opengl - Incorrect preprocessor directive error of IF operator -

i'm writing glsl shader using #if preprocessor directive, i'm getting error incorrect preprocessor directive.

here's code below (just relevant part):

#define thre 20  float s = get_sample_data(sampling_pos); #if s > thre vec4 val = texture(transfer_texture, vec2(s, s)); #endif 

preprocessing 1 of compilation's steps, occurs before runtime. transform source based on # lines finds. doesn't have clue variable, runtime concepts. @ time, variable has no values, , preprocessor don't know them.

knowing that, trivial can't use variable value in preprocessor directive.

you can compare #defined value literal constant :

#define thre 12 #if thre > 15     float x  = 1.; #else     float x  = -1.; #endif 

in glsl, still can use conditionnal structure, 'regular' if.

if(s>thre){     // }else{     // else }