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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.


Completing image registration: (a–c) step 7—image alignment by ECC algorithm; (d) step 8—the image after the homography change due to the sticker thickness; (e) The difference image between images of step 7 and 8.
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sensors-17-00423-f021: Completing image registration: (a–c) step 7—image alignment by ECC algorithm; (d) step 8—the image after the homography change due to the sticker thickness; (e) The difference image between images of step 7 and 8.

Mentions: Although it could seem that the image registration from step 6 or even from step 5 is acceptable, the accuracy of image alignment is not sufficient yet as we aim for subpixel accuracy. By completing step 6 we approached close to the perfect homography transform but not close enough. To achieve the subpixel accuracy we perform parametric image alignment using the enhanced correlation coefficient maximisation algorithm (ECC) [33] implemented in OpenCV library [34]. First, we again create the image mask as in the step 6. The input image of the step 7 is the output of the step 6 and is shown in Figure 20d. The reference image is given by marker sticker pattern in Figure 14. The mask computed from the input image is shown in Figure 21a.


Lightdrum — Portable Light Stage for Accurate BTF Measurement on Site
Completing image registration: (a–c) step 7—image alignment by ECC algorithm; (d) step 8—the image after the homography change due to the sticker thickness; (e) The difference image between images of step 7 and 8.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sensors-17-00423-f021: Completing image registration: (a–c) step 7—image alignment by ECC algorithm; (d) step 8—the image after the homography change due to the sticker thickness; (e) The difference image between images of step 7 and 8.
Mentions: Although it could seem that the image registration from step 6 or even from step 5 is acceptable, the accuracy of image alignment is not sufficient yet as we aim for subpixel accuracy. By completing step 6 we approached close to the perfect homography transform but not close enough. To achieve the subpixel accuracy we perform parametric image alignment using the enhanced correlation coefficient maximisation algorithm (ECC) [33] implemented in OpenCV library [34]. First, we again create the image mask as in the step 6. The input image of the step 7 is the output of the step 6 and is shown in Figure 20d. The reference image is given by marker sticker pattern in Figure 14. The mask computed from the input image is shown in Figure 21a.

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.