Varyings

Varyings are a concept in shader programming used to pass data from the vertex shader to the fragment shader in a graphics pipeline. They represent per-vertex information that is intended to be interpolated across the primitive during rasterization, so that each fragment can receive a suitable value.

In older versions of GLSL, varyings were declared with the varying qualifier in both the vertex and fragment shaders and tied by name and type. The vertex shader assigned a value to the varying, and the fragment shader received an interpolated version of that value. In modern GLSL, the mechanism is replaced by out variables in the vertex shader and corresponding in variables in the fragment shader, with the same interpolation behavior. The data types can be scalar, vector, or more complex GLSL types, and are commonly used for color, texture coordinates, normals, and other per-vertex attributes.

Interpolation is the key feature of varyings. As rasterization converts vertices into fragments, the GPU computes fractional weights (barycentric coordinates) to interpolate the varying values across the primitive. By default, interpolation is smooth and perspective-correct. Qualifiers can modify this: flat disables interpolation and passes a single value per primitive (useful for per-face data); noperspective disables perspective correction and yields a linear interpolation in screen space.

Varyings require consistent declarations across shader stages: the vertex stage outputs a varying, and the fragment stage inputs a matching varying. They are an essential mechanism for conveying shading information such as lighting, texture coordinates, and material indices from vertex processing to fragment shading.