Autodesk 507B1-90A211-1301 User Guide - Page 328

Soft Shadows Presenter includes shadows generated from pre-calculated shadow-maps for each shadow casting light source. The use of shadow maps enables rapid rendering of shadows with soft or graduated edges. The shadow resolution can be controlled to balance performance and image quality. Soft shadows are only suitable for use with small models, and are disabled by default. For large models, the generation of the shadow maps can use excessive amounts of time and memory. Soft shadows generated for large models are often too vague and dispersed without using an excessively high resolution, which uses even more memory and time. Physically Accurate Lights By default, Presenter uses lights with unitless, or empirical intensities. These are physically meaningless and are just chosen to give a visually pleasing result. Presenter can also use physically accurate intensities. These are defined in real world units such as Candela, Lumen or Lux. However, once you start using lights with real world intensities, you start to produce images with a real world variation in luminance values. By default, Presenter uses lights whose intensity remains constant as you move further from the light. In the real world intensity is reduced proportional to the inverse square of the distance from the light. Changing a light's "Fall Off" parameter to "Inverse Square Law" will more accurately model a light's fall off in intensity. However, once you start using lights with real world fall off, you start to produce images with a real world variation in luminance values. In the real world, the human eye is capable of resolving images in extremely varied lighting conditions, ranging from bright sunshine reflecting off snow to a room lit only by a single candle. In computer graphics on the other hand, you need to produce an image on a display device which has a very limited range of luminance values. Therefore, it is necessary to compress the range of luminance values found in a real world scene into the displayable range in such a way as to produce a realistic looking image. Photography, of course, has exactly the same problem. If a photographer (or camera) does not take into account the light levels in a scene before calculating the exposure of the shot, the likely result will be an image which is either over-exposed (everything is too bright) or under-exposed (everything is too dark). A professional photographer will also use different speeds of film for different lighting conditions. The aim is to produce an image on film that is representative of how that scene would have looked to a human observer. Presenter includes the Auto Exposure option (see "Auto Exposure" on page 327). When enabled, Presenter will render the image twice. Once to sample the range of luminance values in the output image, then a second time to render the actual image with the luminance values adjusted to match the behavior of the human eye. In general, when using physically accurate lights, the Auto Exposure option should be on. Volumetric Lights Volumetric lighting allows effects such as the scattering of light, by fog or smoke, in a scene. To use this effect, select the Scattering check box on each light. A Scattering Medium foreground effect must also be in use (see "Foreground Effects" on page 325 for more information). NOTE You may need to adjust the Medium Density and Medium Ambient parameters of the Scattering Medium foreground effect to suit your model. If no volumetric effects are visible, the Medium Density is too low. If the rendered image is entirely white, the Medium Density is too high. The default medium is plain white. Optionally, a density shader may be set to any solid (not wrapped) color shader, to create the effect of a non-uniform (inhomogeneous) medium. Examples of shaders that can be used are Blue Marble and Solid Clouds. A shader that has been designed explicitly for this purpose is the Turbulent shader. The key points when using volumetric lighting are: ■ Remember to turn the Scattering parameter of light sources on if you want to see their volumetric effects. ■ Use Medium Density and Medium Color to define brightness and color of the lit medium. 316 | Chapter 14 Create Photorealistic Visualizations