macos - Using Blending Functions in Scenekit

Question

I can't see an obvious way to change the blending function (glBlendFunc) for a scene kit node or geometry - it doesn't seem to be part of the material, and it isn't very obvious from the scene kit documentation how it organises render passes. Do I need to make a Render delegate for the node which just changes the GLblending mode, or do I need to somehow set up different render passes etc. (It's not obvious from the documentation how I even control things like render passes)?

Answer 1

Will It Blend? - SceneKit

Yes! In iOS 9 and OS X 10.11 (currently in beta), blendMode is an attribute on materials, so you can render any SceneKit content with additive, multiplicative, or other kinds of blending.

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But for while you're still supporting earlier OS versions... SceneKit in iOS 8.x and OS X 10.8 through 10.10 doesn't offer API for blend modes.

There are a couple of options you can look at for working around this.

1. Set the GL state yourself

If you call glBlendFunc and friends before SceneKit draws, SceneKit will render using the blend state you've selected. The trick is setting the state at an appropriate time for drawing your blended content and leaving the state as SceneKit expects for un-blended content.

If you set your GL state in renderer:willRenderScene:atTime: and unset it in renderer:didRenderScene:atTime:, you'll apply blending to the entire scene. Probably not what you want. And you can't use a node renderer delegate for only the node you want blended because then SceneKit won't render your node content.

If you can find a good way to wedge those calls in, though, they should work. Try rendering related nodes with a custom program and set your state in handleBindingOfSymbol:usingBlock:, maybe?

2. Use Programmable Blending (iOS only)

The graphics hardware in iOS devices supports reading the color value of a destination fragment in the shader. You can combine this value with the color you intend to write in any number of ways — for example, you can create Photoshop-style blend modes.

In SceneKit, you can use this with a fragment shader modifier — read from gl_LastFragData and write to _output. The example here uses that to do a simple additive blend.

#pragma transparent
#extension GL_EXT_shader_framebuffer_fetch : require
#pragma body
_output.color = gl_LastFragData[0] + _output.color;