Decoding the ISO 15415 Verification Report: Fixed Pattern Damage
After Modulation, discussed previously, Fixed Pattern Damage (FPD) is likely the next most prevalent problem with 2D barcodes. The “fixed pattern” referenced are the omnipresent locator targets on every symbol. On a QR Code they are those square boxes in three of the corners as well as some other features; on a Data Matrix code they are that characteristic solid line on two adjacent sides and the checkerboard pattern on the opposite adjacent sides. Quiet zones are also part of the fixed pattern on these symbologies. A minimum about of clear space is required on QR and Data Matrix symbols. Quiet zones are not required on some 2D symbologies.
What are “Fixed Patterns” and Why are they Important?
Fixed patterns accomplish two requirements. Fixed patterns contain the finder patterns that orient the scanner to the symbol, by defining what is North/South and what is East/West. This allows a 2D symbol to be mirror-imaged and yet still decode correctly. Fixed patters do not encode any variable data, but they make it possible for the scanner to find and decode the variable data in its correct sequence, from start to finish.
Fixed patterns also provide a clear surround area where the scanner can calibrate itself to the Reflectance Minimum and Reflectance Maximum values that comprise the symbol.
How is the Fixed Pattern Damage Parameter Evaluated?
View a video explanation of Datamatrix fixed patterns below:
A subset of attributes comprise the FPD parameter. On a Datamatrix Code these include:
- L1, the left side of the L track
- L2, the bottom of the L track
- The top Clock Track
- The right Clock Track
- Quiet Zones for Top, Bottom, Right and Left sides
Clock Track Regularity (CTR) is an additional attribute in the fixed pattern subset. CTR evaluates the regularity of the light and dark reflective elements in the clock track. CTR is a pass/fail attribute.
Transition Ratio (TR) is a graded evaluation of the “crispness” of the transition from dark to light in the clock track. Gradient edges are downgraded.
QR Code accomplishes orientation similarly, but with different features.
View a video explanation of QR Code fixed patterns
Here the fixed pattern consists of:
- Upper Left Position Detection Pattern
- Upper Right Position Detection Pattern
- Lower Left Position Detection Pattern
- Alignment Pattern(s)
- Horizontal Clock Track
- Vertical Clock Track
- Quiet Zone
Fixed Patterns in Datamatrix Code and QR Code are Different
QR Code, though similar to Datamatrix Code, differs in several ways.
- The quiet zones for a Datamatrix Code must be no less than one element (X dimension) on each of the four sides. The quiet zone for a QR Code must be no less than four elements (X dimension) per side. However, non-GS1 uses of QR Codes may have a quiet zone of one element.
- The QR Code parameter set for print quality includes Alignment Patterns for QR Version2 and above. These assist the scanner in compensating for distortion on curved surfaces.