Initial Tests and Analysis of the NIST 

Proposed Congruent Matching Cells 

(CMC) Method using Fired Cartridge 

Cases from Consecutively Manufactured 

Pistol Slides

W. Chu, M. Tong, J. Soons, J. Song, 

National Institute of Standards and Technology (NIST)

Gaithersburg, MD 20899

Forensic@NIST 2012

Gaithersburg, MD, Nov 28-30, 2012
Disclaimer

Certain commercial equipment is identified in this

article. Such identification does not imply

recommendation or endorsement by National

Institute of Standards and Technology (NIST), nor

does it imply that the equipment are necessarily the

best available for the purpose

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Thomas Fadul from the Miami Dade Crime Laboratory

initiated a consecutively manufactured slide study.

40 cartridge cases fired from guns with 10 consecutively

manufactured pistol slides are correlated:

• 20 known cartridge cases were fired from the 10 slides

(2 per slide).

• 15 unknown casings and 5 more persistence casings

Material

Motivation

Test the Congruent Matching Cells (CMC) method

proposed by NIST for the FMC2012 project entitled

“Establish the National Ballistics Evidence Search Engine

(NBESE) Based on 3D Topography Measurements on

Correlation Cells.”
Instrument

Nipkow Disk Confocal Microscope

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Data collection and preprocessing

Parameters for data collection:

• Z direction step size: 0.2 μm

• Lateral Resolution: 3.125 μm

• Measured Dimension: 4.8 mm x 4.8 mm

Trim Gaussian 

filter
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Previous test and results using CCF method

Approach: Cross Correlation Function CCF
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Previous test results

Persistence Casing S was able to link Unknown Casing R back to the

correct slide.

 

All ten pairs of KM cases are correctly identified each

other. 19 out of the 20 unknown casings were correctly

identified back to the slide they came from.

Unknown Casing R was deemed non-matching. 

Correlation CCF%

Unknown Casing R  Known Casing 7A 0.5389 (Missed)

Unknown Casing R  Known Casing 7B 0.5448 (Missed)

Unknown Casing R  Unknown Casing Y 0.5972 (Missed)

Unknown Casing R  Unknown Persistence Casing S 0.7125 (Identified)
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Initial tests using the Congruent Matching 

Cells (CMC) method: Correlation scheme

A B

Cell size: 75 pixel x 75 pixel (0.47 0.47) mm2

Total cell number: 7 x 7 or 6 x 6

Rotate image B, find the best matched patch with each cell in A

Rotation range: 30 , 

X- and Y- displacement search range: whole image 
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Control set correlation results

Data input: 20 known matching breech face images (10

pairs)

Total comparisons: 190 (20 x 19/2)

Matching comparisons: 10

Nonmatching comparisons: 180

Removing those cells that do not 

include sufficient valid data points, 

valid correlation cell numbers are:

• 279 cells in total for 10 matching 

comparisons

• 4737 cells in total for 180 matching 

comparisons

Invalid cells for correlation
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Distribution of CCF value of the 

correlated cell pairs
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Statistical distribution after normalization

Angle
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Statistical distribution after normalization

Cell index Cell index
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Statistical distribution after normalization

X-Y Displacement
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For X-displacement, 217 of total 279 results (77.8%) distribute in 

range of ± 15 pixel

For Y-displacement, 227 of total 279 results (81.4%) distribute in 

range of ± 16 pixel

Cell index Cell index

Cell index

Cell index
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Only 12% of these cells have their angle distributed in the range of ± 3°

(compared with cells in matching set: 86%)

12.6 % and 12.2 % have their x- and y-displacement distributed in the 

range of ± 15 pixel respectively

(compared with cells in matching set: 78% and 81%)

Non-matching

Cell index

Distribution of rotation angle
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Selection of threshold parameters

Difference of KM Minimum and KNM Maximum 

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KM Samples’ Minimum CMC (CCFlow = 50%) KNM Samples’ Maximum CMC (CCFlow = 50%)

KM

Min.

CMC

KNM

Max.

CMC

TX and TY (pixel) Tθ

(°) TX and TY (pixel) Tθ

(°)

TX and TY (pixel) Tθ

(°)

Dif.

KMmin − KNMmax = 

12 − 3 = 9 

CMC Distribution 

Difference

(KM

Min.

-

KNM

Max

CMC)

Number of matching cells 

Frequency
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63 comparisons in total between two actually matched

casings, no missed identification

24 -------------> 1, 2

23, 29 -------------> 3, 4

21, 35, 40 -------------> 5, 6

22 -------------> 7, 8

25, 37 -------------> 9, 10

26, 38 -------------> 11, 12

32, 34, 39 -------------> 13, 14

30, 31, 33 -------------> 15, 16

27 -------------> 17, 18

28, 36 -------------> 19, 20

Thresholds selected at 

CCF: 60%;

Angle: 6 ;

x- and y-displacement: 30 pixel

Test results for all 40 casings
Distribution of matched cell number

• A total of 780 

correlations with 63 KM 

and 717 KNM 

correlations.

• No false pos. & neg. 

identifications, with C =

6.

• To be improved.

Non-match

Match

C = 6

N = (7 × 7) cells

n = (0.4 7× 0.47) mm2

Pixel spacing: 6.25 μm

CCFlow = 60 %

Tθ = 6o

Tx & Ty = 0.19 mm 

Number of matching cells 

Frequency
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Conclusion and Discussion

1. The use of the CMC method represents the highest

identification accuracy for the same set of cartridge

cases that we have tested thus far.

2. Optimizing parameter such as cell size, image

processing procedure, parameter thresholds CCFlow, Tθ,

Tx, Tyfor the proposed CMC method improves the

separation between the distributions for the matching

and non-matching set of data.

3. The preliminarily recommendation of the CMC criterion: 

“C = 6” Minimum number of 6 valid cells for a positive

match. 
Acknowledgements

NIST Office of Law Enforcement :

Susan Ballou

Robert Thompson

NIST Surface & Nanostructure Metrology Group:

Richard Silver

Theodore Vorburger

Alan Zheng

Brian Renegar

NIST Statistical Engineering Division:

James Yen