SpriteKit中的默认混合如何工作？

Question

I've started tinkering with SpriteKit for the first time (Xcode 7.1 iOS9), and I'm seeing some unexpected results (both on device and simulator).

I'd like to be able to set pixels in a sprite to being completely transparent via a shader, but for some reason this is not working as I'd expect.

Here's is an example ViewController:

class ViewController: UIViewController {
    @IBOutlet var sceneView: SKView!
    private var scene = SKScene()

    override func viewDidLoad() {
        super.viewDidLoad()
        sceneView.showsDrawCount = true
        sceneView.showsNodeCount = true
        sceneView.showsFPS = true
        scene.scaleMode = .ResizeFill
        scene.backgroundColor = UIColor.redColor()
        let node = SKSpriteNode()
        node.size = CGSizeMake(64, 64)
        node.position = CGPointMake(64, 64)
        node.shader = SKShader(fileNamed: "Test")
        scene.addChild(node)
    }

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)
        self.view.layoutIfNeeded()
        sceneView.presentScene(scene)
    }
}

and here is an example shader:

void main()
{
    if (v_tex_coord.x < 0.5) {
        gl_FragColor = vec4(0.0, 1.0, 0.0, 0.0);
    } else {
        gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
    }
}

With the default blend mode of SKBlendModeAlpha, I would expect to see the left half of the sprite as fully transparent (showing the red background), and the right half as green. But instead, this is what I see:

Shouldn't the blending formula for SKBlendModeAlpha be: R = (D * (1 - A)) + (S * A)

Which in this case should result in:

    D   A   S   R
R   1   0   0   1
G   0   0   1   0
B   0   0   0   0

    D   A   S   R
R   1   1   0   0
G   0   1   1   1
B   0   1   0   0

(red on the left, green on the right)

I've tried manually setting the blending modes on the node, but it makes no difference.

Is this a bug, or am I missing something obvious here?

Thanks for your help.

Answer 1

There was, of course, a simple explanation to my problem. The color returned in gl_FragColor was expected to have pre-multiplied alpha. So rather than returning a color value of:

gl_FragColor = vec4(0.0, 1.0, 0.0, 0.0);

I should have been returning a color value of:

gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);

I incorrectly assumed a zero alpha would mean the color component was ignored.