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gakman

World to Screen Math and Translate/Rotation Transforms

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Hello This is sort-of a two question post. For a while now I''ve been trying to work out how to get a 3d coordinate in space to translate into a 2D screen. I found a topic in this forum where it was answered. I also found a few places on the net about it which included perspective in the caclulations. I''m probably wrong on a few things I say here so please correct me! Anyway at the moment I reset the modelview matrix and do a glTranslatef(0,0,-10) then draw a 3d tileset along the Z axis (so it''s facing the screen). The tiles look like this: for (x = 0; x < w; x++) { for (y = 0; y < h; y++) { glBegin(...) glVertex3f(x,y,0); // .. rest of the tile glEnd() } } To get the 2D screen location of each tile I do this: // 45 is my FOV (run by gluPerspective(45.0f, 1, 1, 100)) // -- but I dont think that''s right... // sw/sh: screen width/height // wx/wy/wz world co-ords // // i get wz because of the translate. wz = 10. // wx, wy is the same as veretx3f(wx,wy,0) when drawing the tile. sx = sw * tan(45.0f / (PI * 2)) * ( wx / wz ) + sw / 2 sy = sh * tan(45.0f / (PI * 2)) * ( wy / wz ) + sw / 2 This works perfectly. sx/sy lines up fine. tan(45.0f / PI2) comes out to be 1.206680. Which is odd because on calc tan 45 = 1 (45 /PI*2 is the radian equiv to 45deg''s shouldn''t it??)... I also still dont think 45 relates to my fov since i change it slightly and the result is completely off. So far so good (almost). The thing that''s completely throwing me is if I do a glTranslatef(0,0,-10) then I do a glRotatef(-30, 1, 0, 0), I have no idea on where my 3D points are. They get drawn in the place I expect -- it looks like a typical RTS layout where the lower tiles are bigger and look closer then the higher drawn ones. Should I work out the position of each tile without doing a glRotatef (and translatef)? My trig maths isn''t too bad, but I have no clue on how to do that... Your help is appreciated! Thanks, Gerald

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Ah hah!

The second part of my question isn''t as hard as it seems.

Yes, I don''t have to use glRotatef. Instead for each vertex I want to rotate on the X axis, I do:

// a being the angle
// px/py/pz being the vertex
py = x
py = cos(a) * y
pz = sin(a) * y

It''s just a simple 2D circle when you look down the X axis!

I still don''t fully understand how the tangent relates to the distance/fov. I''ll post here if I find out

Thanks,
Gerald

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