HalfCylinder.m

//
// a snippet for for iOS scenekit
// generate an open half cylinder in such a way that the texture stretches from 0->1 over the surface
// the idea is that this could be used for a 180' pano
//

- (SCNGeometry*) createCurve:(int)resolution width:(float)radius {

    if(resolution<1)return nil;

    int nquadsaxis = resolution;
    int nvertsaxis = resolution+1;
    int nvertices = nvertsaxis*nvertsaxis;
    int ntriangles = nquadsaxis*nquadsaxis*2;

    SCNVector3 normals[nvertices];
    SCNVector3 vertices[nvertices];
    CGPoint textures[nvertices];
    int indexes[ntriangles*3];

    for(int i = 0; i < nvertsaxis; i++) {
        float x = cos(((float)i)*M_PI/((float)resolution));
        float z = sin(((float)i)*M_PI/((float)resolution));
        for(int j = 0; j < nvertsaxis; j++) {
            normals[i+j*nvertsaxis] = SCNVector3Make(x,0,z);
        }
    }

    for(int i = 0; i < nvertsaxis; i++) {
        float x = -cos(((float)i)*M_PI/((float)resolution))*radius;
        float z = sin(((float)i)*M_PI/((float)resolution))*radius;
        for(int j = 0; j < nvertsaxis; j++) {
            float y = (((float)j)/((float)resolution))*radius*2-radius;
            vertices[i+j*nvertsaxis] = SCNVector3Make(x,y,z);
        }
    }

    for(int i = 0; i < nvertsaxis; i++) {
        float x = ((float)i)/((float)resolution);
        for(int j = 0; j < nvertsaxis; j++) {
            float y = ((float)j)/((float)resolution);
            textures[i+j*nvertsaxis] = CGPointMake(x,y);
        }
    }
    
    for(int i = 0 ; i < nvertices; i++) {
        NSLog(@"t %f %f", textures[i].x, textures[i].y);
    }
    
    for(int i = 0; i < nquadsaxis; i++) {
        for(int j = 0; j < nquadsaxis; j++) {
            indexes[(i+j*nquadsaxis)*2*3+0] = i+j*nvertsaxis;
            indexes[(i+j*nquadsaxis)*2*3+1] = i+j*nvertsaxis+1;
            indexes[(i+j*nquadsaxis)*2*3+2] = i+j*nvertsaxis+nvertsaxis;
            indexes[(i+j*nquadsaxis)*2*3+3] = i+j*nvertsaxis+1;
            indexes[(i+j*nquadsaxis)*2*3+4] = i+j*nvertsaxis+nvertsaxis+1;
            indexes[(i+j*nquadsaxis)*2*3+5] = i+j*nvertsaxis+nvertsaxis;
        }
    }

    SCNGeometrySource *normalSource  = [SCNGeometrySource geometrySourceWithNormals:normals count:nvertices];
    SCNGeometrySource *vertexSource  = [SCNGeometrySource geometrySourceWithVertices:vertices count:nvertices];
    SCNGeometrySource *textureSource = [SCNGeometrySource geometrySourceWithTextureCoordinates:textures count:nvertices];
    
    NSData *indexData = [NSData dataWithBytes:indexes length:ntriangles*3*sizeof(int)];
    
    SCNGeometryElement *element = [SCNGeometryElement geometryElementWithData:indexData primitiveType:SCNGeometryPrimitiveTypeTriangles
                                 primitiveCount:ntriangles bytesPerIndex:sizeof(int)];

    SCNGeometry *geometry = [SCNGeometry geometryWithSources:@[vertexSource, normalSource, textureSource] elements:@[element]];
    
    return geometry;
}