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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.


The distribution of 134 LED modules on the hemispherical skeleton. (a) the simulation model result shown projected onto an X-Y plane: bigger red points are for 50 deterministically set positions and 84 blue smaller points correspond to positions computed by randomised algorithm; (b) the manufactured skeleton from PMMA with holes for LED modules and slot for cameras. Note that 5 additional LEDs are mounted between the cameras and are not shown in this figure.
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sensors-17-00423-f012: The distribution of 134 LED modules on the hemispherical skeleton. (a) the simulation model result shown projected onto an X-Y plane: bigger red points are for 50 deterministically set positions and 84 blue smaller points correspond to positions computed by randomised algorithm; (b) the manufactured skeleton from PMMA with holes for LED modules and slot for cameras. Note that 5 additional LEDs are mounted between the cameras and are not shown in this figure.

Mentions: In our design the cameras are located along the meridian from the pole up to the required lowest zenithal direction () of an incomplete hemisphere. We had to distribute the LED modules described above on the hemispherical surface. We studied the properties of such a distribution with additional geometrical constraints, given by the LED module size and the zenithal slot for the cameras at the azimuthal angle . While the problem can be understood as packing and/or sampling, it is not completely the case as our concern was also the uniformity of the modules’ distribution expressed by discrepancy of points on the sphere. A further consideration was that we wanted to accurately measure the specular reflection for highly reflective samples which requires the positioning of the cameras in the direction of ideal reflection from a luminaire. For all these reasons, we used a semi-deterministic algorithm to distribute the LED modules on the hemispherical skeleton. Some of the LED modules are positioned deterministically along the geometrical border of the slot with the cameras and around the border of the incomplete hemisphere (for zenith angle ). In addition, we added 6 LED modules positioned optically opposite the cameras based on the reflection at the mirrored sample, at an azimuthal angle . The remaining LED modules are positioned on the hemispherical surface by a randomised algorithm using Lloyd’s relaxation [27] working with the minimum distance between two samples given by the LED module size. The randomised algorithm first puts the samples randomly where possible with respect to already positioned LED modules. Then, by further relaxation, it optimises the positions of the LED modules to make their distribution as uniform as possible. The sampling algorithm was run hundreds of times and the best solution with the maximum number of LEDs was used. The best solution has 134 LEDs tightly packed on the hemispherical skeleton with an outer radius of 234 mm. The distribution of the LED modules is shown in Figure 12. Further, an additional 5 LED modules were put between the cameras so that we can measure backscattering efficiently. These move together with the cameras along the meridian.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
The distribution of 134 LED modules on the hemispherical skeleton. (a) the simulation model result shown projected onto an X-Y plane: bigger red points are for 50 deterministically set positions and 84 blue smaller points correspond to positions computed by randomised algorithm; (b) the manufactured skeleton from PMMA with holes for LED modules and slot for cameras. Note that 5 additional LEDs are mounted between the cameras and are not shown in this figure.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f012: The distribution of 134 LED modules on the hemispherical skeleton. (a) the simulation model result shown projected onto an X-Y plane: bigger red points are for 50 deterministically set positions and 84 blue smaller points correspond to positions computed by randomised algorithm; (b) the manufactured skeleton from PMMA with holes for LED modules and slot for cameras. Note that 5 additional LEDs are mounted between the cameras and are not shown in this figure.
Mentions: In our design the cameras are located along the meridian from the pole up to the required lowest zenithal direction () of an incomplete hemisphere. We had to distribute the LED modules described above on the hemispherical surface. We studied the properties of such a distribution with additional geometrical constraints, given by the LED module size and the zenithal slot for the cameras at the azimuthal angle . While the problem can be understood as packing and/or sampling, it is not completely the case as our concern was also the uniformity of the modules’ distribution expressed by discrepancy of points on the sphere. A further consideration was that we wanted to accurately measure the specular reflection for highly reflective samples which requires the positioning of the cameras in the direction of ideal reflection from a luminaire. For all these reasons, we used a semi-deterministic algorithm to distribute the LED modules on the hemispherical skeleton. Some of the LED modules are positioned deterministically along the geometrical border of the slot with the cameras and around the border of the incomplete hemisphere (for zenith angle ). In addition, we added 6 LED modules positioned optically opposite the cameras based on the reflection at the mirrored sample, at an azimuthal angle . The remaining LED modules are positioned on the hemispherical surface by a randomised algorithm using Lloyd’s relaxation [27] working with the minimum distance between two samples given by the LED module size. The randomised algorithm first puts the samples randomly where possible with respect to already positioned LED modules. Then, by further relaxation, it optimises the positions of the LED modules to make their distribution as uniform as possible. The sampling algorithm was run hundreds of times and the best solution with the maximum number of LEDs was used. The best solution has 134 LEDs tightly packed on the hemispherical skeleton with an outer radius of 234 mm. The distribution of the LED modules is shown in Figure 12. Further, an additional 5 LED modules were put between the cameras so that we can measure backscattering efficiently. These move together with the cameras along the meridian.

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.