max planck institut informatik
Countershading: Colour contrast used to enhance the impression of global contrast.
In this work we take a non-standard approach to the problem of depicting HDR images for LDR display. Instead of developing yet another algorithm, we provide the means to enhance the depiction of an HDR image produced by an arbitrary tone mapping algorithm, thus restoring original contrast information. Based on experience and conclusions from previous work we identify Global Contrast Change and Detail Visibility Change as the two major tone mapping distortions, and we create objective perceptual metrics to measure these contrast distortions between an HDR image and its LDR depiction. Driven by these metrics, we present techniques for creating enhanced images that restore the original HDR contrast information with colour contrast achieved by chroma scaling. The increased colour contrast augments detail visibility, and countershading encourages the prominence of foreground objects, thus reclaiming the loss in perceived global contrast.
High Dynamic Range (HDR) images capture the full range of luminance present in real world scenes, and unlike Low Dynamic Range (LDR) images, can simultaneously contain detailed information in the deepest of shadows and the brightest of light sources. For display or aesthetic purposes, it is often necessary to perform tone mapping, which creates LDR depictions of HDR images at the cost of contrast information loss. The purpose of this work is two-fold: to analyze a displayed LDR image against its original HDR counterpart in terms of perceived contrast distortion, and to enhance the LDR depiction with perceptually driven colour adjustments to restore the original HDR contrast information. For analysis, we present a novel algorithm for the characterization of tone mapping distortion in terms of observed loss of global contrast, and loss of contour and texture details. We classify existing tone mapping operators accordingly. We measure both distortions with perceptual metrics that enable the automatic and meaningful enhancement of LDR depictions. For image enhancement, we identify artistic and photographic colour techniques from which we derive adjustments that create contrast with colour. The enhanced LDR image is an improved depiction of the original HDR image with restored contrast information.
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Details restoration: Café image resulting from photoreceptor tone mapping (left) and with enhanced details (right). Notice that the flowers, chairs and umbrellas are more visible and the distant landscape contrains more details and depth.
Details restoration: Gamma correction leads to a blanched sky without sun or cloud details (left). These details are restored with our technique (right).
Countershading: Tone mapping leads to light global contrast loss (left). We use countershading to emphasize the separation between the sky and the mountains (right).