How Scanners work

Line scanner cameras

All current scanners use a line scanner.  A line scanner is a special camera that scans a single line, one pixel wide.  To capture a larger picture, the camera is moved over surface while continuously capturing individual lines.  When the scan lines are combined, this produces the larger image.

Line scanners are typically used in machine vision applications.   When the camera is combined with the correct lens, and the camera is properly aligned, the majority but not all of the distortion along the scan line can be accommodated by the lens.

Slab scanners use a combination of two line scanners to produce the final image.   One mounted directly over the other.  The field of view of the cameras overlaps in the middle.  When a scan is completed, there are two separate images, one from each camera.  These images need to be “Stiched” together to produce a single final image.

Stitching and Parallax

This diagram illustrates the general layout of a scanner.   The field of view of each camera overlaps at the center.  This is the “Stitching Area” where the two independent images must be merged for the final image of the entire line scan.

Stitching

There are various methods of stitching the images at the overlap area of the cameras.  One method is to look for similarly colored pixels from the bottom area of the top camera and the top area of the bottom camera.  When similar pixels are found the images are moved together and blended in the overlap, based on the similar pixels.   This works reasonably well, but can fail if there is not enough color information at the overlap (for instance if the overlap area is a uniform color).

This is not the method used on the Stone Vision scanner.  The Stone Vision scanner implements a calibration bar that is imaged on every scan.  The bar has a series of black dots that are a know distance apart.  Based on a known thickness of slab, this calibration bar  provides a mathematical method to determine the correct stitching line between the images.  This works regardless of having enough detail to graphically stitch the images.  More detail on the calibration bar and how it works is discussed below.

At the optical centerline of each camera, the thickness of the slab is not a issue.  However as you progress toward the edge of the camera’s field of view, the thickness must be allowed for.   If you look at the parallax detail in the diagram, you will see that at the edge of the field of view, there is a significant difference between the position of the from the camera’s view between the top and bottom of the slab.  As previously noted, a good camera and lens with a “Flat field correction” will do a reasonable job of correcting for this difference.  This explains why it’s important to specify the correct thickness when scanning a slab to arrive at accurate dimensions on the final image.

The Stone Vision scanner calibraiton bar has two depths, one at 6mm and one at 30mm.  Each scan is auto-corrected for the parallax distortion at each of these depths, then interpolated between them for any thickness scanned between 6mm and 30mm.  This allows for any additional corrections needed beyond what the camera and lens are inherently capable of.

Slabsmith integration

The Start

In late 2024 Slabsmith was approached by the USA Helios representative “Total Stone Solutions”, to guage a potential collaboration to create the next generation of slab scanners.  SLAB VISION is the result.  

Slabsmith was first released in 2005 and has been constantly improved for over 20 years.   Slabsmith is arguably the reason that creating digital slabs and doing digital layouts is now considered standard fare in the stone industry.

Development Goals

Once we agreed to work together with Helios building the hardware, and Slabsmith in control of the image processing, we applied our experience in slab imaging to the unique needs of a scanner.  This lead to the following goals.

• Industry leading dimensional accuracy
• Industry leading color accuracy and repeatability
• Extremely low maintenance

Each of these goals are intertwined in the approach Slabsmith implemented.

1. Automatic and continuous calibration

All three of these goals are accomplished with a custom fabricated calibration bar and advanced programming within Slabsmith.   

The calibration bar is 85 inches long to cover the largest slabs and consists of:

1. Three color bars, low, medium and high tones on each level
2. A series of dots every 50mm
3. Two heights, 6mm and 30mm

Each of these features will be discussed below.

6mm and 30mm heights

There are two heights to 

Color bars

The color bars are used to build a curve to create accurate colors.   Each time the scanner scans, it looks at every pixels from both cameras