OpenGL ES 3.0和APPLE_clip_distance扩展

根据 this document OpenGL ES 3.0和iPhone 6+硬件上存在APPLE_clip_distance扩展。我试图通过首先在代码中启用它,然后在着色器中使用来使用它。

我可以通过

GLint i;
glGetIntegerv(GL_MAX_CLIP_DISTANCES_APPLE, &i);
printf("i: %d\n", i);

这个打印出8。

尝试在着色器中使用此选项时,请执行以下操作:

const mediump int gl_MaxClipDistances = 8;
out highp float gl_ClipDistance[gl_MaxClipDistances];

我收到以下错误:

ERROR: 0:24: Identifier name 'gl_MaxClipDistances' cannot start with 'gl_'
ERROR: 0:25: Use of undeclared identifier 'gl_MaxClipDistances'
ERROR: 0:33: Use of undeclared identifier 'gl_ClipDistance'

This document 描述了扩展,据我所知,我应该重新声明剪辑距离数组,因为我使用的是ES 3.0:

(1) GLSL 300 doesn't support unsized arrays, how should gl_ClipDistance
be sized?

  RESOLVED: For maximal compatibility, it works like ES2/desktop,
  remaining unsized until sized by direct access or explicitly
  redeclared.  Language describing this behavior must be added to the
  extension specification since it's gone from the base language
  specification.

同时声明如下:

out highp float gl_ClipDistance[];

给出此错误:

ERROR: 0:25: Unsized array 'gl_ClipDistance' requires sized initializer under GLSL 300
ERROR: 0:33: Use of undeclared identifier 'gl_ClipDistance'

有人有这个扩展的工作示例吗?