gl_Position - OpenGL 4 Reference Pages - Khronos Group

The value of gl_Position (or the gl_Position member of the gl_out[] array, in the case of the tessellation control shader) is undefined after the vertex, ... Name gl_Position—containsthepositionofthecurrentvertex Declaration gl_Positionisamemberofthe gl_PerVertexnamedblock: outgl_PerVertex{ vec4gl_Position; floatgl_PointSize; floatgl_ClipDistance[]; }; Description Inthevertex,tessellationevaluationandgeometrylanguages,asingle globalinstanceofthegl_PerVertexnamedblockisavailableand itsgl_Positionmemberisanoutputthatreceivesthe homogeneousvertexposition.Itmaybewrittenatanytimeduringshaderexecution. Thevaluewrittentogl_Positionwillbeusedbyprimitiveassembly, clipping,cullingandotherfixedfunctionalityoperations,ifpresent,thatoperateon primitivesaftervertexprocessinghasoccurred. Inthetessellationcontrollanguage,thegl_PerVertexnamedblock isusedtoconstructanarray,gl_out[],whosegl_Position membersholdthehomogeneouscontrolpointposition,whichbecomeavailableasinputstothesubsequent tessellationevaluationshader. Thevalueofgl_Position(orthegl_Positionmemberofthegl_out[] array,inthecaseofthetessellationcontrolshader) isundefinedafterthevertex,tessellationcontrol,andtessellationevaluation shadingstagesifthecorrespondingshaderexecutabledoes notwritetogl_Position.Itisalsoundefinedafterthegeometryprocessingstageifthegeometryshaderexecutablecalls EmitVertexwithouthaving writtengl_PositionsincethelastcalltoEmitVertex (orhasn'twrittenitatall). Inthetessellationcontrol,tessellationevaluationandgeometrylanguages, thegl_PerVertexnamedblockisusedtoconstructanarray,gl_in[] ofper-vertexorper-controlpointinputswhosecontentrepresentsthecorresponding outputswrittenbythepreviousstage. VersionSupport OpenGLShadingLanguageVersion FunctionName 1.10 1.20 1.30 1.40 1.50 3.30 4.00 4.10 4.20 4.30 4.40 4.50 gl_Position ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ Versions1.10to1.40-vertexshaderonly. Versions1.50to3.30-vertexandgeometryshadersonly. SeeAlso gl_PointSize,gl_ClipDistance Copyright Copyright©2011-2014KhronosGroup. Thismaterialmaybedistributedsubjecttothetermsandconditionssetforthin theOpenPublicationLicense,v1.0,8June1999. http://opencontent.org/openpub/.