Bubble Sheet Multiple Choice Scanner

Bubble sheet multiple choice scanner example with OpenCV Java (opencv-3.4.0). Not ready for production usage yet.

Install

Download OpenCV from official site. Then add library to project and set VM options as following.

// native library path 

-Djava.library.path=/opencv/build/lib

Intellij

File > Project Structure (Ctrl + Alt + Shift + S) > Libraries > + (Alt + Insert) > Select OpenCV jar file 

Run/Debug Configuration -> Application -> VM options

Steps

• Dilate source image for better recognition
• Transform to Grayscale format
• Threshold operation (for recognizing mask/conjuction with bitwise_and)
• Blur filter
• Canny edge algorithm
• Adaptive Thresh (for find main wrapper rectangle & bubbles)
• Recognize main wrapper rectangle according to hierarchy
• Find bubbles with estimated ratio (~17/15.5)
• Sort bubbles by coordinate points
• Recognize which option is filled or empty with bitwise_and and countNonZero

Sources

• Pdf - Bubble Sheet Form
• Inputs - Example Sheet 1 & 2
• Outputs for Sheet 1 & 2

Running

Run the "main" method of Main class.

    public static void main(String[] args) throws Exception {

        sout("...started");

    (1) Mat source = Imgcodecs.imread(getSource("sheet-1.jpg"));

        Scanner scanner = new Scanner(source, 20);
    (2) scanner.setLogging(true);
        scanner.scan();

        sout("...finished");
    }

(1) change source file name

(2) if logging is

• enabled, you can see processing flow and some detailed logs.

• disabled, you can see only output/result file.

Output (for sheet-2)

...started
*************************************
*************************************
answer is ....
*************************************
*************************************
1. A
2. D
3. B
4. EMPTY/INVALID
5. D
6. A
7. D
8. C
9. A
10. EMPTY/INVALID
11. B
12. A
13. D
14. EMPTY/INVALID
15. B
16. EMPTY/INVALID
17. EMPTY/INVALID
18. C
19. EMPTY/INVALID
20. D
...finished