GLSL sampler2D in struct

Question

In GLSL there seems to be linking error of shaders when I try to pass a uniform struct with a sampler2D attribute to a function which is forward declared. The code works if

Answer 1

This actually is not supposed to work because you cannot instantiate a struct that contains an opaque data type (sampler, image, atomic counter). It is acceptable to have a struct with an opaque type as a uniform, but you cannot implement the function add (...) by passing an instance of Material.

Data Type (GLSL) - Opaque Types

Variables of opaque types can only be declared in one of two ways. They can be declared at global scope, as a uniform​ variables. Such variables can be arrays of the opaque type. They can be declared as members of a struct, but if so, then the struct can only be used to declare a uniform​ variable (or to declare a member of a struct/array that itself a uniform variable). They cannot be part of a buffer-backed interface block or an input/output variable, either directly or indirectly.

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This change to your code should work across all compliant implementations of OpenGL:

// Must be in, because you cannot assign values to an opaque type.
vec4 add (in sampler2D tex) {
  return vec4(texture(tex, texcoords));
}

This would also work, since it uses the sampler in the material.tex uniform:

vec4 add (void) {
  return vec4(texture(material.tex, texcoords));
}