Abstract
High Dynamic Range (HDR) imaging using a fisheye lens has provided new opportunities to evaluate the luminous environment in visual comfort research. For glare analysis, strict calibration is necessary to extract accurate luminous maps to achieve reliable glare results. Most studies have focused on correcting the vignetting effect in HDR imaging during post-calibration. However, the lens projection also contributes to luminous map errors because of its inherent distortion. To date, there is no simple method to correct this distortion phenomenon for glare analysis. This paper presents a parametric-based methodology to correct the projection distortion from fisheye lenses for the specific use in glare analysis. HDR images were captured to examine two devices: a 190° equisolid SIGMA 8 mm F3.5 EX DG fisheye lens mounted on a Canon 5D camera, and a 195° fisheye commercial lens with an unknown projection, mounted on the rear camera of a Samsung Galaxy S7. A mathematical and geometrical model was developed to remap each pixel to correct the projection distortion using Grasshopper and MATLAB. The parametric-based method was validated using Radiance and MATLAB through checking the accuracy of pixel remapping and measuring color distortion with Structural Similarity Index (SSIM). Glare scores were used to compare the results between both devices, which validates the use of mobile phones in photometric research. The results showed that this method can be used to correct HDR images projection distortion for more accurate evaluation of the luminous environment in glare research.
Highlights
Luminance maps captured using a fisheye lens have advanced pertinent knowledge in glare analysis
This study provided a new remapping method for glare analysis through defining and remapping fisheye projections through a geometrical model which was validated through laboratory measurements
The developed method resolved the issue of using fisheye lenses of unrecognized image projections
Summary
Luminance maps captured using a fisheye lens have advanced pertinent knowledge in glare analysis. Many post-occupancy studies in visual comfort research have adopted this approach to assess the subjective experiences of discomfort glare in buildings [4,5,6,7,8]. This is because a camera with fisheye lens can capture Low Dynamic Range (LDR) images to generate HDR images with a field of view similar to a human’s field of view (FOV). The step, in order to calibrate the fisheye images for glare analysis, is to remap the image’s projection into an equidistance or hemispherical projection
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