Question

我有一个带有SCNVector3顶点的数组，用于描述轨迹路径。我想画出它的道路。

- (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数量元素的数组时，它工作正常。类似的东西：

// 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]];

是否存在使用非预定义数量的顶点创建线节点的其他方法？

Answer 1

你面临的问题是int指针的大小是指针本身的大小，而不是它指向的内存。

因此，当动态分配内存时，大小（当我运行它时）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，即在创建NSData对象时，只能得到它的前8个字节。

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

sizeof(int)*numberOfVertices; // 48

我给了http://sqlfiddle.com/#!15/11711/1/0。

因此，更改NSData创建以使用

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

如何创建具有非预定义顶点数的SCNode？

1 个答案: