Volume 17 (1998 ), number 4 pp. 235–247 COMPUTER forum GRAPHICS Incident Light Metering in Computer Graphics L´ aszl´ o Neumann†, Kreˇ simir Matkovi ´ c‡, Attila Neumann†and Werner Purgathofer‡ † Maros u.36,H-1122 Budapest, Hungary ‡ Institute of Computer Graphics, Vienna University of Technology, Karlsplatz 13/186/2, A-1040 Vienna, Austria Abstract Every rendering process consists of two steps. The first is the computing of luminance values by methods like ray tracing or radiosity, and the second step is the mapping of the computed values to values appropriate for displaying. In the last years, as alternative to simple linear scaling which maps the average value to the medium luminance, some new ways of mapping were introduced. These new methods are based on photography analogies and on human vision models. All existing methods follow, implicitly or explicitly, the reflected light metering principle. The method introduced in this paper is the first that follows the incident light metering used in professional photography and in the movie industry. Actually the irradiances are measured using a set of diffusors, which are placed automatically in the scene, and a linear scale factor based on these measurements is used to map the computed radiances to the display device. The diffusors act as half space integrators, they collect the lightenergy from all half space directions. The lightcomes from the primary light sources, or it is the result of various interreflections. The newly introduced method reproduces original colors faithfully even for scenes with very low or very high average reflectivity. Keywords: tone mapping, light metering, color reproduction, irradiance computation, rendering, radiosity. 1. Introduction The mapping of computed luminance values to display devices is an important step in the rendering process. The mapping depends on device characteristics, therefore, the device should be calibrated first. In this paper we assume the use of a calibrated display device with linear response and the input range [0,1] for each color channel. All existing mapping methods follow the reflected light metering principle. That means the mapping parameters are based on the radiance values arriving at the view point.Most of these methods normalize an image us- ing the average value, and then apply some linear or non-linear mapping function. Some non-linear mapping techniques take into account viewing conditions. These methods work fine as long as the average objects’ reflec- tivity is close to 0.5. If objects are dark or bright these methods fail. In the case of reflected light metering the product of incident light and reflectivity is measured. Therefore, a strongly lit dark gray surface is equivalent to a weakly lit white surface. According to some research 1 this is true only for the laboratory settings with a small view angle. Human perception can somehow distinguish these two cases in a real, complex environment. In our opinion, this is due to the recognition of various typical interreflection effects. Using the new method introduced in this paper it is possible to separately handle incident light intensity and local BRDF properties. Our intention is to reproduce the objects’ colors as faithfully as possible. We want to preserve not only the original r : g : b ratio by displaying, but the lightness (Y) too.E.g.,we want an originally selected orange color to be reproduced as orange, and not as a kind of brown. Of course, due to the inhomogeneous lighting and BRDFs of objects, objects can not be displayed in the exactly defined colors. We assume that the lighting, and adaptation level of the scene are normal in the range 300 to 16000 cd/m 2 . Using a linearmapping,the perception in this range can be approximated with sufficient accuracy, and the ratio of the color components remains invariant. We will c The Eurographics Association and Blackwell Publishers Ltd 1998.Published by Blackwell Publishers,108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA. 235