[OpenGL] Earth atmosphere effect

[User137]

What i was thinking to fix that is calculating the rotation matrix differently. Find out x,y,z for the center of planet as it is transformed with Modelview matrix and then calculate a "look-at" matrix from that point to eye center, that is in the middle of near plane in frustum... or something

[Brainer]

What i was thinking to fix that is calculating the rotation matrix differently. Find out x,y,z for the center of planet as it is transformed with Modelview matrix and then calculate a "look-at" matrix from that point to eye center, that is in the middle of near plane in frustum... or something

What do you think of this? Is this of any help maybe?

[User137]

Yes, billboarding is exactly the technique used in the case. The glow is sort of sprite that you turn towards camera.

[Brainer]

Yip, I agree, but isn't it the technique that is used in my case? Why doesn't it work?

[Mirage]

Yip, I agree, but isn't it the technique that is used in my case? Why doesn't it work?

Obviously, because 3D sphere and 2D billboard will be deformed differently by perspective. Try to flattern the sphere in vertex shader along view vector.

[Brainer]

I don't know anything about vertex shaders, so that's all black magic to me. I found a little demo on Nehe's website, here's the conversion:
[pascal]
{ .: GetCameraPosAndUp :. }
procedure GetCameraPosAndUp(var camPos, camUp: TAffineVector);
var
ViewMatrix: array[0..15] of Single;
M: TMatrix;
begin
glGetFloatv(GL_MODELVIEW_MATRIX, @ViewMatrix);

camPos := AffineVectorMake(-ViewMatrix[12], -ViewMatrix[13], -ViewMatrix[14]);
camUp := AffineVectorMake(ViewMatrix[1], ViewMatrix[5], ViewMatrix[9]);
ViewMatrix[12] := 0.0;
ViewMatrix[13] := 0.0;
ViewMatrix[14] := 0.0;
M := ConvertArrayToMatrix(ViewMatrix);
TransposeMatrix(M);

camPos := AffineVectorMake(VectorTransform(VectorMake(camPos ), M));
end;

{ .: CreateBillboardMatrix :. }
procedure CreateBillboardMatrix(var AMat: TMatrix; const ARight, AUp, ALook,
APos: TAffineVector);
begin
AMat[0][0] := ARight[0];
AMat[0][1] := ARight[1];
AMat[0][2] := ARight[2];
AMat[0][3] := 0.0;
AMat[1][0] := AUp[0];
AMat[1][1] := AUp[1];
AMat[1][2] := AUp[2];
AMat[1][3] := 0.0;
AMat[2][0] := ALook[0];
AMat[2][1] := ALook[1];
AMat[2][2] := ALook[2];
AMat[2][3] := 0.0;
// Add the translation in as well.
AMat[3][0] := APos[0];
AMat[3][1] := APos[1];
AMat[3][2] := APos[2];
AMat[3][3] := 1.0;
end;

{ .: BillboardAxis :. }
procedure BillboardAxis(const Pos, Axis, camPos: TAffineVector);
var
Look, Up, Right: TAffineVector;
begin
Look := VectorSubtract(camPos, Pos);
NormalizeVector(Look);

Up := Axis;
Right := VectorCrossProduct(Up, Look);
NormalizeVector(Right);

Look := VectorCrossProduct(Right, Up);

CreateBillboardMatrix(M, Right, Up, Look, Pos);
end;
[/pascal]
But this doesn't work either. Here are the screenshots - Screen #1, Screen #2.

Any more ideas?

[Mirage]

I did this once. Sphere (planet) is volumetric and due to perspective its projection to the screen is not always a circle. You need to make your planet flat too. This most easily can be done by a vertex shader. But you can flattern it while drawing since you are using glBegin/glEnd, not VBO.
In other words, for each vertice of the planet you do the following:

DirP := DotProduct(Vertices[i], Direction);
DrawVertices[i] := SubVector(Vertices[i], ScaleVector(Direction, DirP);

Direction is normalised direction vector from camera to center of the planet.

[Brainer]

Don't get me wrong, but what's the point in doing so? I think that the problem is with billboard matrix calculation, not the sphere itself.

[farcodev_]

just try to take a look, in the GLScene CVS branch, for GLAtmosphere, i use it in my game and work well.

[Brainer]

Yip, I tried to do so, but no success. Besides, as you can read in the comments, it uses up a lot of processing resources, and for all I can see it's possible to achieve the same result in a more efficient way with that glow texture.