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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) Marker sticker design with a 51 mm diameter hole shown by the green colour; the size of marker sticker is 85 × 85 mm, the angular distribution of chequerboard pattern is intentionally irregular; (b) Red circles mark the regions that allow for visual detection and checking of correctness of acquired images. This enables proper sample orientation in case of any image transformation by camera processing or for incorrect positioning the orientation of the gantry towards the measured sample.
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sensors-17-00423-f014: (a) Marker sticker design with a 51 mm diameter hole shown by the green colour; the size of marker sticker is 85 × 85 mm, the angular distribution of chequerboard pattern is intentionally irregular; (b) Red circles mark the regions that allow for visual detection and checking of correctness of acquired images. This enables proper sample orientation in case of any image transformation by camera processing or for incorrect positioning the orientation of the gantry towards the measured sample.

Mentions: Our marker sticker design is shown in Figure 14a and the source code generating the marker sticker pattern is in Appendix C. The marker sticker is a simplification of the sample holder used in stationary devices. It is made of a 0.10 mm thick aluminium foil of size 85 × 85 mm with a hole of diameter 51 mm in the centre where the sample being measured is located. The information for the registration is put as a black and white pattern on the non-measured border on the upper side of the marker sticker. The bottom side of the marker sticker has a thin coat of glue. The cameras on the spinning gantry aim at the centre of the sample. As the camera sensor chip is rectangular, there is space remaining outside the circular sample area in which we place the information needed for image registration. For this purpose we use a radial chequerboard pattern on the marker sticker outside the circular hole.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
(a) Marker sticker design with a 51 mm diameter hole shown by the green colour; the size of marker sticker is 85 × 85 mm, the angular distribution of chequerboard pattern is intentionally irregular; (b) Red circles mark the regions that allow for visual detection and checking of correctness of acquired images. This enables proper sample orientation in case of any image transformation by camera processing or for incorrect positioning the orientation of the gantry towards the measured sample.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f014: (a) Marker sticker design with a 51 mm diameter hole shown by the green colour; the size of marker sticker is 85 × 85 mm, the angular distribution of chequerboard pattern is intentionally irregular; (b) Red circles mark the regions that allow for visual detection and checking of correctness of acquired images. This enables proper sample orientation in case of any image transformation by camera processing or for incorrect positioning the orientation of the gantry towards the measured sample.
Mentions: Our marker sticker design is shown in Figure 14a and the source code generating the marker sticker pattern is in Appendix C. The marker sticker is a simplification of the sample holder used in stationary devices. It is made of a 0.10 mm thick aluminium foil of size 85 × 85 mm with a hole of diameter 51 mm in the centre where the sample being measured is located. The information for the registration is put as a black and white pattern on the non-measured border on the upper side of the marker sticker. The bottom side of the marker sticker has a thin coat of glue. The cameras on the spinning gantry aim at the centre of the sample. As the camera sensor chip is rectangular, there is space remaining outside the circular sample area in which we place the information needed for image registration. For this purpose we use a radial chequerboard pattern on the marker sticker outside the circular hole.

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