# OpenGL Sphere/Cylinder mapping with OpenGL

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Tommato    237
Hi All

I need spherical/cylindrical map for arbitrary objects (not a reflection mapping). Actually I'm calculating UV manually. If a polygon crosses UV seam or pole, then I split it into several new polygons. It works but it's a big piece of code that makes app much slower. Are there better ways with OpenGL?

Thanks
Tom

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blueshogun96    2264

Tbh, I don't know the exact algorithms used to do this, but I have managed to do this (at least the spherical) in OpenGL by copying some code from the DirectX 9 SDK (the older ones)  and changing the necessary features to work for OpenGL.  It's been years since I've done this so I can't remember exactly what I changed for the sphere, but it should be obvious enough IMO.

Here's the sphere:

// Establish constants used in sphere generation
DWORD dwNumSphereRings    = m_bHighTesselation ? 15 :  5;
DWORD dwNumSphereSegments = m_bHighTesselation ? 30 : 10;
FLOAT fDeltaRingAngle = ( D3DX_PI / dwNumSphereRings );
FLOAT fDeltaSegAngle  = ( 2.0f * D3DX_PI / dwNumSphereSegments );

D3DXVECTOR4 vT;
FLOAT fScale;

// Generate the group of rings for the sphere
for( DWORD ring = 0; ring < (dwNumSphereRings/2); ring++ )
{
FLOAT r0 = sinf( (ring+0) * fDeltaRingAngle );
FLOAT r1 = sinf( (ring+1) * fDeltaRingAngle );
FLOAT y0 = cosf( (ring+0) * fDeltaRingAngle );
FLOAT y1 = cosf( (ring+1) * fDeltaRingAngle );

// Generate the group of segments for the current ring
for( DWORD seg = 0; seg < (dwNumSphereSegments+1); seg++ )
{
FLOAT x0 =  r0 * sinf( seg * fDeltaSegAngle );
FLOAT z0 =  r0 * cosf( seg * fDeltaSegAngle );
FLOAT x1 =  r1 * sinf( seg * fDeltaSegAngle );
FLOAT z1 =  r1 * cosf( seg * fDeltaSegAngle );

// Add two vertices to the strip which makes up the sphere
// (using the transformed normal to generate texture coords)
(*vtx).p   = (*vtx).n   = D3DXVECTOR3(x0,y0,z0);
D3DXVec3Transform( &vT, &(*vtx).n, &matWorldView );
fScale = 1.37f / D3DXVec4Length( &vT );
(*vtx).tu1 = 0.5f + fScale*vT.x;
(*vtx).tv1 = 0.5f - fScale*vT.y;
vtx++;

(*vtx).p   = (*vtx).n   = D3DXVECTOR3(x1,y1,z1);
D3DXVec3Transform( &vT, &(*vtx).n, &matWorldView );
fScale = 1.37f / D3DXVec4Length( &vT );
(*vtx).tu1 = 0.5f + fScale*vT.x;
(*vtx).tv1 = 0.5f - fScale*vT.y;
vtx++;
}
}


Then draw as a triangle strip.  You'll have to use your own matrix transformation and vector length functions to do the texture coordinates, but that shouldn't be hard.

The cylinder is much easier.  I don't know if you want an open or closed cylinder, I've never done the latter.

for( DWORD i=0; i<50; i++ )
{
FLOAT theta = (2*D3DX_PI*i)/(50-1);

pVertices[2*i+0].position = D3DXVECTOR3( sinf(theta),-1.0f, cosf(theta) );
pVertices[2*i+0].color    = 0xffffffff;
pVertices[2*i+0].tu       = ((FLOAT)i)/(50-1);
pVertices[2*i+0].tv       = 1.0f;

pVertices[2*i+1].position = D3DXVECTOR3( sinf(theta), 1.0f, cosf(theta) );
pVertices[2*i+1].color    = 0xff808080;
pVertices[2*i+1].tu       = ((FLOAT)i)/(50-1);
pVertices[2*i+1].tv       = 0.0f;
}


Then draw as a triangle strip.  This example uses 100 vertices btw.

Not sure if this helped, but I tried.

Shogun.

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Tommato    237

blueshougun96. thx for your reply. I've no problems with UV calculation itself, my question was about "should I do that" or, in other words. "can I avoid manual UV calcs?". An answer "no, you can't"  would be also informative for me, if so I'm doing things right

Thanks

Tom

Edited by Tommato

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blueshogun96    2264

Okay, my mistake.

Generally speaking, it should be fine, that's what I'd do.  I don't know of any way to do it automatically, unless OpenGL has an equivalent of D3DTSS_TCI_x.

Shogun

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