Graphics Reference
In-Depth Information
(a) Results rendered using the traditional cosine lobe model.
(b) Results rendered using the Lafortune BRDF.
Figure 8.15
Renderings of a simple scene viewed from increasing angles. (a) The cosine lobe model
fails to capture the expected increasing specularity near grazing reflection. (b) The Lafor-
tune model is an improvement. (From [Lafortune et al. 97] c
1997 ACM, Inc. Included
here by permission.)
the variation of reflectance across a surface. The idea to render the surface with
varying reflectance can also be extended to “bidirectional texture map” rendering
as introduced in the next chapter.
8.1.7 Image-Based BRDFs
Advances inmeasurement devices.
Ward's image-based BRDF measure-
ment method does not measure the BRDF values directly; the true BRDF values
are derived by comparing the image pixels with the those of a reference material.
The dynamic range of the camera and the relative measure of the reference mate-
rial limit the range of measurements. This is a serious issue, because the range of
BRDF values can be very large, especially for specular surfaces. The emergence
of HDR imaging opened the door on better image-basedmeasurement. One devel-
opment was described in the paper “Image-Based Bidirectional Reflectance Dis-
tribution Function Measurement” by Stephen R. Marschner, Stephen H. Westin,
Eric P. F. Lafortune, and Kenneth E. Torrance [Marschner et al. 00]. Their ap-
proach extracts BRDF values from captured HDR images of surface samples.
Unlike earlier measurement systems, which use a flat surface sample, Marsch-
ner's method attaches a sample of the surface to be measured onto a curved
surface, such as a cylinder or sphere. The reflection property of the material is
