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  1.  
  2. <H1><A ID="SECTION00820000000000000000">
  3. Atmospheric Effects</A>
  4. </H1>
  5.  
  6. <P>
  7. Any number of atmospheric effects may be associated with the default
  8. medium (``air'').
  9.  
  10. <P>
  11. <DL>
  12. <DT><STRONG><#4948#><#4948#></STRONG></DT>
  13. <DD><#1443#><TT>fog</TT><#1443#> <tex2html_verbatim_mark>#math142#<tex2html_image_mark>#tex2html_wrap_inline4950#⇧ <tex2html_verbatim_mark>#math143#<tex2html_image_mark>#tex2html_wrap_inline4952#⇧ 
  14. <BR>Add exponential fog with the specified <#606#><EM>thinness</EM><#606#> and <#607#><EM>color</EM><#607#>.
  15. </DD>
  16. </DL>Fog is simulated by blending the color of the fog with the color of
  17. each ray.  The amount of fog color blended into a ray color is an exponential
  18. function of the distance from the ray origin to the point of intersection
  19. divided by the specified <#609#><EM>thinness</EM><#609#> for each color channel.
  20. If the distance is equal to <#610#><EM>thinness</EM><#610#>,
  21. a ray's new color will be half of the fog color plus half its
  22. original color.
  23.  
  24. <P>
  25. <DL>
  26. <DT><STRONG><#4953#><#4953#></STRONG></DT>
  27. <DD><#1448#><TT>mist</TT><#1448#> <tex2html_verbatim_mark>#math144#<tex2html_image_mark>#tex2html_wrap_inline4955#⇧ <tex2html_verbatim_mark>#math145#<tex2html_image_mark>#tex2html_wrap_inline4957#⇧ <#1451#><EM>zero scale</EM><#1451#> 
  28. <BR>Add global low-altitude mist of the specified color.  The color of
  29. a ray is modulated by a fog with density that varies linearly with
  30. the difference in <I>z</I> coordinate<A NAME="tex2html4" HREF="footnode_mn.html#foot616" TARGET="footer"><SUP>6.1</SUP></A>between the ray origin and
  31. the point of intersection.  The thinness values specify the transmissivity
  32. of the fog for each color channel.
  33. The base altitude of the
  34. mist is given by <#617#><EM>zero</EM><#617#>, and the apparent height of the mist can
  35. be modulated using <#618#><EM>scale</EM><#618#>, which scales the difference in
  36. altitude used to compute the fog.
  37. </DD>
  38. </DL>
  39. <P>
  40. <DL>
  41. <DT><STRONG><#4961#><#4961#></STRONG></DT>
  42. <DD><#1454#><TT>fogdeck</TT><#1454#> <#1455#><EM>altitude</EM><#1455#> <#1456#><EM>offset</EM><#1456#> <tex2html_verbatim_mark>#math146#<tex2html_image_mark>#tex2html_wrap_inline4963#⇧ <#1458#><EM>chaoscale</EM><#1458#>
  43.  <tex2html_verbatim_mark>#math147#<tex2html_image_mark>#tex2html_wrap_inline4965#⇧ <tex2html_verbatim_mark>#math148#<tex2html_image_mark>#tex2html_wrap_inline4967#⇧ 
  44. <BR>Add low-altitude fog, with transmissivity modulated by
  45. a chaotic function.
  46. </DD>
  47. </DL>
  48. <P>
  49.