Vector algebra working strangely in a geometry shader

Question

My relatively simple geometry shader

#version 330 core
layout (lines) in;
layout (triangle_strip, max_vertices = 3) out;

void main() {    
gl_Position = gl_in[0].gl_Position; 
EmitVertex();

gl_Position = gl_in[1].gl_Position; 
EmitVertex();

gl_Position = 3 * (vec4 (0, 1, 0, 1) + vec4 (0, 0, 0, 1) + vec4 (1, 1, 0, 1)); 
EmitVertex();

EndPrimitive();
}

for some reason, computes the third vertex by vector interpolation instead of vector addition, multiplication by scalar does not work either.

Answer 1

Not sure what you're asking, but your maths seems off. Note that

3 * (vec4(0, 1, 0, 1) + vec4(0, 0, 0, 1) + vec4(1, 1, 0, 1))
== 3 * (vec4(1, 2, 0, 3))
== vec4(3, 6, 0, 9)
== vec4(1/3, 2/3, 0, 1)

You probably want this:

vec4(3 * (vec3(0, 1, 0) + vec3(0, 0, 0) + vec3 (1, 1, 0)), 1)
== vec4(3 * vec3(1, 2, 0), 1)
== vec4(3, 6, 0, 1)