How to add transparency with a shader in SceneKit?

Question

I would like to have a transparency effect from an image, for now I just test with a torus, but the shader does not seem to work with alpha. From what I understood from this

Answer 1

SceneKit uses premultiplied alpha (r, g and b fields should be multiplied by the desired a) :

vec4(0.0, 0.2, 0.0, 0.2); // `vec4(0.0, 1.0, 0.0, 1.0) * alpha` with alpha = 0.2