gl_BaryCoordEXT vs gl_BaryCoordNoPerspEXT in GLSL

In OpenGL and Vulkan shading via the GL_EXT_fragment_shader_barycentric extension, gl_BaryCoordEXT and gl_BaryCoordNoPerspEXT are built-in fragment shader input variables that expose the barycentric coordinates of a fragment within a primitive. The fundamental difference between them lies in how interpolation is handled: gl_BaryCoordEXT provides perspective-correct barycentric coordinates, whereas gl_BaryCoordNoPerspEXT provides screen-space, linearly interpolated coordinates without perspective correction.

Understanding Barycentric Coordinates

Barycentric coordinates describe a point inside a triangle as a weighted combination of the triangle's three vertices. Represented as a vec3 \((u, v, w)\) where \(u + v + w = 1.0\), each component corresponds to the weight or relative distance of the current fragment with respect to one of the vertices.

gl_BaryCoordEXT: Perspective-Correct Interpolation

gl_BaryCoordEXT interpolates coordinates across the triangle taking perspective projection into account.

• Mechanism: The values are interpolated linearly in clip space (divided by \(W\) at each vertex) and then recovered via perspective division per pixel, matching the behavior of vertex attributes qualified with smooth.
• Visual Result: The rate of change of the coordinates matches the perspective distortion of the 3D surface. Features rendered using these coordinates compress as the primitive recedes into the distance.
• Common Use Cases:
  • Procedural textures applied directly to geometry surfaces.
  • Wireframes or inner-triangle patterns that should shrink with distance and respect 3D depth perspective.
  • Custom attribute interpolation where perspective projection must be preserved.

gl_BaryCoordNoPerspEXT: Linear Screen-Space Interpolation

gl_BaryCoordNoPerspEXT interpolates coordinates linearly across the screen-space projection of the triangle, ignoring the depth (\(W\)) component.

• Mechanism: The coordinates are interpolated directly across the 2D window coordinate space, matching the behavior of vertex attributes qualified with the noperspective qualifier.
• Visual Result: The coordinates change at a constant rate per pixel across the primitive, regardless of the angle, depth, or tilt of the primitive relative to the camera.
• Common Use Cases:
  • Constant-width wireframe rendering, where line thickness along the edges of triangles must remain consistent in screen pixels regardless of camera distance.
  • Edge-detection algorithms, anti-aliasing techniques, and screen-aligned stylized outlines.
  • 2D user interface overlays or HUD elements mapped across 3D meshes.

Summary of Differences