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Billboarding problem - Billboards work but I dont want them to rotate on the y axis

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I got billboards working properly (to use ShaderX's texture perturbation fire effect) in my 3d engine and I only want it to rotate on the x and z axis (horizontally, not vertically). Code used DXmatWorld = Matrix.Identity DXmatTemp = Matrix.Identity DXmatTemp = Matrix.TransposeMatrix(DXmatView) With DXmatTemp .M14 = 0 .M24 = 0 .M34 = 0 .M41 = 0 .M42 = 0 .M43 = 0 End With D3DDevice.SetTransform(TransformType.World, DXmatTemp) D3DDevice.DrawUserPrimitives(PrimitiveType.TriangleStrip, FireVert.Length / 2, FireVert) DXmatWorld = Matrix.Identity D3DDevice.SetTransform(TransformType.World, DXmatWorld) It works great for particles (the matrix), but I want it to preserve the y axis using this code. Any ideas? Edit: Fire is rendered at 0,0,0 (for now). That is why M41,M42,M43 are 0,0,0.

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Well i admit i'm having a guess here but have you tried removing the y component of the view matrix before you transpose it, and then adding it back in after the view has been transposed?

This might make sense, but im actually in a class at uni now, so havn't got the time to look it up.

ace

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I have just recently been working on billboarding myself. I posted something here not too long ago about not being able to use the D3DXMatrixTranspose function, but I found a workaround. I only used it on the y axis, but I don't think it would be too hard to adapt to just the x and z.

Firstly I found the position of the viewport (x, y, and z) and the position of the thing to be rotated (x, y, and z). Using these values, I found the difference in positions for all the axis (no idea what the plural of axis is). So here I would have something like this (this is in C++):

float x, y, z, x1, y1, z1; // x, y, and z are the viewport
float XDiff, YDiff, ZDiff;
float XRot, ZRot;

XDiff = (float)x - x1;
YDiff = (float)y - y1;
ZDiff = (float)z - z1;

It's late-ish here in Australia, and I have school tomorrow, so I may have missed out some steps (you might need to use some Pythagoras in here to find the differences, but I'm not sure). But anyway, then I used inverse tan to figure out the angle it had to rotate:

XRot = -1*(atanf(YDiff/ZDiff));
ZRot = -1*(atanf(YDiff/XDiff));

Once again, I only used this while using the y-axis, so I don't know how it will work here. You might find that if the viewport is in a certain position in relation to the object, it might be flipped so it is facing away from you. To solve this you just need to put in a line like:

if(blahblahblah) XRot += 3.14;

Then when you have the rotation, use D3DXMatrixRotationX, and D3DXMatrixRotationZ, then multiply them with the world matrix, and it should work. It might take a little while to get it working properly, but that is the general gist of it.

Like I said at the beginning, I only used this because I couldn't get D3DXMatrixTranspose working. For all I know this might do something bad, but it is how I do it now (it's also how I make my enemies face towards the player!), and it works quite well for me (even if it's only on the y axis).

I really hope this helps.

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Thanks, but I figured it out...
DXmatView = Matrix.LookAtLH(RenderCamPos, New Vector3(RenderCamViewDir.X + RenderCamPos.X, RenderCamViewDir.Y + RenderCamPos.Y, RenderCamViewDir.Z + RenderCamPos.Z), CamUp)

RenderCamPos.Y = 0

DXmatViewFlat = Matrix.LookAtLH(RenderCamPos, New Vector3(RenderCamViewDir.X + RenderCamPos.X, RenderCamViewDir.Y + RenderCamPos.Y, RenderCamViewDir.Z + RenderCamPos.Z), CamUp)

I use DXmatView as the world View matrix and DXmatViewFlat as the transpose matrix...

:)

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