Metal 着色语言编程指南 五

(.) 操作符访问矢量成员 

    Metal 也支持(.) 操作符来访问矢量成员， (.) 操作符后面跟随字符来表示坐标或者颜色： <Vector_data_type>.xyzw 或者 <Vector_data_type>.rgba。

   示例代码如下

int4 test = int4(0, 1, 2, 3);int a = test.x;  //  a = 0int b = test.y;  //  b = 1int c = test.z;  //  c = 2int d = test.w;  //  d = 3int e = test.r;  //  e = 0int f = test.g;  //  f = 1int g = test.b;  //  g = 2int h = test.a;  //  h = 3

  (.) 操作符也可以同时选择多个成员:

float4 c;c.xyzw = float4(1.0f, 2.0f, 3.0f, 4.0f);c.z = 1.0f;c.xy = float2(3.0f, 4.0f);c.xyz = float3(3.0f, 4.0f, 5.0f);

应用(.)操作符来复制或者交换矢量成员：

float4 pos = float4(1.0f, 2.0f, 3.0f, 4.0f);float4 swiz = pos.wzyx; // swiz = (4.0f, 3.0f, 2.0f, 1.0f)float4 dup = pos.xxyy;  // dup = (1.0f, 1.0f, 2.0f, 2.0f)

应用(.)操作符给矢量成员复制，  当矢量成员成为左值(lvalue)的时候， 用来标示其成员索引的字母(xyzw或者rgba)是不可重复的:

float4 pos = float4(1.0f, 2.0f, 3.0f, 4.0f);// pos = (5.0, 2.0, 3.0, 6.0)pos.xw = float2(5.0f, 6.0f);// pos = (8.0, 2.0, 3.0, 7.0)pos.wx = float2(7.0f, 8.0f);// pos = (3.0, 5.0, 9.0, 7.0)pos.xyz = float3(3.0f, 5.0f, 9.0f);