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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.


Overcoming the limitation of highly glossy materials by restricting the field of view—visible disturbing seams in directional domain on the tile. Material sample basketball from The MAM 2014 sample set: (a) the original tile size; (b) the decreased tile size usage. Measured material sample fabric003, physical sample courtesy of UTIA BTF Database: (c) the original tile size; (d) the decreased tile size.
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sensors-17-00423-f028: Overcoming the limitation of highly glossy materials by restricting the field of view—visible disturbing seams in directional domain on the tile. Material sample basketball from The MAM 2014 sample set: (a) the original tile size; (b) the decreased tile size usage. Measured material sample fabric003, physical sample courtesy of UTIA BTF Database: (c) the original tile size; (d) the decreased tile size.

Mentions: We wrote a PCA decompression software plugin to the renderer Mitsuba [38] for environment map illumination. We have used for rendering the environment map Grace-New (http://gl.ict.usc.edu/Data/HighResProbes/probes/grace-new.hdr, courtesy of Paul Debevec). The rendered images for 36 measured material samples (selected out of 50 to present various materials, similar or diffuse material samples are not shown) are shown in Figure 25 (MAM 2014 sample set) and in Figure 26 (upholstery and textile) and in Figure 27 (other materials). To get the final images that are used in this paper (Figure 25, Figure 26, Figure 27 and Figure 28) we had to adjust the lightness of images, as the paper and display screens only exhibit a low dynamic range. To keep the background of images the same, we individually adjusted the intensity of BTF by a single multiplicative constant (darker materials are brighter, brighter materials are darker). This is necessary, if we want to depict the material features better, due to the dark and light adaption mechanism of the human eye, since it is impossible to transfer the high dynamic range data in the limited intensity range of computer output devices such as displays and printers. The rendered images are shown for applying the full sample size measured (the square mm on the measured sample pixels). We scaled the used BTF dataset 6 times in both U and V directions of the used UV texture mapping over the 3D object’s surface. For rendering we used far-field assumption for the measured BTF data, even though this is only an approximation, as neither the camera is orthographic nor is the illumination collimated. This may result in artifacts as we discuss further.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
Overcoming the limitation of highly glossy materials by restricting the field of view—visible disturbing seams in directional domain on the tile. Material sample basketball from The MAM 2014 sample set: (a) the original tile size; (b) the decreased tile size usage. Measured material sample fabric003, physical sample courtesy of UTIA BTF Database: (c) the original tile size; (d) the decreased tile size.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f028: Overcoming the limitation of highly glossy materials by restricting the field of view—visible disturbing seams in directional domain on the tile. Material sample basketball from The MAM 2014 sample set: (a) the original tile size; (b) the decreased tile size usage. Measured material sample fabric003, physical sample courtesy of UTIA BTF Database: (c) the original tile size; (d) the decreased tile size.
Mentions: We wrote a PCA decompression software plugin to the renderer Mitsuba [38] for environment map illumination. We have used for rendering the environment map Grace-New (http://gl.ict.usc.edu/Data/HighResProbes/probes/grace-new.hdr, courtesy of Paul Debevec). The rendered images for 36 measured material samples (selected out of 50 to present various materials, similar or diffuse material samples are not shown) are shown in Figure 25 (MAM 2014 sample set) and in Figure 26 (upholstery and textile) and in Figure 27 (other materials). To get the final images that are used in this paper (Figure 25, Figure 26, Figure 27 and Figure 28) we had to adjust the lightness of images, as the paper and display screens only exhibit a low dynamic range. To keep the background of images the same, we individually adjusted the intensity of BTF by a single multiplicative constant (darker materials are brighter, brighter materials are darker). This is necessary, if we want to depict the material features better, due to the dark and light adaption mechanism of the human eye, since it is impossible to transfer the high dynamic range data in the limited intensity range of computer output devices such as displays and printers. The rendered images are shown for applying the full sample size measured (the square mm on the measured sample pixels). We scaled the used BTF dataset 6 times in both U and V directions of the used UV texture mapping over the 3D object’s surface. For rendering we used far-field assumption for the measured BTF data, even though this is only an approximation, as neither the camera is orthographic nor is the illumination collimated. This may result in artifacts as we discuss further.

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.