Scientific proof of NEGA’s SVF (Smart ViewFinder) filter

In the field of forensic document analysis, determining the order of execution in crossed handwritten strokes is a key issue. To address this challenge, we have developed the SVF (Smart ViewFinder) digital filter within our NEGA forensic software, based on a high-precision scientific study conducted by the Department of Electronic Engineering at the University degli Studi “Roma Tre” (Italy).

This study, carried out using a confocal laser microscope, analyzes in three dimensions the deformation of paper caused by writing instruments. When writing or signing, the tip of the pen not only deposits ink, but also creates a groove whose depth depends on the pressure exerted. When two strokes cross, the upper stroke cuts into the groove of the lower one, causing a unique deformation: side walls or “bumps” that are filled with ink at the point of intersection, leaving a characteristic pattern.

You can view the complete study by the University degli Studi “Roma Tre” (Italy) here: 3D laser profilometry on writing strokes…

How to scan right to use the SVF filter

Important: Tests have shown that images obtained with digital cameras do not provide valid results for analysis with the SVF filter. This is because cameras do not capture the texture of the paper or the ink load with sufficient accuracy, due to several factors such as:

• The lighting conditions of the environment, which alter the light reflected on the paper and modify the colors captured..
• The possibility of blurring due to movement or vibration during shooting.
• The lack of uniformity in lighting, which can cause unwanted shadows or glare..

For this reason, we recommend using only optical desktop scanners, which guarantee uniform, accurate, and reproducible digitization.

Only images scanned with a desktop optical scanner provide conclusive results.

To ensure maximum fidelity when capturing handwritten strokes, follow these recommendations when digitizing your documents:

Recommended scanning parameters for the optical scanner

• Color depth: Select the maximum depth available (24 or 32 bit). This ensures that all the color information necessary to detect ink load is captured.
• Color mode: Use Color or RGB mode. Avoid “Grayscale” or “Black and White,” as these alter the original colors and make analysis difficult.
• Automatic adjustments: Disables any automatic adjustments to color, brightness, or contrast. These can modify the image and affect ink pattern detection.
• Optical resolution (without interpolation): Scan using only the actual resolution of the scanner. Interpolation can generate artifacts that distort details. Recommendation: 1200 dpi for maximum accuracy. Resolutions starting at 300 dpi, such as 600 dpi or 800 dpi, are also acceptable in less demanding cases.
• File format: Save the image in uncompressed TIFF format, which preserves all color and detail information. Avoid formats such as JPG or PNG, which apply compression and may lose data critical for analysis.

With these parameters, the SVF filter can operate at maximum efficiency, allowing for accurate detection of the execution order in crossed handwritten strokes. If you have questions about how to configure your scanner or need help preparing your documents, please don’t hesitate to contact us.

What does the SVF filter do?

The SVF filter is designed to digitally highlight the ink load pattern at intersections, allowing the top stroke, i.e., the one that was executed last, to be identified. Its effectiveness depends on several factors:

• The pressure exerted by the writing instrument.
• The type of ink (most effective with low-flow inks such as those used in BIC pens or similar).
• The quality of the document scan.
• The color of the ink analyzed.

Filter performance levels

To adapt to different types of ink, we have developed two levels of analysis, each with an independent slider to adjust its intensity depending on the image.:

Applied scientific methodology

To validate the SVF filter, we conducted an empirical analysis of handwritten stroke crossings, following this methodology:

• Writing instrument: BIC ballpoint pens, standard tip, blue and black ink.
• Support: 80 g/m² white paper.
• Test design: Crosses between 4 lines (2 horizontal and 2 vertical), analyzed in all possible permutations of writing sequences, which total 24. See the complete analysis and images of the test here: Empirical analyses using scientific methodology applied to the SVF digital filter… 
• Digitization:
  • • Scanner: EPSON XP-520 multifunction.
    • Resolution: 1200 dpi actual (not interpolated), to avoid alterations caused by the scanner software.
    • Color settings: Disabled, ensuring a faithful reproduction of the original document.
• Statistical treatment: These data allow us to evaluate the reliability of the method and establish acceptable margins of error.
  • • Percentage of correct sequence determinations.
    • Standard error of the mean.
    • Comparison between different types of ink and stroke direction.

Each sample was numbered to facilitate identification and subsequent analysis with the SVF filter in NEGA, ensuring the traceability of the experiment.

This analysis is part of a scientific methodology applied to the study of the sequence of strokes in handwritten documents, using the SVF (Smart ViewFinder) digital filter. This technique is based on physical, optical, and statistical principles that allow the interaction between writing instruments and paper to be evaluated, with the aim of determining the order in which the strokes were made.

SVF filter performance according to tests carried out

The following results show the effectiveness of the SVF filter in identifying stroke sequences, evaluated at different levels and with different ink colors (blue and black):

Interpretation of results

• Mean values close to 4 indicate high quality in stroke identification, especially in black ink with the SVF Level 2 filter.
• The low standard error suggests good consistency between the tests performed.
• The accuracy rate, all above 87%, demonstrates that the SVF filter is highly reliable under optimal scanning and ink contrast conditions.

Conclusive SVF filter cross-talk results

It is extremely important to know how to identify the conclusive patterns indicated by the filter, since, although the filter highlights the ink loads at the intersections to identify them, we will have to test examples where we know the result in advance to train our predictions. Here are some examples with useful clarifications according to the filter level..

SVF Level 1

As already mentioned, this level is specially designed for blue inks because conclusive results cannot be obtained with black ink.

Conclusive ink patterns for level 1 would be:

Example 4

SVF Level 1 Blue ink.

The ink pattern indicates that the vertical line is the over one.

SVF Level 2

This level is designed for black inks, but it can also be used for blue inks, as demonstrated in the tests carried out.

Conclusive ink patterns for level 2 would be:

Example 4

SVF Level 2 Black ink.

The ink pattern indicates that the horizontal line is the over one.

SVF filter (CONS)

• No tests have been conducted with red and green inks: Although these colors are less common in signatures and writing, they cannot be ruled out. Currently, there is no conclusive data on their behavior with the SVF filter. The possibility of including more filter levels or adapting the technique to evaluate its effectiveness with these colors in subsequent investigations is being studied.
• Not suitable for images taken with digital cameras: Light variations, blurring, and lack of precision in the capture negatively affect the analysis..
• Dependence on scan quality: Requires high-resolution, uncompressed optical scans to function properly..
• Sensitivity to paper texture: Papers with irregular or very rough textures may interfere with filter reading..
• Not designed for photocopies: On original documents from traditional or digital photocopiers (laser, inkjet), the filter may not provide conclusive results.
• It depends on the stroke depth on the paper: its effectiveness decreases in writing instruments that do not require as much pressure to write, such as markers and fountain pens, for example.

Final conclusion

The results obtained by applying the SVF filter demonstrate that it is a reliable, non-invasive, and highly effective tool for analyzing stroke sequences in handwritten documents. Its ability to preserve the integrity of the sample and provide categorical results (“conclusive” or “non-conclusive”) makes it a particularly valuable technique in the forensic and documentary fields. The statistical tests carried out show a high percentage of correct results, reaching up to 91.7% positive identification with black ink at Level 2 of the filter, and with low standard errors, confirming the consistency of the method. These data support its application in contexts where accuracy and evidence preservation are critical. Although no tests have been conducted with red and green inks, as these colors are less common in signatures and writing, their use is not ruled out. In fact, the possibility of incorporating new filter levels or adapting the technique to obtain conclusive results with these colors in future studies is being evaluated. In summary, the SVF filter is positioned as a robust, accurate, and complementary tool, capable of providing real value in the identification of overlapping strokes, provided it is used under optimal scanning conditions and with previously validated inks. Although it is a powerful tool, if inconclusive results are obtained, it is advisable to use it as part of a set of techniques, not as the sole evaluation criterion.