在iOS上使用OpenGL ES 2.0时，如何在两点之间绘制圆柱体? [英] Using OpenGL ES 2.0 with iOS, how do I draw a cylinder between two points?

问题描述

我给了两个GLKVector3，分别代表圆柱体的起点和终点.使用这些点和半径，我需要构建和渲染一个圆柱体.我可以构建一个圆柱体，使它们之间的点之间的距离正确，但方向固定(当前始终沿y(0，1，0)向上).我不确定我需要进行哪种计算才能使圆柱体位于两点之间的正确平面上，从而使一条线穿过两个端点.我在考虑使用方向矢量或角度创建顶点数据时可以应用某种计算，这些计算将创建指向正确方向的圆柱体.有没有人有一种算法，或者知道一种算法会有所帮助?

I am given two GLKVector3's representing the start and end points of the cylinder. Using these points and the radius, I need to build and render a cylinder. I can build a cylinder with the correct distance between the points but in a fixed direction (currently always in the y (0, 1, 0) up direction). I am not sure what kind of calculations I need to make to get the cylinder on the correct plane between the two points so that a line would run through the two end points. I am thinking there is some sort of calculations I can apply as I create my vertex data with the direction vector, or angle, that will create the cylinder pointing the correct direction. Does anyone have an algorithm, or know of one, that will help?

推荐答案

检查该真棒文章;它已经过时，但是在修改算法后，它就像一个魅力.一个提示，OpenGL ES 2.0仅支持三角形，因此，与其使用GL_QUAD_STRIP(而不是方法)，而是使用GL_TRIANGLE_STRIP，结果是相同的. 站点还包含许多其他有关OpenGL几何形状的有用信息.

Check this awesome article; it's dated but after adapting the algorithm, it works like a charm. One tip, OpenGL ES 2.0 only supports triangles so instead of using GL_QUAD_STRIP as the method does, use GL_TRIANGLE_STRIP instead and the result is identical. The site also contains a bunch of other useful information regarding OpenGL geometries.

有关解决方案，请参见下面的代码. Self代表网格，并包含顶点，索引等.

See code below for solution. Self represents the mesh and contains the vertices, indices, and such.

- (instancetype)initWithOriginRadius:(CGFloat)originRadius
                   atOriginPoint:(GLKVector3)originPoint
                    andEndRadius:(CGFloat)endRadius
                      atEndPoint:(GLKVector3)endPoint
                   withPrecision:(NSInteger)precision
                        andColor:(GLKVector4)color
{
self = [super init];

if (self) {
    // normal pointing from origin point to end point
    GLKVector3 normal = GLKVector3Make(originPoint.x - endPoint.x,
                                       originPoint.y - endPoint.y,
                                       originPoint.z - endPoint.z);

    // create two perpendicular vectors - perp and q
    GLKVector3 perp = normal;
    if (normal.x == 0 && normal.z == 0) {
        perp.x += 1;
    } else {
        perp.y += 1;
    }

    // cross product
    GLKVector3 q = GLKVector3CrossProduct(perp, normal);
    perp = GLKVector3CrossProduct(normal, q);

    // normalize vectors
    perp = GLKVector3Normalize(perp);
    q = GLKVector3Normalize(q);

    // calculate vertices
    CGFloat twoPi = 2 * PI;        
    NSInteger index = 0;
    for (NSInteger i = 0; i < precision + 1; i++) {
        CGFloat theta = ((CGFloat) i) / precision * twoPi; // go around circle and get points

        // normals
        normal.x = cosf(theta) * perp.x + sinf(theta) * q.x;
        normal.y = cosf(theta) * perp.y + sinf(theta) * q.y;
        normal.z = cosf(theta) * perp.z + sinf(theta) * q.z;

        AGLKMeshVertex meshVertex;
        AGLKMeshVertexDynamic colorVertex;

        // top vertex
        meshVertex.position.x = endPoint.x + endRadius * normal.x;
        meshVertex.position.y = endPoint.y + endRadius * normal.y;
        meshVertex.position.z = endPoint.z + endRadius * normal.z;
        meshVertex.normal = normal;
        meshVertex.originalColor = color;

        // append vertex
        [self appendVertex:meshVertex];

        // append color vertex
        colorVertex.colors = color;
        [self appendColorVertex:colorVertex];

        // append index
        [self appendIndex:index++];

        // bottom vertex
        meshVertex.position.x = originPoint.x + originRadius * normal.x;
        meshVertex.position.y = originPoint.y + originRadius * normal.y;
        meshVertex.position.z = originPoint.z + originRadius * normal.z;
        meshVertex.normal = normal;
        meshVertex.originalColor = color;

        // append vertex
        [self appendVertex:meshVertex];

        // append color vertex
        [self appendColorVertex:colorVertex];

        // append index
        [self appendIndex:index++];
    }

    // draw command
    [self appendCommand:GL_TRIANGLE_STRIP firstIndex:0 numberOfIndices:self.numberOfIndices materialName:@""];
}

return self;
}

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