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skystorm
10-14-2001, 07:28 PM
hi, i am new to opengl and i need some help.

suppose i am looking at a circular disc placed on its flat edge with my line of sight on the same plane as the disc is lying. at this point, i should see a straight flat line. when i increase my angle of elevation, my line of sight is now at an angle from the plane in which the disc is lying (ie, i am slowly moving upward), i should gradually see a curve forming. what i am interested in knowing is that are there any algorithms that relate the angle of elevation to the curvature of the edge. Is it possible to get the coordinates of the curve formed with each angle of elevation? Have been searching the net for algorithms but nothing similar came up.

mikael_aronsson
10-15-2001, 11:44 PM
Hi !

I guess you could try to use OpenGL feedback to get information about it maybe ?

Mikael

JML
10-16-2001, 12:03 AM
What are you trying to achieve?
Perhaps it is enough to just draw the disc in 3D and let OpenGL create the result you want?

HTH

Jean-Marc

skystorm
10-16-2001, 12:56 AM
hi,
actually, i am trying to isolate the edge of the disc. at 0 deg, i will get a straight line, at maybe 5 deg, i get a slight curve, then at 10deg, another curve, but with greater amount of curvature. so i am trying to see if there is a relationship that defines how the angle of elevation will affect the curve u get. true, i can get it done in opengl, but i am really interested in knowing if such a relationship exists, or if some algo exists.

thks for ur feedback.

T
10-16-2001, 01:56 AM
Hi,

By disk, you mean circle, right?

When looking along the disk's plane you see a line. The major axis is normal length but the minor axis is 0.

When looking straight down on the disk (at 90 degrees to the original plane) you see a circle. The major and minor axis are normal length

Does it follow then that at 45 degrees you will see an elipse where the major axis is normal and the minor is exactly radius/2.

This does not take perspective into account. Its orthogonal projection or something.

The shape will always be an elipse.

Tim.

skystorm
10-16-2001, 08:18 PM
hi,
my mistake in referring to the ellipse as a curve, but that's because I am only interested in the near edge, which should give me a curve. anyway, i am having a difficult time seeing why at 45 deg, u should see the minor axis with a length of radius/2. Here, I am assuming that when the person's line of sight is elevated, it is along the y axis ie, the distance between the eye and the edge increases and does not remain constant as in moving in a circle. i thought there should be something that relates the curve u see, to the angle of elevation, but there seems to be nothing coming up.