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Started by Sean Doherty Nov 22, 2003 at 3:16 PM 2 replies 700+ views

Original Post

Nov 22, 2003 at 3:16 PM

Since I know next to nothing about OpenGL, would someone please explain what this is doing? The following is partial source code if from Free Space:

trail_calc_facing_pts( &topv, &botv, fvec, &pos, w );

g3_rotate_vertex( ⊤, &topv );
g3_rotate_vertex( ⊥, &botv );
top.a = bot.a = l;	

vertex *vlist[4];
vlist[0] = &last_bot;
vlist[1] = ⊥
vlist[2] = ⊤
vlist[3] = &last_top;

vlist[0]->u = 0.0f;  vlist[0]->v = 0.0f;
vlist[1]->u = 1.0f;  vlist[1]->v = 0.0f;
vlist[2]->u = 1.0f;  vlist[2]->v = 1.0f;
vlist[3]->u = 0.0f;  vlist[3]->v = 1.0f;

gr_set_bitmap(ti->bitmap, GR_ALPHABLEND_FILTER, GR_BITBLT_MODE_NORMAL, l/255.0f );
if ( D3D_enabled )	{
    g3_draw_poly( 4, vlist, TMAP_FLAG_TEXTURED|TMAP_FLAG_ALPHA|TMAP_FLAG_GOURAUD );
}
else
{
    g3_draw_poly( 4, vlist, TMAP_FLAG_TEXTURED );
}

It looks similar to locking the vertex buffer in DirectX and manipulating the vertices; but I have no idea what "g3_rotate_vertex( ⊤, &topv );" is doing? _______________________________________ Understanding is a three edged sword...

OrangyTang

333

OrangyTang

Nov 22, 2003 at 3:48 PM

quote:
Original post by Sean Doherty I have no idea what "g3_rotate_vertex( ⊤, &topv );" is doing?

And neither have I, since none of that is actually OpenGL code. All those underscored_function_names must be somewhere else in the source.

Sean Doherty

Author

Sean Doherty

Nov 22, 2003 at 5:26 PM

quote:

And neither have I, since none of that is actually OpenGL code. All those underscored_function_names must be somewhere else in the source.

Fair enough; how about describing this in terms of DirectX (it is OpenGL):


void VectorAlignedBillBoard::display(
        const Vector &cameraLocation,
        const Vector &billBoardLocation,
        const Vector &billBoardAxis,
        float halfLength,
        float halfWidth)
    {
    //the camera location is the camera view point (naming issues)


    //we define the four corners based on the direction of

    //travel and the cross product of the direction of travel

    //with the camera direction. so that the billboard is allways

    //showing as much as it can towards the camera

    Vector cameraDirection = cameraLocation - billBoardLocation;
    Vector lateralVector = crossProduct(cameraDirection,billBoardAxis);
        
    //the travel direction and the lateral vector will

    //be used to construct the four points of the quad that

    //defines the objects display primitive

    lateralVector = lateralVector.withLength(halfWidth);
    Vector forwardVector = billBoardAxis.withLength(halfLength);
    
    glBegin(GL_TRIANGLE_STRIP);
        //top right

        glTexCoord2f(1.0,0.0);
        gVertex(forwardVector + lateralVector + billBoardLocation);
        //top left

        glTexCoord2f(0.0,0.0);
        gVertex(lateralVector - forwardVector + billBoardLocation);
        //bottom right

        glTexCoord2f(1.0,1.0);
        gVertex(forwardVector - lateralVector + billBoardLocation);
        //bottom left

        glTexCoord2f(0.0,1.0);
        gVertex(-lateralVector - forwardVector + billBoardLocation);
    glEnd();
    }

_______________________________________
Understanding is a three edged sword...

benjamin bunny

144

benjamin bunny

Nov 22, 2003 at 5:39 PM

This is the OpenGL forum. If you need to know how to display a billboard with D3D, perhaps you should ask on the appropriate forum.

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