[SceneKit][Metal] Is there is ability to render transparent nodes based on depth buffer and not by rendering order

Question

I have a lot of transparent nodes in the scene. And if scene rotates in some orientations entire object blink and not rendering

1. some suggest to adjust rendering order but this solving issue partially and visual bug eventually appears

2. it's possible to uncheck "read depth" but it cause blinking and I am almost sure that this is not solution

Question:

is there is some correct shader that is using correct depth buffer so that transparent nodes not hiding randomly

or is there is another proper way to solve this issue?

PS: used Metal rendering, so in case you mention to advise some technics from OpenGL, just leave it

thanks

Here is sample screenshots:

In this example to make it simple used 3 flat double sided planes with opacity 0.5, transparency can also be set with alpha Chanel of the texture. In the real project objects may have any shape, can be transparent or not, placed inside of each other or not

crossing planes visibility issue 1:

crossing planes visibility issue 2:

Ok, looks like "Order Independent Transparency", A-buffer and changing fragments count per pixel should fix this but common, how this can be achieved? we are ok with GLSL shader until it works with SceneKit and Metal rendering

http://www.openglsuperbible.com/2013/08/20/is-order-independent-transparency-really-necessary/

Answer 1

For Metal rendering there is possible way to render the scene and I succeeded.

I suppose you have reference to id <MTLRenderPipelineState>, so what I did is I've created another and disabled writing into the color buffer (added setup for writeMask to none).

After that I call the renderer to render the model twice (first time with the secondly created id <MTLRenderPipelineState> where writing to color buffer is disabled) per one pass.

First pipeline:

MTLRenderPipelineDescriptor *volumetricPipelineDescriptor = [MTLRenderPipelineDescriptor new];
volumetricPipelineDescriptor.vertexFunction = vertexFunction;
volumetricPipelineDescriptor.fragmentFunction = fragmentFunction;
volumetricPipelineDescriptor.vertexDescriptor = vertexDescriptor;

volumetricPipelineDescriptor.colorAttachments[0].blendingEnabled             = YES;
volumetricPipelineDescriptor.colorAttachments[0].rgbBlendOperation           = MTLBlendOperationAdd;
volumetricPipelineDescriptor.colorAttachments[0].alphaBlendOperation         = MTLBlendOperationAdd;
volumetricPipelineDescriptor.colorAttachments[0].sourceRGBBlendFactor        = MTLBlendFactorSourceAlpha;
volumetricPipelineDescriptor.colorAttachments[0].sourceAlphaBlendFactor      = MTLBlendFactorSourceAlpha;
volumetricPipelineDescriptor.colorAttachments[0].destinationRGBBlendFactor   = MTLBlendFactorOneMinusSourceAlpha;
volumetricPipelineDescriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactorOneMinusSourceAlpha;

volumetricPipelineDescriptor.colorAttachments[0].pixelFormat = _renderDestination.colorPixelFormat;
volumetricPipelineDescriptor.depthAttachmentPixelFormat = _renderDestination.depthStencilPixelFormat;
volumetricPipelineDescriptor.stencilAttachmentPixelFormat = _renderDestination.depthStencilPixelFormat;

error = nil;
_volumetricPipelineState = [_device newRenderPipelineStateWithDescriptor:volumetricPipelineDescriptor
                                                                   error:&error];

if (!_volumetricPipelineState) {
    NSLog(@"Error occurred when creating render pipeline state: %@", error);
}

Second pipeline:

MTLRenderPipelineDescriptor *volumetricPipelineDescriptor = [MTLRenderPipelineDescriptor new];
volumetricPipelineDescriptor.vertexFunction = vertexFunction;
volumetricPipelineDescriptor.fragmentFunction = fragmentFunction;
volumetricPipelineDescriptor.vertexDescriptor = vertexDescriptor;
volumetricPipelineDescriptor.colorAttachments[0].writeMask = MTLColorWriteMaskNone;
volumetricPipelineDescriptor.colorAttachments[0].pixelFormat = _renderDestination.colorPixelFormat;
volumetricPipelineDescriptor.depthAttachmentPixelFormat = _renderDestination.depthStencilPixelFormat;
volumetricPipelineDescriptor.stencilAttachmentPixelFormat = _renderDestination.depthStencilPixelFormat;

error = nil;
_volumetricPipelineState2 = [_device newRenderPipelineStateWithDescriptor:volumetricPipelineDescriptor
                                                                   error:&error];

if (!_volumetricPipelineState2) {
    NSLog(@"Error occurred when creating render pipeline state: %@", error);
}

Depth state:

MTLDepthStencilDescriptor *volumetricDepthStateDescriptor = [MTLDepthStencilDescriptor new];
volumetricDepthStateDescriptor.depthCompareFunction = MTLCompareFunctionLessEqual;
volumetricDepthStateDescriptor.depthWriteEnabled = YES;
_volumetricDepthState = [_device newDepthStencilStateWithDescriptor:volumetricDepthStateDescriptor];