GLPOSITION

gl_Position is a built-in output variable of the vertex shader in the OpenGL Shading Language (GLSL). It is of type vec4 and specifies the vertex’s position in clip space. The vertex shader must assign a value to gl_Position for every processed vertex. The x, y, and z components indicate the vertex position after applying the vertex transformation, while w provides the homogeneous coordinate used for perspective division.

In the graphics pipeline, coordinates in clip space are subject to clipping against the canonical view volume. After clipping, the coordinates are divided by w to produce normalized device coordinates (NDC), which are then mapped to window coordinates by the viewport transform. Therefore, gl_Position directly influences visibility, projection, and the final screen position of each vertex.

A common usage pattern is to compute a model–view–projection transform and assign the result to gl_Position, for example: gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4(vertexPosition, 1.0); The exact form depends on the shader and the available matrices, but the concept remains that gl_Position encodes the vertex’s clip-space position.

gl_Position is a GLSL vertex-shader feature and is not accessible in the fragment shader. If it is not written in a vertex shader, or if w is zero, the resulting screen position is undefined or erroneous. In practice, ensuring a valid, nonzero w and correct transformation is essential for correct rendering.