View on GitHub

VulkanSceneGraphPrototype

Prototype of the VulkanSceneGraph/VkSceneGraph

Design Principles and Philosophy

“Successful Software Lives For Ever”

This document introduces the design principles/philosophy of the project lead Robert Osfield. The principles draw upon the experience of being project lead of the OpenSceneGraph and lessons learned from the real-time compute graphics, C++, and open source/free software communities.

First lets break down the opening phrase, “Successful Software Lives For Ever”, one I coined for a training course over a decade ago:

For the OpenSceneGraph project what made it a success was it achieved performance levels that competed well with the best proprietary and open source scene graphs, and was well designed enough to help developers be Productive over long term application development and deployment. Responsive of contributors to support the community has also been crucial for the OpenSceneGraph remaining relevant to real-time graphics application developers for nearly two decades.

Technology moves on, Vulkan is displacing OpenGL as the graphics API of choice for high performance cross platform graphics development. C++ has begun evolving more rapidly and now C++17 offers many features that make applications faster, more robust and easier to develop and maintain. Ideas on scene graphs have also advanced, a well designed clean room scene graph has the potential for improving graphics and compute performance and developer productivity beyond the previous start of art. The VulkanSceneGraph project goal is to embody the potential of Vulkan and modern C++ and create middle-ware for the next generation of graphics applications, and for it to remain a valued tool for the next decade and beyond.

There are two broad areas that will determine the success of the VulkanSceneGraph : Performance and Productivity. The rest of this document will break these areas down and discuss the design principles that will aim to deliver in these areas. I’ll coin another phrase to emphasise how both are equally crucial to success:

“You live for Performance, but die a slow, painful death without Productivity”

Making allowances for living at the Bleeding Edge

The below discussion outlines what is guiding the path forward - it’s our compass heading, as of the fall of 2018, it’s still very early days so the code is in flux and is not ready for wide use out in the industry. During this phase of rapid development early adopters will have to accept lower Performance and Productivity associated with working with alpha software.

Early adopters are hugely important for the success of the project, the extra work you have to put in will benefit us all in helping shape and refine the software so that it’s fit for purpose by the time we reach our first stable release and genuinely deliver on what we are striving for.

Performance

Performance can be mean different things to different applications, some applications maximum frame-rate may be the goal, others avoiding frame drops when targetting a fixed frame rate is critical, for VR minimizing latency is crucial, while others may look to minimize power demands required to achieve a target frame rate and visual quality. In all these cases overall efficiency of taking a representation of a 2D or 3D world and rendering it on a GPU is the key determiner, the lower overhead on the complete computer system the better the efficiency and potential performance.

The stages of work that are crucial to undertake efficiently with a graphics application are:

  1. Creation and destructions of scenes, paged scenes must run in a parallel to rendering
  2. Updating the scene(s)
  3. Cull traversal - view frustum etc. culling to generate a dispatch graph
  4. Dispatch traversal - sending the data in the dispatch graph to the GPU(s)
  5. Graphics Processing - graphics and compute work done on the GPU(s)

The adoption of Vulkan provides a significant reduction in CPU overhead with dispatching data compared to OpenGL and Direct3D(prior to 11), this immediately reduces the cost of stage 4 - dispatch traversal. However, the benefits are only fully realized if the scene graph overhead involved in dispatch traversal stage and cost of paging, updating and cull traversal are proportionally reduced. To deliver on all the potential benefits that Vulkan has the scene graph must take similar strides forwards in reducing overheads and improving efficiency.

The principles used as a guide to achieving efficiency include:

An example of minimizing footprint, non essential conditions and data storage has already impacted the design and implementation can be seen in minimizing the size of the core vsg::Object that is backbone of the scene graph by moving all optional data out into an optional vsg::Auxiliary class. This change reduces the size of vsg::Object to 24 bytes, compared to the OpenSceneGraph’s osg::Object class that is 72 bytes. The vsg::Node class adds no extra data members overhead so remains at 24 bytes, while the OpenSceneGraph’s osg::Node class footprint is 20 bytes. These memory footprint reductions are carried over to all objects in the scene graph.

Scene graphs are fundamentally a graph of objects connected by pointers between those objects. The size of those objects is inextricably connected to the size of pointers. C++11 onwards provides the std::shared_ptr<> which on 64-bit systems has a size of 16 bytes, while the VSG’s intrusive reference counting enables the vsg::ref_ptr<> to have a size of 8 bytes. In experiments with creation of a quad tree scene graph using std::shared_ptr<> vs vsg:::ref_ptr<>, the shared_ptr<> results in a 75% more memory used overall, and 65% slower traversal speeds. This significant difference illustrates that one should not assume that C++ core features are always the most efficient tool.

Traversals of a scene graph and dispatch graphs are the main operations that a scene graph undertakes each frame, cache misses and lowering the cycle overhead per object visited is key to reducing traversal times. The memory footprint reduction immediately reduces the number of page faults that occur and avoiding non essential conditionals provides a second improvement as it reduces number of cycles required per object visited.

The way that avoiding non essential conditionals has been addressed is to drop the NodeMask and TraversalMode parameters that are found in the OpenSceneGraph’s osg::Node and osg::NodeVisitor respectively. When NodeMask functionality is required in a scene graph the task will fall to a MaskGroup that will have a local mask and undertake the conditional during traversals that is required, so that only scene graphs that require a mask will pay the penalty for it. Decision of what type of traversal to undertake is also moved to the Visitor subclass rather than the Visitor base class. Finally the NodePath that is automatically accumulated by the OpenSceneGraph’s NodeVisitor is also dispensed with, if Visitor implementation require this functionality then they are left to implement it.

To test the effectiveness of these design differences a test program, vsgroups (found in vsgFrameworks project) was used. The results of these seemingly small design changes over the OpenSceneGraph have a dramatic improvement in performance.

The reason for this dramatic improvement is due to:

These are benefits even before we compared Vulkan vs OpenGL improvements, it’s still too early in the projects life to be able to compare on realistic scenes, as things progress we’ll provide more results. We can be confident that the improvements in efficiency of the scene graph traversals combined with the efficiency of Vulkan will substantially improve the ability to have large and complex worlds, and reduce the power over-head required to achieve a specific level of visual quality.

Productivity

The tools and middle-ware you choose for your projects are key determinants of the amount of work needed to achieve required functionality and to maintain and enhance that functionality through the software’s life. The approach that we take within the VulkanSceneGraph project not only determines how productive it’s own development is, it will have a great influence on how productive users of it will be. The following are principles that we are adopting to help us all achieve better productivity.

General project development principles that aid Productivity:

Software quality principles that aid Productivity: