Inverse Tone Mapping

Abstract

In recent years many Tone Mapping Operators (TMOs) have been presented in order to display High Dynamic Range Images (HDRI) on typical display devices. TMOs compress the luminance range while trying to maintain contrast. The dual of tone mapping, inverse tone mapping, expands a Low Dynamic Range Image (LDRI) into a HDRI. HDRIs contain a broader range of physical values that can be perceived by the human visual system. The majority of today’s media is stored in low dynamic range. Inverse Tone Mapping Operators (iTMOs) could thus potentially revive all of this content for use in high dynamic range display and image-based lighting. We propose an approximate solution to this problem that uses mediancut to find the areas considered of high luminance and subsequently apply a density estimation to generate an Expand-map in order to extend the range in the high luminance areas using an inverse Photographic Tone Reproduction operator.

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LIGHTPROBE RESULTS:

Paul Debevec's Grace Cathedral	Median Cut Sampling using original Grace HDRI
Median Cut Sampling using clamped Grace Cathedral	Median Cut Sampling using Inversed Tone Mapped clamped Grace Cathedral

RENDER RESULTS:
Image Based Lighting examples rendered using 256 directional lights sampled using Median Cut algorithm.

Lucy render using original HDRI of St. Peter lightprobe	Lucy render using original clamped St. Peter lightprobe
Lucy render using Inversed Tone Mapped clamped St. Peter lightprobe

RENDER RESULTS WITH IMPROVED METHOD FOR IMAGE BASED LIGHTING(new):

In the previous results hard shadows were presented as artifacts of the current algorithm. This problem can be solved applying a larger radius in the density estimation. We found that a radius 25% times bigger than usual is enough to attenuate hard shadows.

Lucy render using original HDRI of St. Peter lightprobe

Lucy render using Inversed Tone Mapped clamped St. Peter lightprobe