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Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site

View Article: PubMed Central - PubMed

ABSTRACT

We propose a miniaturised light stage for measuring the bidirectional reflectance distribution function (BRDF) and the bidirectional texture function (BTF) of surfaces on site in real world application scenarios. The main principle of our lightweight BTF acquisition gantry is a compact hemispherical skeleton with cameras along the meridian and with light emitting diode (LED) modules shining light onto a sample surface. The proposed device is portable and achieves a high speed of measurement while maintaining high degree of accuracy. While the positions of the LEDs are fixed on the hemisphere, the cameras allow us to cover the range of the zenith angle from 0∘ to 75∘ and by rotating the cameras along the axis of the hemisphere we can cover all possible camera directions. This allows us to take measurements with almost the same quality as existing stationary BTF gantries. Two degrees of freedom can be set arbitrarily for measurements and the other two degrees of freedom are fixed, which provides a tradeoff between accuracy of measurements and practical applicability. Assuming that a measured sample is locally flat and spatially accessible, we can set the correct perpendicular direction against the measured sample by means of an auto-collimator prior to measuring. Further, we have designed and used a marker sticker method to allow for the easy rectification and alignment of acquired images during data processing. We show the results of our approach by images rendered for 36 measured material samples.

No MeSH data available.


Visualization of 120 camera positions used for BTF data acquisition for 1020 s duration. (a) In XY projection from unit hemisphere The black dots represent camera positions for rotating servo motor clockwise, the green dots for rotating the geared servo motor anticlockwise; (b) 3D perspective visualization of camera positions as spheres.
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sensors-17-00423-f018: Visualization of 120 camera positions used for BTF data acquisition for 1020 s duration. (a) In XY projection from unit hemisphere The black dots represent camera positions for rotating servo motor clockwise, the green dots for rotating the geared servo motor anticlockwise; (b) 3D perspective visualization of camera positions as spheres.

Mentions: While the number of camera directions can be set arbitrarily over the surface (with the limitation that all six cameras are always moved together) in practice we have to choose the number of images captured by the cameras as that influences the total measurement time. After initial testing of the system performance we decided to approximately match the number of camera directions on the hemisphere to the number of LED modules used in the gantry. The number of rotational positions given by the servo motor is 20, with the shift of the stepper motor for a half of those positions. The number of acquired images for this setting is 6. The lightdrum first rotates to 10 positions in steps of 36 (0, 36, 72…, 324). It then moves the cameras using the stepper motor and rotates back using a different 10 rotational positions (342, …, 90, 54, 18). After it finishes the last data acquisitions, the servo motor rotates the drum back to the initial position ready for the next measurement. The camera directions on the hemisphere in projection to XY plane are shown in Figure 18.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
Visualization of 120 camera positions used for BTF data acquisition for 1020 s duration. (a) In XY projection from unit hemisphere The black dots represent camera positions for rotating servo motor clockwise, the green dots for rotating the geared servo motor anticlockwise; (b) 3D perspective visualization of camera positions as spheres.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5375714&req=5

sensors-17-00423-f018: Visualization of 120 camera positions used for BTF data acquisition for 1020 s duration. (a) In XY projection from unit hemisphere The black dots represent camera positions for rotating servo motor clockwise, the green dots for rotating the geared servo motor anticlockwise; (b) 3D perspective visualization of camera positions as spheres.
Mentions: While the number of camera directions can be set arbitrarily over the surface (with the limitation that all six cameras are always moved together) in practice we have to choose the number of images captured by the cameras as that influences the total measurement time. After initial testing of the system performance we decided to approximately match the number of camera directions on the hemisphere to the number of LED modules used in the gantry. The number of rotational positions given by the servo motor is 20, with the shift of the stepper motor for a half of those positions. The number of acquired images for this setting is 6. The lightdrum first rotates to 10 positions in steps of 36 (0, 36, 72…, 324). It then moves the cameras using the stepper motor and rotates back using a different 10 rotational positions (342, …, 90, 54, 18). After it finishes the last data acquisitions, the servo motor rotates the drum back to the initial position ready for the next measurement. The camera directions on the hemisphere in projection to XY plane are shown in Figure 18.

View Article: PubMed Central - PubMed

ABSTRACT

We propose a miniaturised light stage for measuring the bidirectional reflectance distribution function (BRDF) and the bidirectional texture function (BTF) of surfaces on site in real world application scenarios. The main principle of our lightweight BTF acquisition gantry is a compact hemispherical skeleton with cameras along the meridian and with light emitting diode (LED) modules shining light onto a sample surface. The proposed device is portable and achieves a high speed of measurement while maintaining high degree of accuracy. While the positions of the LEDs are fixed on the hemisphere, the cameras allow us to cover the range of the zenith angle from 0∘ to 75∘ and by rotating the cameras along the axis of the hemisphere we can cover all possible camera directions. This allows us to take measurements with almost the same quality as existing stationary BTF gantries. Two degrees of freedom can be set arbitrarily for measurements and the other two degrees of freedom are fixed, which provides a tradeoff between accuracy of measurements and practical applicability. Assuming that a measured sample is locally flat and spatially accessible, we can set the correct perpendicular direction against the measured sample by means of an auto-collimator prior to measuring. Further, we have designed and used a marker sticker method to allow for the easy rectification and alignment of acquired images during data processing. We show the results of our approach by images rendered for 36 measured material samples.

No MeSH data available.