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 thread (Using Blending Functions in Scenekit) and this wiki link about transparency : (http://en.wikibooks.org/wiki/GLSL_Programming/GLUT/Transparency), GLBlendFunc is replaced by pragma transparency in SceneKit.

Would you know what is wrong with this code?

I created a new project with SceneKit, and I changed the ship mesh for a torus.

---

EDIT :
I am trying with a plane, but the image below does not appear inside the plane, instead I get the image with the red and brownish boxes below.

My image with alpha :

The result (the image with alpha should replace the brownish color) :

let plane = SCNPlane(width: 2, height: 2)
var texture = SKTexture(imageNamed:"small")
texture.filteringMode = SKTextureFilteringMode.Nearest
plane.firstMaterial?.diffuse.contents = texture
let ship = SCNNode(geometry: plane) //SCNTorus(ringRadius: 1, pipeRadius: 0.5)
ship.position = SCNVector3(x: 0, y: 0, z: 15)
scene.rootNode.addChildNode(ship)

let myscale : CGFloat = 10
let box = SCNBox(width: myscale, height: myscale, length: myscale, chamferRadius: 0)
box.firstMaterial?.diffuse.contents = UIColor.redColor()
let theBox = SCNNode(geometry: box)
theBox.position = SCNVector3(x: 0, y: 0, z: 5)
scene.rootNode.addChildNode(theBox)


let scnView = self.view as SCNView
scnView.scene = scene
scnView.backgroundColor = UIColor.blackColor()


var shaders = NSMutableDictionary()
shaders[SCNShaderModifierEntryPointFragment] = String(contentsOfFile: NSBundle.mainBundle().pathForResource("test", ofType: "shader")!, encoding: NSUTF8StringEncoding, error: nil)
var material = SCNMaterial()
material.shaderModifiers = shaders
ship.geometry?.materials = [material]

The shader :

#pragma transparent
#pragma body
_output.color.rgba = vec4(0.0, 0.2, 0.0, 0.2);

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