Moving on a sphere facing center

Posted on 2001-08-05
Last Modified: 2013-12-26

Is there an easy way (a supplied function or so) in DX8 to set the view matrix so that the viewer will be able to move around (as on a sphere) an object in all directions always facing the centre of the sphere?

What I do now is this (this allows me to move horizontally around the object in a circle and straight up and down always facing the object):

long lElapsedTime = timeGetTime() - m_dwTime;
m_dwTime = timeGetTime();

fHorzPos += fHorzSpeed * lElapsedTime;
vFromPt.x = sinf(fHorzPos) * 60.0f;
vFromPt.z = cosf(fHorzPos) * 60.0f;

fVertPos += fVertSpeed * lElapsedTime;
vFromPt.y = fVertPos * 60.0f;

D3DXMatrixLookAtLH(&matView, &vFromPt, &vLookatPt, &vUpVec);
m_pd3dDevice->SetTransform(D3DTS_VIEW, &matView);

What I want is that when I move up or down, I always have the same distance to the object.
Do I have to do the calculations myself or is there already a function for this in DX8?

Question by:jacobkristensen
  • 2

Expert Comment

ID: 6358167
There are two methods of specifying your position in space relative to another. The one is to use an x,y,z offset to a reference point, the other is to use the x angular, y angular and range offset. I use the following math to determine a camear position when panning and tilting around an object.

    eye.X = posx(sel) + Sin(((hang) / 57.2958 - heading(sel) / 57.29)) * zoom * Cos(vang / 57.2958)
    eye.Y = posy(sel) + Sin(vang / 57.2958 - roly(sel) / rad) * zoom
    eye.z = posz(sel) + Cos(((hang) / 57.2958 - heading(sel) / 57.29)) * zoom * -1 * Cos(vang / 57.2958)

In the above code snippet. POSX, POSY, POSZ are then x,y,z coordinates of the object in space. ZOOM is the distance to the object. VANG = vertical angular offset (the tilt) and HANG = horizontal angular offset. When I want to pan and tilt the camera around an object, I merely assign the mouse x and y co ordinates to HANG and VANG. ROLY is the pitch of the object being viewed. In other words, if in this case, the aircraft goes nose down, the camera angle in increased so as to keep a constant vertical viewing angle to the craft. Likewise, heading is the direction that the target craft is facing. If the target turns right or left, so the camera turns, maintaining a constant orientation to the target.

In order to translate an X,Y,Z coordinate into a XANG, YANG, RANGE co ordinate, use the following routine.

ixdif = eyeposx - targetposx
iydif = eyeposz - targetposz
xang = itnang
ixdif = itargetrange
iydif = eyeposy - targetposy
yang = itnang
range = itargetrange

Sub getcord()
        itargetrange = Sqr(ixdif ^ 2 + iydif ^ 2)
        If iydif <> 0 Then itgan = Atn(ixdif / iydif) * 57.2958
        If iydif = 0 And ixdif = 0 Then itgan = 0: itgang = 0
        If iydif = 0 And ixdif < 0 Then itgan = 270
        If iydif = 0 And ixdif > 0 Then itgan = 90
        itgang = itgan
        If iydif < 0 Then itgang = 180 + itgan
        If ixdif < 0 And iydif > 0 Then itgang = 360 + itgan
        itnang = ina + itgang
        If itnang > 360 Then itnang = itnang - 360
        If itnang < 0 Then itnang = itnang + 360
End Sub

Unfortunately the code is in visual basic ... but the math is correct. Let me know if u need any further assistance

Accepted Solution

cybermike3d earned 100 total points
ID: 6358201
Just an afterthought ... is your aplication for a camera or for an object moving around an object that must allways point at the targets centre like a compass. If your app. is for a camera ... take a look at the LOOKAT function.

    eye.X = posx(sel) + Sin(((hang) / 57.2958 - heading(sel) / 57.29)) * zoom * Cos(vang / 57.2958)
    eye.Y = posy(sel) + Sin(vang / 57.2958 - roly(sel) / rad) * zoom
    eye.z = posz(sel) + Cos(((hang) / 57.2958 - heading(sel) / 57.29)) * zoom * -1 * Cos(vang / 57.2958)
    tar.X = posx(sel)
    tar.Y = posy(sel)
    tar.z = posz(sel)
    rollz = roll(sel) * 0.2 * rollmag
'roll the camera as the rarget banks
    orr.X = (Sin((heading(sel) - 90) / 57.29)) * rollz / 30
    orr.Y = Cos(rollz / 57.29)
    orr.z = (Cos((heading(sel) - 90) / 57.29)) * rollz / 30
    D3DXMatrixLookAtLH matView, eye, tar, orr
    g_D3DDevice.SetTransform D3DTS_VIEW, matView
    D3DXMatrixPerspectiveFovLH matProj, fov, 1, 0.001, 100000000
    g_D3DDevice.SetTransform D3DTS_PROJECTION, matProj

If it is an object ... then use the top algorithm.  then apply the HANG, VANG variables to the object that is moving on the YAW and PITCH of the SETYAWPITCHROLL angular translation of that object.

Author Comment

ID: 6371360
Hi CyberMike3D

Thanks for your answers.

What I was doing, was moving the camera around the object and as you can see I was also using the D3DXMatrixLookAtLH function.

I ended up making a lot of changes to my game, so now I'm actually rotating the objects instead of moving the camera.

I'm happy to give you the points though, as you (of cource) gave me the answer for what I asked in the first place...

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