cell divided from 

A is correlate to every rotated image B. The rotation 

range is ±30° with 3° increments. Each correlation cell 

in image A scans the whole area of each rotated 

reference image B to find the best matching position. 

Once the procedure is completed, the similarity metrics 

consisting of the CCFmax value, the registration angle θ

and the translation distances in x and y are recorded.

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Implementation of CMC Method, cont.

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Case-A Case-B

Fig.2 Correlation scheme using the CMC method
Fast CMC Method

Cell correlations against the whole reference image are 

time consuming. To shorten exam time, a fast correlation 

method was proposed. In most modern handguns, the 

barrel, bullet, cartridge case and slide are concentric (see 

Fig. 3). Thus the matching patterns from a pair of 

matching cases will appear in the same relative position in 

each breech face image.

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Fig.3 Barrel, breech face (cartridge cases) and 

slide are concentric 
Fast CMC Method

• Observer is in Reference position, Carousel is being 

compared

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Fast CMC Method

With this feature, the whole reference area from image B 

could be reduced to a small “reference area” (see the red 

dotted areas in Fig. 4 right) which is little bigger than the 

cell from image A. The central position of the reference area 

is the same as the corresponding cell position of the image 

A (cell A2 in Fig. 4 left). The reference area keeps stationary, 

and independent of image B’s rotation during the 

correlation process. The correlation speed can now be 

increased ten times.

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A1 A2 A3 A4

A5 A6 A7 A8

A9 A10 A11 A12

A13 A14 A15 A16

B2

Independent of

Image B rotation

A2, B2 matched in

the same position

Fig.4 Correlation scheme using the quick CMC method
Determination of the CMC Numbers

The 20 known cartridge cases were correlated first and statistically 

analyzed. Excluding those cell pairs that do not contain sufficient 

data points, the correlations were implemented for 266 cell pairs of 10 

KM correlations and 4794 cell pairs of 180 KNM correlations. The 

CCFmax distribution histograms for all cell pairs are shown in fig. 5. To 

allow the most KM and KNM cell to participate in the determination of 

the CMC numbers, the CCFlow = 25% was selected.

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Fig. 5 Correlation scheme 

using the quick CMC 

method
Determination of the CMC Numbers

Based on the CMC definition, if all the three 

identification parameters of the correlated cell 

pairs CCFmax, θ and x, y show positive results with 

three thresholds (CCFlow, TƟ and Tx, y), these 

correlated cell pairs are considered congruent 

matching cell pairs. 

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Fig. 5 Correlation 

scheme using the quick 

CMC method
3. Analysis of Correlation Results

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Based on the CMC definition, a program was made

which can search the difference between the min 

CMC number of KM group and the max CMC 

number of KNM group in all different TƟ and Tx, y

combination with a given CCFlow value. 
 Analysis of Correlation Results

For the 20 known cartridge cases, Fig. 6 shows the 

distribution of the CMC differences for the selected 190 

correlations with CCFlow = 25%. The CMC difference 

achieves the maximum as shown in the area of white frame 

(see Fig. 6). By selecting the thresholds in this area (TƟ = 

3°, Tx, y = 25 pixel), Fig. 7 shows the CMC distribution 

based on these thresholds. 

The KM and KNM distributions are well separated. The 

maximum CMC number of the 180 KNM correlations is 5, 

and the minimum CMC number for the 10 KM correlations 

is 12. There is a gap of 6 CMCs, which indicates no 

misidentification or missed identification.

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Cell size: (100 x 100) pixel2

Number of cells: 7 x 7

CCFlow = 25%

Tθ = 3°

Tx, y = 25 pixel

C = 6

Analysis of Correlation Results

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Fig. 6 Difference of KM Minimum and KNM 

Maximum

(190 correlations; CCFlow = 25%) Fig. 7 CMC Distribution

(190 correlations; top-ring light)

 

Tx, y (pixel)

TƟ (degree)
Analysis of Correlation Results

• The same processing was now conducted on all 

of the 40 cartridge cases. The difference 

distribution of 63 KM and the 717 KNM is shown 

in Fig. 8. The max difference is -1. It means there 

has been a misidentification or missed 

identification. 

• The max CMC difference area of 780 correlations 

(see Fig. 8) is the same as the max CMC 

difference area of 190 correlations (see Fig. 6). By 

using the same thresholds, the CMC distribution 

of 780 correlations is showed in Fig. 9. In this 

result, the distribution of KNM doesn’t have an 

obvious change. 

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Analysis of Correlation Results 

 

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Tx, y (pixel)

TƟ (degree)

C = 6

Cell size: (100 x 100) pixel2

Number of cells: 7 x 7

CCFlow = 25%

Tθ = 3°

Tx, y = 25 pixel

2 1

Fig. 8 Difference of KM Minimum and KNM 

Maximum

(780 correlations; CCFlow = 25%)

Fig. 9 CMC Distribution

(780 correlations; top-ring light)
4. Correlation Analysis by Angle

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(a) Cell-A (b) Cell-B (c) CCF distribution

Fig. 10 Correlation Processing of KM Cell Correlation

Fig. 11 CCF Distribution of KNM Cell 

Correlation

To further analyze the “misidentification” and the “CMC 

close to 6” results, the correlation processing of a typical 

pair of matching cells is shown in Fig. 10. 
Correlation Analysis by Angle (continued)

 

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(a) Forward Comparison (A vs. B)

(b) Reverse Comparison (B vs. A)

Fig. 12 CMC Distribution of a Typical KM by Angle

(a) Forward Comparison (A vs. B)

(b) Reverse Comparison (B vs. A)

Fig. 13 CMC Distribution of a Typical KNM by 

Angle
Correlation Analysis by Angle 

We propose two methods to increase the 

accuracy:

1) Using “High CMC Number” replace the max 

CMC number to find the matched angle. In KM 

correlations, if existing angles which CMC number 

is a little less than the maximum (less 1 or 2), 

these angles are usually close to the peak angle. 

But in KNM correlations, these angles are usually 

randomly distribute in the whole range.

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Correlation Analysis by Angle 

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AA vs. B BB vs. A 

Fig. 14 Forward and Reverse Correlations

2. Combining forward and reverse correlations (A vs. B, 

then B vs. A). According to the analysis, some random 

factors could make KNM correlations show a high CMC 

number in some angles. Making a “reverse” correlation 

means making two correlations with different 

parameters (cell size, cell direction) in same angle (see 

Fig. 14 below).
Result of Correlation Analysis by Angle

Based on the lower result of a pair of 

correlations (forward and reverse correlations) 

with the original CMC method, and using the 

assisted analysis results then an optimized 

result with no misidentification or missed 

identification is shown in Fig. 15 following. 

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Result of Correlation Analysis by Angle

Cell size: (100 x 100) pixel2

Number of cells: 7 x 7

CCFlow = 25%

Tθ = 3°

Tx, y = 25 pixel

High CMC Number: Max - 1

C = 6

Fig. 15 Optimized CMC Distribution

(780 correlations; top-ring light)
Side-light using controllable ring-light

Leica FS M with LED Ring-Lights Leica Microsystems

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5. Side Light Test and Result

Examiners typically use a side light when they are 

reviewing comparisons. The side light adds surface 

contrasts that in turn give a better sense of depth and 

texture of the primer surface.*

We made an initial test using the CMC method for side 

light images. Images from all 40 cases were captured 

using side light. The correlations gave much better 

results than ring light (see Fig. 16).

(* National Research Council of the National Academies, Ballistic 

Imaging:182)

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Side Light Test and Result

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Cell size: (100 x 100) pixel2

Number of cells: 7 x 7

CCFlow = 35%

Tθ = 3°

Tx, y = 15 pixel

High CMC Number: Max - 2

C = 6

Fig. 16 Optimized CMC Distribution 

(780 correlations; 3 o’clock side light)
Comparison of results, top ring light 

vs. side-light

Fig. 15 Optimized CMC Distribution

(780 correlations; top-ring light)

Fig. 16 Optimized CMC Distribution 

(780 correlations; 3 o’clock side light)

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6. Conclusions and Future Work 

The CMC method and the proposed numerical 

identification criterion (C ≥ 6) for ballistic 

identification were proven effective in threedimensional topography images of cartridge case 

by NIST. Results were obtained without any false 

positive or false negative identification. 

In the optical images of the same 40 samples test, 

one case was “Falsely excluded” using the 

original CMC method. This might indicate that 

topography data is still better suited for ballistics 

identification than optical images.

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Conclusions and Future Work

However, the proposed assisted analysis method 

based on features of CMC can effectively enhance 

this identification accuracy. The correlation 

results show a significant separation between the 

KM and KNM distributions without any false 

positive or false negative identification. As an 

assisted technology, initial tests show promise. 

Additionally, these improvements can also used 

for topography comparisons and identification.

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Conclusions and Future Work

In the initial test, the side light images 

demonstrate better results than ring light, and 

close to topography correlation results. 

However the side light approach lacks a unified 

illumination standard that may limit the side light 

applications in search systems. In our future 

work, more lighting variables will be tested and 

analyzed.

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Conclusions and Future Work

In our future plans, more impression “signatures” 

on cartridge case will be tested, including firing 

pin and ejector marks, using derivatives of the 

CMC method. 

We are also finalizing an error rate reporting 

method for ballistics identification based on the 

CMC method.

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Thanks for your attention!

Questions?

jun-feng.song@nist.gov

robert.m.thompson@nist.gov

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