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

Adding transparency can be quite easily done in the SCNShadable Surface or Fragment entry point

The SCNShaderModifierEntryPointSurface entry point version

#pragma transparent
#pragma body

_surface.diffuse.a = 0.5;

The SCNShaderModifierEntryPointFragment entry point version

#pragma transparent
#pragma body

_output.color.a = 0.5;