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Custom Calibrations

Authentica Creator comes with a list of already included calibrations for officially supported illumination devices. Currently these consist of the Truvis Helios S53 and the Broncolor Scope D50.

For generating Authentica Materials from image sequences which were captured with other devices or with manual positioning of the light sources (e.g. Highlight RTI Method) Authentica Creator supports the use of a custom calibration file. Such a calibration files describes the number and direction of the light sources. We currently support two different formats: the LP (.lp) file format, and the JSON (.json) file format. These formats are described in more detail further below.

How to use a custom calibration file

  • Follow the instructions on how to generate a digital material. In the pop-up dialog select 'Custom Calibration' as 'Device'.
  • Click 'Choose File' and locate the .json of .lp file you want to use.
  • All other steps and settings for creating a digital material are described in detail under the general instructions at Generate a digital material.

How to generate a custom calibration file

There are two main ways to generate a custom calibration file. Either through the 'Highlight RTI Method' by placing a reflective sphere in the image or by calculating the directions of the light sources directly from the known geometry of a illumination setup.

For generating a .lp file from images of a reflective sphere, tools such as RTIBuilder can be used. See RTIBuilder by CHI and Guide to Highlight Image Processing by CHI

When dealing with a illumination setup with known light positions, please refer to the description of the supported calibration file formats below.

The LP (.lp) calibration file format

The LP file format is used in different tools for generating RTI data, among others also the RTIBuilder software. It is a simple white space separated text file. The first row has only one column consisting of a number N which indicates the number of images in the image sequence. The following N rows consist of 4 columns each. Each row is associated with a image in the corresponding image sequence.

The first column holds the path to the corresponding image from the image sequence which was used to generate the calibratiton file.

Warning

Note: The first column holding the image path is ignored in Authentica Creator. Authentica Creator assumes the rows in the file to be sorted alpha-numerically by the file names of the individual images. This is consistent with the standard behaviour that results from generating a .lp file with RTIBuilder. Authentica Creator will read the images in the image folder provided through the graphical user interface also in alpha-numerical order and in this way associate each of them with the corresponding row in the .lp file. This allows to reuse a calibration for multiple image sequences, assuming the geometry of the illumination setup does not change. Images of the reflective sphere need only be captured once for calibration, and not subsequently on all images of the objects of interest.

The following 3 columns hold the x-, y- and z-components of the (normalized) vector describing the direction of the corresponding light source.

The filename's extension needs to be '.lp'.

For illumination setups with fixed geometries we recommend capturing an image sequence of a reflective sphere once, placing the sphere in the center of the image, and then generating an LP file with RTIBuilder.

Example file exampleCalibration.lp:

4
/Users/Thomas/rtiSequences/sequence42/image_01.tif 0.7071067812 0.0 0.7071067812
/Users/Thomas/rtiSequences/sequence42/image_02.tif 0.0 0.7071067812 0.7071067812
/Users/Thomas/rtiSequences/sequence42/image_03.tif -0.7071067812 0.0 0.7071067812
/Users/Thomas/rtiSequences/sequence42/image_02.tif 0.0 -0.7071067812 0.7071067812

This example file has 4 light sources positioned above the x- and y- coordinate axes at a constant height resulting in 45° angle with the x/y plane.

The JSON (.json) calibration file format

The Authentica JSON file format for describing the light directions is a simple text file in JSON notation. It holds a single JSON array, with a number of items equal to the number images in the sequence. Each entry holds the x-, y- and z-components of the (normalized) vector describing the direction of the corresponding light source.

Authentica Creator will read the images in the image folder in alpha-numerical order and in this way associate each of them with the corresponding item in the array.

The filename's extension needs to be '.json' or '.JSON'.

Example file exampleCalibration.json:

[
  {
    "x": 0.7071067812,
    "y": 0.0,
    "z": 0.7071067812
  },
  {
    "x": 0.0,
    "y": 0.7071067812,
    "z": 0.7071067812
  },
  {
    "x": -0.7071067812,
    "y": 0.0,
    "z": 0.7071067812
  },
  {
    "x": 0.0,
    "y": -0.7071067812,
    "z": 0.7071067812
  },
]

This example file has 4 light sources positioned above the x- and y- coordinate axes at a constant height resulting in 45° angle of the light direction with the x/y plane.