Vertex Shader/Defined Outputs

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< Vertex Shader
Revision as of 00:09, 26 July 2013 by Alfonse (talk | contribs) (Better description of them.)
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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.

gl_Position is the clip-space output position of the current vertex.

gl_PointSize is the pixel width/height of the point being rasterized. It is only necessary to write to this when rendering point primitives.

gl_ClipDistance allows the shader to set the distance from the vertex to each clip plane. A positive 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. In order to use this variable, the user must manually redeclare it with an explicit size.