Use a matrix for the tilted camera stuff.

data/dome.vs

@@ -18,16 +18,21 @@ float ystretch = (aspect > 1.0) ? 1.0/aspect : 1.0;
 float stilt = sin(radians(tilt));
 float ctilt = cos(radians(tilt));
 
+mat4 Ttilt = mat4( 1.0, 0.0,   0.0,    0.0,
+		   0.0,  ctilt, -stilt, 0.0,
+		   0.0,  stilt,  ctilt, 0.0,
+		   0.0, 0.0,   0.0,    1.0 );
+
+mat4 TtiltModelView = Ttilt * gl_ModelViewMatrix;
+
 void main() {
-   vec4 p = gl_ModelViewMatrix * gl_Vertex;
-   float yt = p.y*ctilt - p.z*stilt;
-   float zt = p.z*ctilt + p.y*stilt;
+   vec4 p = TtiltModelView * gl_Vertex;
 
-   float r = sqrt(p.x*p.x + yt*yt);
-   float arc = atan( r / zt );
-   float w = (zt < -nearclip && abs(arc) < pi_2) ? pi_2*zoom : -pi_2*zoom;
+   float r = sqrt(p.x*p.x + p.y*p.y);
+   float arc = atan( r / p.z );
+   float w = (p.z < -nearclip && abs(arc) < pi_2) ? pi_2*zoom : -pi_2*zoom;
    float arc_r = arc / r;
-   gl_Position = vec4( xstretch * p.x * arc_r, ystretch * yt * arc_r, 0.5, w );
+   gl_Position = vec4( -xstretch * p.x * arc_r, -ystretch * p.y * arc_r, 0.5, w );
    gl_FrontColor = gl_Color;
    gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
 }