Add depth peeling example for order-independent transparency (OIT)

[swp-ariaci]

This pull request adds a depth peeling example to VulkanSceneGraph, intended both as a demonstration and as a prototype for potential integration of order-independent transparency (OIT) techniques into VSG.

The example implements a multi-pass depth peeling approach to correctly render overlapping transparent geometry without requiring manual sorting. It is designed with clarity in mind and aims to explore how such a technique could fit into VSG’s rendering architecture.

This work is related to the ongoing discussion about transparency handling in VSG:
vsg-dev/VulkanSceneGraph#1290

The example can serve as:

• A reference implementation of depth peeling in VSG
• A starting point for further discussion and refinement towards a native solution
• A basis for evaluating API design and integration strategies

Key aspects:

• Multi-pass depth peeling pipeline
• Use of VSG render graph, render passes, and framebuffers
• Focus on correctness and integration feasibility rather than performance optimization

Usage:

The example can be built and run via the standard vsgExamples workflow.

Notes:

• This is an experimental implementation and may evolve based on feedback and design discussions.
• Feedback regarding integration into VSG core is highly appreciated.

[robertosfield]

Thanks for the example. I have now merged with vsgExamples master, turns out a bit premature as it broke the build on non Windows systems :-|

I have fixed the build error - was simply a capitalization issue with an include.

There are quite a few warnings that will need fixing too:

$ make
[ 25%] Building CXX object examples/oit/vsgdepthpeeling/CMakeFiles/vsgdepthpeeling.dir/depthpeeling/Bindings.cpp.o
[ 25%] Building CXX object examples/oit/vsgdepthpeeling/CMakeFiles/vsgdepthpeeling.dir/depthpeeling/Builder.cpp.o
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp: In member function ‘vsg::ref_ptr<vsg::ClearAttachments> vsg::oit::depthpeeling::Builder::createClearAttachments() const’:
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:111:26: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
  111 |     attachments.push_back({ VK_IMAGE_ASPECT_COLOR_BIT, 0, {0.0f, 0.0f, 0.0f, 0.0f} });
      |     ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:111:26: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:111:26: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:111:26: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:111:26: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:111:26: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:112:26: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
  112 |     attachments.push_back({ VK_IMAGE_ASPECT_DEPTH_BIT, 0, {0.0f, 0} });
      |     ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:112:26: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:112:26: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:112:26: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:112:26: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Builder.cpp:112:26: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
[ 50%] Building CXX object examples/oit/vsgdepthpeeling/CMakeFiles/vsgdepthpeeling.dir/depthpeeling/PipelineConfigurator.cpp.o
[ 50%] Building CXX object examples/oit/vsgdepthpeeling/CMakeFiles/vsgdepthpeeling.dir/depthpeeling/RenderGraph.cpp.o
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.cpp: In constructor ‘vsg::oit::depthpeeling::RenderGraph::RenderGraph(vsg::ref_ptr<vsg::Window>, vsg::ref_ptr<vsg::View>)’:
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.cpp:39:42: warning: declaration of ‘window’ shadows a member of ‘vsg::oit::depthpeeling::RenderGraph’ [-Wshadow]
   39 | RenderGraph::RenderGraph(ref_ptr<Window> window, ref_ptr<View> view)
      |                          ~~~~~~~~~~~~~~~~^~~~~~
In file included from /home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.h:5,
                 from /home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.h:3,
                 from /home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.cpp:1:
/home/robert/install/include/vsg/app/RenderGraph.h:43:25: note: shadowed declaration is here
   43 |         ref_ptr<Window> window;
      |                         ^~~~~~
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.cpp: In constructor ‘vsg::oit::depthpeeling::RenderGraph::RenderGraph(vsg::ref_ptr<vsg::Window>, vsg::ref_ptr<vsg::View>)’:
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.cpp:39:42: warning: declaration of ‘window’ shadows a member of ‘vsg::oit::depthpeeling::RenderGraph’ [-Wshadow]
   39 | RenderGraph::RenderGraph(ref_ptr<Window> window, ref_ptr<View> view)
      |                          ~~~~~~~~~~~~~~~~^~~~~~
/home/robert/install/include/vsg/app/RenderGraph.h:43:25: note: shadowed declaration is here
   43 |         ref_ptr<Window> window;
      |                         ^~~~~~
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.cpp: In constructor ‘vsg::oit::depthpeeling::RenderGraph::RenderGraph(vsg::ref_ptr<vsg::Window>, vsg::ref_ptr<vsg::View>)’:
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/RenderGraph.cpp:39:42: warning: declaration of ‘window’ shadows a member of ‘vsg::oit::depthpeeling::RenderGraph’ [-Wshadow]
   39 | RenderGraph::RenderGraph(ref_ptr<Window> window, ref_ptr<View> view)
      |                          ~~~~~~~~~~~~~~~~^~~~~~
/home/robert/install/include/vsg/app/RenderGraph.h:43:25: note: shadowed declaration is here
   43 |         ref_ptr<Window> window;
      |                         ^~~~~~
[ 75%] Building CXX object examples/oit/vsgdepthpeeling/CMakeFiles/vsgdepthpeeling.dir/depthpeeling/Resources.cpp.o
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp: In member function ‘vsg::ref_ptr<vsg::RenderPass> vsg::oit::depthpeeling::Resources::createTransparencyPass(const CreateRenderPassInfo&)’:
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:72:76: warning: missing initializer for member ‘vsg::SubpassDescription::inputAttachments’ [-Wmissing-field-initializers]
   72 |         SubpassDescription renderSubpass{0, VK_PIPELINE_BIND_POINT_GRAPHICS};
      |                                                                            ^
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:72:76: warning: missing initializer for member ‘vsg::SubpassDescription::colorAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:72:76: warning: missing initializer for member ‘vsg::SubpassDescription::resolveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:72:76: warning: missing initializer for member ‘vsg::SubpassDescription::depthStencilAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:72:76: warning: missing initializer for member ‘vsg::SubpassDescription::preserveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:72:76: warning: missing initializer for member ‘vsg::SubpassDescription::depthStencilResolveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:90:77: warning: missing initializer for member ‘vsg::SubpassDescription::inputAttachments’ [-Wmissing-field-initializers]
   90 |         SubpassDescription combineSubpass{0, VK_PIPELINE_BIND_POINT_GRAPHICS};
      |                                                                             ^
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:90:77: warning: missing initializer for member ‘vsg::SubpassDescription::colorAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:90:77: warning: missing initializer for member ‘vsg::SubpassDescription::resolveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:90:77: warning: missing initializer for member ‘vsg::SubpassDescription::depthStencilAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:90:77: warning: missing initializer for member ‘vsg::SubpassDescription::preserveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:90:77: warning: missing initializer for member ‘vsg::SubpassDescription::depthStencilResolveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:106:73: warning: missing initializer for member ‘vsg::SubpassDescription::inputAttachments’ [-Wmissing-field-initializers]
  106 |     SubpassDescription combineSubpass{0, VK_PIPELINE_BIND_POINT_GRAPHICS};
      |                                                                         ^
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:106:73: warning: missing initializer for member ‘vsg::SubpassDescription::colorAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:106:73: warning: missing initializer for member ‘vsg::SubpassDescription::resolveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:106:73: warning: missing initializer for member ‘vsg::SubpassDescription::depthStencilAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:106:73: warning: missing initializer for member ‘vsg::SubpassDescription::preserveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:106:73: warning: missing initializer for member ‘vsg::SubpassDescription::depthStencilResolveAttachments’ [-Wmissing-field-initializers]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp: In static member function ‘static const vsg::RenderGraph::ClearValues& vsg::oit::depthpeeling::Resources::clearValues()’:
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
  267 |     };
      |     ^
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘VkClearColorValue’ [-Wmissing-braces]
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/depthpeeling/Resources.cpp:267:5: warning: missing braces around initializer for ‘float [4]’ [-Wmissing-braces]
[ 75%] Building CXX object examples/oit/vsgdepthpeeling/CMakeFiles/vsgdepthpeeling.dir/depthpeeling/ShaderSet.cpp.o
[100%] Building CXX object examples/oit/vsgdepthpeeling/CMakeFiles/vsgdepthpeeling.dir/vsgdepthpeeling.cpp.o
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/vsgdepthpeeling.cpp: In function ‘vsg::ref_ptr<vsg::Group> createScene(vsg::oit::depthpeeling::Builder&, vsg::ref_ptr<vsg::Data>, bool)’:
/home/robert/dev/vsgExamples/examples/oit/vsgdepthpeeling/vsgdepthpeeling.cpp:135:38: warning: missing braces around initializer for ‘std::__array_traits<vsg::t_vec4<float>, 5>::_Type’ {aka ‘vsg::t_vec4<float> [5]’} [-Wmissing-braces]
  135 |         vsg::vec4(0.0, 0.0, 1.0, 0.5)};
      |                                      ^
[100%] Linking CXX executable ../../../bin/vsgdepthpeeling
[100%] Built target vsgdepthpeeling

[robertosfield]

I have fixed the remaining warnings and have moved the example from examples/oit/vsgdepthpeeling into examples/app as a single example doesn't warrent it's own dedicated directory category.