EUROGRAPHICS 2012 / P. Cignoni, T. Ertl (Guest Editors) Volume 31 (2012), Number 2 Unsharp Masking, Countershading and Halos: Enhancements or Artifacts? Matthew Trentacoste 1 and Rafal Mantiuk 2 and Wolfgang Heidrich 1 and Florian Dufrot 2 1 University of British Columbia, Canada 2 Bangor University, United Kingdom Abstract Countershading is a common technique for local image contrast manipulations, and is widely used both in auto- matic settings, such as image sharpening and tonemapping, as well as under artistic control, such as in paintings and interactive image processing software. Unfortunately, countershading is a double-edged sword: while cor- rectly chosen parameters for a given viewing condition can significantly improve the image sharpness or trick the human visual system into perceiving a higher contrast than physically present in an image, wrong parameters, or different viewing conditions can result in objectionable halo artifacts. In this paper we investigate the perception of countershading in the context of a novel mask-based contrast enhancement algorithm and analyze the circum- stances under which the resulting profiles turn from image enhancement to artifact for a range of parameters and viewing conditions. Our experimental results can be modeled as a function of the width of the countershading profile. We employ this empirical function in a range of applications such as image resizing, view dependent tone mapping, and countershading analysis in photographs and works of fine art. 1. Introduction Local contrast enhancement is a powerful image processing technique, fundamental to many aspects of computer graph- ics such as image editing and tonemapping of HDR images. The countershading operation is one of the most common approaches to enhancing local contrast in images, where the local edge contrast is increased by adding gradients to either side of the edges. These gradients, known as countershad- ing profiles, result from numerous algorithms. Regardless of the method of introduction, the result resembles that of the unsharp masking (UM) operator. Shown in Figure 1-red, un- sharp masking increases local contrast by adding a high-pass image H σ( Y ) to the original image Y = Y + λHσ( Y )=(1 + λ) Y - λgσ * Y (1) where λ is the contrast of the countershading and the high- pass image Hσ( Y ) is produced by subtracting a Gaussian blurred image gσ * Y from the original image Y . Unsharp masking with a narrow high-pass filter can in- crease the apparent sharpness [Ney93] of the image, mak- ing fine details easier to identify. On the other hand, unsharp masking with a wide high-pass filter can increase the con- trast of the regions adjacent to the edge [SKMS06], alter- ing the overall impression of contrast in the image. Suffi- ciently wide unsharp masking profiles can even induce the Cornsweet illusion [KM88], where the entirety of adjacent regions change in appearance. However, unsharp masking can also introduce objection- able countershading around an edge, frequently referred to as haloes. In these cases, the contrast enhancement detracts from the image, providing neither improved understanding nor aesthetic quality. The same operation can be interpreted as an enhancement in some cases, while it’s interpreted as an artifact in others. We present a method of introducing perceptually-symmetric countershading profiles and conduct a perceptual study to determine the amount of local contrast that can be introduced by our operator without becoming ob- jectionable. The goal is to find a boundary between where the result is considered contrast enhancement and artifact. We discover the perception of countershading profiles fol- lows a U-shaped characteristic, where certain width pro- files are considered unacceptable even if only slightly vis- ible, separating adjacent regions of both wider and narrower © 2012 The Author(s) Computer Graphics Forum © 2012 The Eurographics Association and Blackwell Publish- ing Ltd. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA. DOI: 10.1111/j.1467-8659.2012.03056.x