如何创建具有非预定义顶点数的 SCNode？

Question

我有一个包含 SCNVector3 顶点的数组来描述轨迹路径。我想画出它的路径。

- (SCNNode *)lineNodeFromVertices:(SCNVector3 *)vertices count:(NSUInteger)count
{
    NSInteger numberOfVertices = (count - 1) * 2;
    int *verticesSequence = calloc(numberOfVertices, sizeof(int));

    for (int index = 0; index < count; index ++)
    {
        if (index > 0 && index < count - 1)
        {
            verticesSequence[index * 2 - 1] = index;
            verticesSequence[index * 2] = index;
        }
        else if (index == 0)
        {
            verticesSequence[0] = index;
        }
        else if (index == count - 1)
        {
            verticesSequence[index * 2 - 1] = index;
        }
    }

    NSData *sequenceData = [NSData dataWithBytes:verticesSequence
                                          length:sizeof(verticesSequence)];

    free(verticesSequence);

    SCNGeometryElement *element = [SCNGeometryElement geometryElementWithData:sequenceData                                                                primitiveType:SCNGeometryPrimitiveTypeLine
                                                               primitiveCount:(count - 1)
                                                                bytesPerIndex:sizeof(int)];

    SCNGeometrySource *source = [SCNGeometrySource geometrySourceWithVertices:vertices
                                                                        count:count];

    SCNGeometry *line = [SCNGeometry geometryWithSources:@[source]
                                                elements:@[element]];

    return [SCNNode nodeWithGeometry:line];
}

当我将动态创建的数组作为创建 SCNGeometryElement 的参数传递时，它不起作用。

但是当我传递具有预定义元素数量的数组时，它可以正常工作。 比如：

// Sample data
SCNVector3 vertices[] =
{
    SCNVector3Make(1.0, 1.5, 0.5),
    SCNVector3Make(0.8, 1.0, 0.2),
    SCNVector3Make(0.4, 2.0, 1.2),
    SCNVector3Make(0.0, 2.5, 2.7),
    SCNVector3Make(-0.2, 2.0, 4.0),
    SCNVector3Make(-0.4, 0.5, 5.0),
    SCNVector3Make(-0.3, 3.0, 3.0)
};

int count = sizeof(vertices) / sizeof(vertices[0]);

// Array with predefined vertices sequence 
int verticesSequence[] =
{
    0, 1,
    1, 2,
    2, 3,
    3, 4,
    4, 5,
    5, 6
};

NSData *sequenceData = [NSData dataWithBytes:verticesSequence
                                      length:sizeof(verticesSequence)];

SCNGeometryElement *element = [SCNGeometryElement geometryElementWithData:sequenceData                                                            primitiveType:SCNGeometryPrimitiveTypeLine
                                                           primitiveCount:(count - 1)
                                                            bytesPerIndex:sizeof(int)];

SCNGeometrySource *source = [SCNGeometrySource geometrySourceWithVertices:vertices
                                                                    count:count];

SCNGeometry *line = [SCNGeometry geometryWithSources:@[source]
                                            elements:@[element]];

[self.rootSceneNode addChildNode:[SCNNode nodeWithGeometry:line]];

是否存在其他方法来创建具有非预定义顶点数的线节点？

Answer 1

您面临的问题是 int 指针的大小是指针本身的大小，而不是它指向的内存。

所以在动态分配内存的时候，大小（我跑的时候）是8

int *verticesSequence = calloc(numberOfVertices, sizeof(int));
sizeof(verticesSequence); // 8

与当时预定义的数组相比，它的大小为 48（当我运行它时）

int verticesSequence[] = { 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6 };
sizeof(verticesSequence); // 48

这会导致一个问题，当你创建一个 NSData 对象时，你只能得到它的前 8 个字节。

相反，您想要的是分配内存的大小 (sizeof(int) * numberOfVertices)

sizeof(int)*numberOfVertices; // 48

我给了a similar explanation for Swift in this answer。

---

所以，更改 NSData 创建以使用它

NSData *sequenceData = [NSData dataWithBytes:verticesSequence
                                      length:sizeof(int)*numberOfVertices];