Yellow Printer Dots -- using Video ToolBox Ultra software for enhanced viewing, analysis and document comparison

Summary

Yellow printer dots can be difficult to detect using normal RGB color space. Using the advanced color space tools in Video ToolBox Ultra, these yellow dots are much easier to see.  A pdf download of this blog posting is available at:Yellow Printer Dots and Video ToolBox Ultra 

Standard RGB color	CIE Lab Color (channel a)

You can quickly compare the dot patterns on different documents or pages using the live image overlay tools of Video ToolBox Ultra, and additional tools are helpful if you are trying to decode the yellow dot patterns using available online tools.

Adjustable sized grid makes it easier to identify and analyze yellow dots	Comparison between yellow dots on two different documents. Dots are recolored to red on first document and blue on the second one for easier comparison.

A video demonstration of a workflow using Video ToolBox Ultra to examine yellow printer dots can be seen at: https://vimeo.com/1204266160 . If you are already using Video ToolBox Ultra software then you can jump directly to the workflow instructions is available at: Yellow Printer Dots - workflow.

Outline

1. Introduction

2. Detection

3. Document Comparison

4. Analysis

5. Examples of Yellow Dot Patterns

6. Workflow Design -- Configuring Video ToolBox Ultra

   
   

Introduction

Tiny yellow dots are often used by advanced color printers to encode information about the printer being used, and can include additional information such as the time and date the document was created . These dots are barely visible to the human eye unless viewed at a higher magnification The image above was taken at 40X. At lower magnifications than that, these dots become more difficult to detect and analyze.

In the image below, taken at 11X, the dots (100 - 120 microns in diameter) are barely detectable.

The reason that tiny yellow dots are so difficult to detect has to do with the way our eyes detect the color yellow. Our eye breaks colors into three channels -- red, green and blue -- using three different types of cells called cone cells. Each cone cell is sensitive to either red, green or blue light. Yellow sits opposite to blue on the color wheel -- that is yellow represents an absence of blue. The number of blue-sensitive cone cells in our eyes are only about 5% of the total, which means that we see blue and yellow objects with very low resolution. If the brightness of the yellow dots matches the brightness of the white paper, they will practically disappear onto the page.

Detection

To make the printer dots easier to see, we want to switch color spaces to increase the contrast between the yellow dots and the white paper background. One good starting option is to use only the blue channel from an RGB (red, green, blue) color image. Since yellow represents the lack of blue, yellow dots should appear dark in the blue color channel.

The blue channel of figure (3) above provides an improvement over the raw color image (in figure (2)), but there are other color spaces (available in Video ToolBox Ultra) that make the dots more visible for analysis.

Saturation from the HSV color space is another good color space choice. The yellow dots are highly saturated compared to paper white and black printed letters, so they stand out in saturation more clearly than just the blue channel, as shown here:

Another color space that produces good contrast for printer dots is the CIE Lab space, channel a -- seen below:

With some contrast adjustment, the printer dots stand out sharply against the background.

Note: Use of alternate wavelengths of light, such as blue or ultraviolet light, can also improve the visibility of yellow printer dots.  However, we find that switching to an alternative color space with white alieviates the need to use more specialized light sources.  The two options can be used in tandem.

Document Comparison

The image overlay tools in Video ToolBox Ultra software make it easy to compare the yellow dot patterns from different documents. You can perform this comparison to determine if different documents or even different pages within the same document have been created on the same printer.

Consider this printed document shown below. This document contains a yellow dot pattern

Figure (6) Yellow dots in white light	Figure (7) Same yellow dots from Figure (6), shown in CIE Lab color space (channel a).

that is more easily seen in CIE Lab color space (viewing channel a).

Someone examining documents might be interested in comparing the printer dots from different pages of a multi-page document, to ascertain if they were all printed on a common printer. Or an examiner might compare different documents to look for evidence that they were created on the same printer.

Video ToolBox Ultra has a range of tools that simplify this task of image comparison. Consider these two printer dot samples (displayed in CIE Lab channel a):

Figure (8) Yellow dots on sample #1 (CIE Lab Color channel a)	Figure (9) Yellow dots on sample #2 (CIE Lab color channel a)

From a quick look, these dot patterns appear different, but Video ToolBox Ultra offers tools to speed up your workflow when comparing documents live under your camera or microscope.

Overlay tool lets you display an image as a transparent overlay on the live image. You can move the live sample, or you can shift and rotate the overlay to align it with your live image.

Here we overlay Figure (8) and Figure (9) one the same screen:

With all the dots displayed as the same color, it is hard to distinguish which dots are from which document. Video ToolBox Ultra has overlay options to display the documents in different colors, with added annotations to help distinguish between them.

In this above example, we take the same two sample images but place the live image into the red color channel and the reference image into the blue color channel. This produces dark dots for one document, and lavender dots for the other. We then add annotations in yellow to make the dot pattern easier to distinguish.

Once an overlay is created, you can check other documents by quickly sliding them under the camera or microscope system. The Video ToolBox Ultra software will display a live, full speed video image without video lag or motion blur.

Analysis

Analysis of yellow dot patterns is complicated by the fact that there is no official interpretation of the printer dot patterns. Different online sights suggest that document information such as printed date and the printer serial number can be extracted from the pattern, but there is no way to confirm if these interpretations are correct. Furthermore, many yellow dot patterns do not match the basic pattern described on these web sights. Extreme care should be given in attempting to interpret yellow dot patterns, beyond determining if the patterns from different documents match completely, or partially.

For analyzing the "standard" yellow dot patterns, Video ToolBox Ultra has a number of added tools to help view the dot patterns. These tools include measurement tools, adjustable grids and live onscreen markers.

For example,

this figure shows a yellow dot pattern from a typical laser-printed document. Video ToolBox Ultra has tools that make it easier to isolate the repeating yellow dot pattern for further analysis.

In figure (13) above we use the measurement crosshair cursor tool to position a rectangle around the section of repeating yellow dot pattern. This makes it easier to evaluate the pattern or to overlay a different document to see if the dots match.

Video ToolBox Ultra will also let you place a user-configurable grid as an overlay on the live image.

In figure (14) above, the crosshair cursor is re-drawn as a 15 x 8 grid -- which matches perfectly with the spacing of the yellow dots on this document.

These overlays are displayed on the live image so that they can be used while working with multiple documents, or for comparison and documentation.

Examples of Yellow Dot Patterns

There are many different patterns of yellow printer dots which are found on a range of documents -- from official identity documents such as US driver's licenses, to common office laser printers to high-end printers for plaques and certificates. Here are some samples that we have observed. Note that the descriptions are ours, and are not in common use.

15 x 8 Grid

The most commonly described yellow dot pattern on internet sites is the 15 x 8 grid pattern as shown here (with an added grid for viewing) :

There are many online sights that claim they can decode the printer information encoded within this pattern. Be careful, as there is no definitive method to confirm the validity of the decoded information.

Two dot landmark

We have observed some printers that create a yellow dot pattern that incorporates a pair of closely spaced dots which can be used as a landmark to locate the repeating pattern.

The two-dot landmarks can be used to align the patterns from different pages to compare different documents using the live overlay feature.

Embedded, extra wide dot pattern

An unusual printer of official documents and certificates, prints a fine resolution yellow halftone pattern that obscures the yellow dot pattern.

When we zoom back, switch to CIE Lab color space, channel a, and stitch multiple images together, we can see the repeating dot pattern within the hallfone.

Non-rectangular dot pattern

We found this yellow dot pattern on the literature of OVD Kinegram, a document security company. The yellow dot pattern from their printer is atypical.

This pattern does not follow a rectangular grid but prints the dots in a wavy pattern. The dimensions of the repeating pattern are smaller than most others we have seen, indicating that potentially less information is stored. It is highly unlikely that documents from this printer would be confused with those from other devices we have reviewed.