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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.


(a) A BTF parameterization corresponding to the ideal measurement arrangement of a planar sample of width w and height h of a texel at coordinates . The illumination is provided by collimated incident light in the direction  over the whole measured sample.  is the normal over the whole plane with the measured sample,  is the outgoing light direction; (b) rendered image example, showing a 3D object covered with a measured BTF.
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sensors-17-00423-f002: (a) A BTF parameterization corresponding to the ideal measurement arrangement of a planar sample of width w and height h of a texel at coordinates . The illumination is provided by collimated incident light in the direction over the whole measured sample. is the normal over the whole plane with the measured sample, is the outgoing light direction; (b) rendered image example, showing a 3D object covered with a measured BTF.

Mentions: We illustrate the BTF parameterization that corresponds to an ideal measurement arrangement in Figure 2a and the example output with a rendered object in Figure 2b. Assuming collimated incident light to a sample, the BTF expressed as a ratio of incoming irradiance in and outgoing radiance in is then unitless (i.e., ):(1)BTF(x,y,ωo,ωi)[sr−1]=L(x,y,ωo)E(ωi)


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
(a) A BTF parameterization corresponding to the ideal measurement arrangement of a planar sample of width w and height h of a texel at coordinates . The illumination is provided by collimated incident light in the direction  over the whole measured sample.  is the normal over the whole plane with the measured sample,  is the outgoing light direction; (b) rendered image example, showing a 3D object covered with a measured BTF.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f002: (a) A BTF parameterization corresponding to the ideal measurement arrangement of a planar sample of width w and height h of a texel at coordinates . The illumination is provided by collimated incident light in the direction over the whole measured sample. is the normal over the whole plane with the measured sample, is the outgoing light direction; (b) rendered image example, showing a 3D object covered with a measured BTF.
Mentions: We illustrate the BTF parameterization that corresponds to an ideal measurement arrangement in Figure 2a and the example output with a rendered object in Figure 2b. Assuming collimated incident light to a sample, the BTF expressed as a ratio of incoming irradiance in and outgoing radiance in is then unitless (i.e., ):(1)BTF(x,y,ωo,ωi)[sr−1]=L(x,y,ωo)E(ωi)

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.