Vertex Shader/Defined Outputs

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Vertex Shaders have the following predefined outputs.

out gl_PerVertex
  vec4 gl_Position;
  float gl_PointSize;
  float gl_ClipDistance[];

gl_PerVertex defines an interface block for outputs. The block is defined without an instance name, so that prefixing the names is not required.

These variables only take on the meanings below if this shader is the last active Vertex Processing stage, and if rasterization is still active (ie: GL_RASTERIZER_DISCARD is not enabled). The text below explains how the Vertex Post-Processing system uses the variables. These variables may not be redeclared with interpolation qualifiers.

the clip-space output position of the current vertex.
the pixel width/height of the point being rasterized. It only has a meaning when rendering point primitives. It will be clamped to the GL_POINT_SIZE_RANGE.
allows the shader to set the distance from the vertex to each user-defined clipping half-space. A non-negative distance means that the vertex is inside/behind the clip plane, and a negative distance means it is outside/in front of the clip plane. Each element in the array is one clip plane. In order to use this variable, the user must manually redeclare it with an explicit size. With GLSL 4.10 or ARB_separate_shader_objects, the whole gl_PerVertex block needs to be redeclared. Otherwise just the gl_ClipDistance built-in needs to be redeclared.