RE: [Algorithms] Obscuring objects
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From: Tony C. <to...@mi...> - 2003-07-28 22:27:01
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We're talking about projection onto the screen plane, which involves a perspective division, so it's not a simple plane-sphere intersection (which would always give you a circle anyway). Rendering a sphere, and computing the silhouette is an example in Jim Blinn's book "Notation, notation, notation". Jim seems to think it's a general conic section, and even gives the example that the Earth's horizon as seen from someone standing on the Earth is a hyperbola. Who am I to argue? Tony Cox - Development Lead, Hockey Microsoft Games Studios - Sports -----Original Message----- From: gda...@li... [mailto:gda...@li...] On Behalf Of Mat Noguchi (BUNGIE) Sent: Monday, July 28, 2003 2:37 PM To: gda...@li... Subject: RE: [Algorithms] Obscuring objects Unless my math is incorrect, the projection of a sphere onto a plane of any orientation in world space has the same shape as the intersection of a plane and a sphere (assuming the two intersect). MSN -----Original Message----- From: gda...@li... [mailto:gda...@li...] On Behalf Of Tony Cox Sent: Monday, July 28, 2003 1:15 PM To: gda...@li... Subject: RE: [Algorithms] Obscuring objects Can't the projection be any conic-section (not just an ellipse), in cases where the sphere straddles the plane of projection? Tony Cox - Development Lead, Hockey Microsoft Games Studios - Sports -----Original Message----- From: gda...@li... [mailto:gda...@li...] On Behalf Of Mat Noguchi (BUNGIE) Sent: Monday, July 28, 2003 12:05 PM To: gda...@li... Subject: RE: [Algorithms] Obscuring objects The projection of the sphere onto the viewing plane onto the viewport in normalized screen coordinates is an ellipse. The projection of the sphere onto the viewing plane just taking into account z_near and z_far will be a circle. MSN -----Original Message----- From: gda...@li... [mailto:gda...@li...] On Behalf Of Tom Forsyth Sent: Monday, July 28, 2003 11:39 AM To: gda...@li... Subject: RE: [Algorithms] Obscuring objects Trick question! The projection of a sphere onto the viewing plane is actually an ellipse. :-) Easiest thing is just to do a check for intersection between the occluding sphere and the line from the viewer to the point. Tom Forsyth - Muckyfoot bloke and Microsoft MVP. This email is the product of your deranged imagination, and does not in any way imply existence of the author. > -----Original Message----- > From: Lars Wilke [mailto:la...@ch...] > Sent: 28 July 2003 19:25 > To: gda...@li... > Subject: [Algorithms] Obscuring objects >=20 >=20 > This may be a dumb question, but here goes. >=20 > How do I determine if an object in the scene is obscuring another=20 > point in 3D in screen coordinates? It could be something as simple > in my case > of determining whether the bounding sphere obscures the point. So, > posing the question in another way, ho do I find the equation of the > projected circle in screen coordinates of a sphere in 3D? >=20 > Thanks in Advance. >=20 > Lars Wilke ------------------------------------------------------- This SF.Net email sponsored by: Free pre-built ASP.NET sites including Data Reports, E-commerce, Portals, and Forums are available now. Download today and enter to win an XBOX or Visual Studio .NET. http://aspnet.click-url.com/go/psa00100003ave/direct;at.aspnet_072303_01 /01 _______________________________________________ GDAlgorithms-list mailing list GDA...@li... https://lists.sourceforge.net/lists/listinfo/gdalgorithms-list Archives: http://sourceforge.net/mailarchive/forum.php?forum_ida88 ------------------------------------------------------- This SF.Net email sponsored by: Free pre-built ASP.NET sites including Data Reports, E-commerce, Portals, and Forums are available now. Download today and enter to win an XBOX or Visual Studio .NET. http://aspnet.click-url.com/go/psa00100003ave/direct;at.aspnet_072303_01 /01 _______________________________________________ GDAlgorithms-list mailing list GDA...@li... https://lists.sourceforge.net/lists/listinfo/gdalgorithms-list Archives: http://sourceforge.net/mailarchive/forum.php?forum_ida88 |