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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.

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(a) The principle of six camera motion using two short linear guides 60 mm long carrying an arc with the cameras; the black circular arc is for the  position in the centre of linear guides, the blue dashed arc and the red dashed arc are the low and high dead centre positions of the motion, respectively; (b) solid drawing of the camera motion mechanism; (c) camera positions in relation to stepper motor position; (d) radial camera position error taken as the distance of camera axis from the centre of hemisphere as a function of the stepper motor position.
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sensors-17-00423-f007: (a) The principle of six camera motion using two short linear guides 60 mm long carrying an arc with the cameras; the black circular arc is for the position in the centre of linear guides, the blue dashed arc and the red dashed arc are the low and high dead centre positions of the motion, respectively; (b) solid drawing of the camera motion mechanism; (c) camera positions in relation to stepper motor position; (d) radial camera position error taken as the distance of camera axis from the centre of hemisphere as a function of the stepper motor position.

Mentions: As the selection of circular guides is very limited, and additionally these are heavy, we have proposed a light-weight linear motion mechanism approximating the circular motion. The idea behind our solution is based on the assumption that we need the circular motion only in a limited angular range. As we use 6 cameras mounted on the arc set into required position by motion mechanism, its required angular motion range is only 12.5. The principle of the light-weight motion mechanism used is shown in Figure 7a, its solid drawing in Figure 7b. It was described by Hošek et al. [22]. The idea of putting the cameras along the arc above a measured object was proposed by Tong et al. [23].


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
(a) The principle of six camera motion using two short linear guides 60 mm long carrying an arc with the cameras; the black circular arc is for the  position in the centre of linear guides, the blue dashed arc and the red dashed arc are the low and high dead centre positions of the motion, respectively; (b) solid drawing of the camera motion mechanism; (c) camera positions in relation to stepper motor position; (d) radial camera position error taken as the distance of camera axis from the centre of hemisphere as a function of the stepper motor position.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f007: (a) The principle of six camera motion using two short linear guides 60 mm long carrying an arc with the cameras; the black circular arc is for the position in the centre of linear guides, the blue dashed arc and the red dashed arc are the low and high dead centre positions of the motion, respectively; (b) solid drawing of the camera motion mechanism; (c) camera positions in relation to stepper motor position; (d) radial camera position error taken as the distance of camera axis from the centre of hemisphere as a function of the stepper motor position.
Mentions: As the selection of circular guides is very limited, and additionally these are heavy, we have proposed a light-weight linear motion mechanism approximating the circular motion. The idea behind our solution is based on the assumption that we need the circular motion only in a limited angular range. As we use 6 cameras mounted on the arc set into required position by motion mechanism, its required angular motion range is only 12.5. The principle of the light-weight motion mechanism used is shown in Figure 7a, its solid drawing in Figure 7b. It was described by Hošek et al. [22]. The idea of putting the cameras along the arc above a measured object was proposed by Tong et al. [23].

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.


Related in: MedlinePlus