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OpenGL Engine design questions

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Thus far I've found the gamedev forums, and this forum in particular, to provide very useful information on everything that has to do with game engines. I've read many 'classic' threads where Yann L. explains everything from shadow mapping to scene graph implementations, and they've proven very helpful. However, in the process of working on my own rendering, I'm finding it difficult to ty all these theories and 'best designs' together in a coherent design that makes sense for me. I'll list the issues I'm running into and my thoughts about them one by one below... Also note that I'm using OpenGL, and though the renderer is designed to easily allow support for other libraries such as Direct3D I see no need to this yet, so I may sometimes refer to OpenGL specific things.
  • The scenegraph Everything I've read about the subject has led me to believe that spatial ordering and other optimization techniques don't belong in a scene graph. Currently I have a very limited SceneGraph class that consists of GroupNode and GeometryNode objects only (both are derived from SceneNode). GroupNodes can have children, GeometryNodes can't have children, so all geometry nodes are leaves of the tree. This works exactly the way I thought a scene graph is supposed to work (I can add parent-child relations between objects in a scene, and transformations in each node are relative to the parent node). Though many threads I've read about scene graphs add more types of nodes, I probably won't be doing that in the near future because I want things to remain as simple as possible until I've grown more comfortable with the design. So far so good, but my understanding starts lacking when it comes to passing objects from the scene graph to the Renderer. This has to do with the way in which GeometryNodes are defined in my engine:
    • GeometryNode The only thing that makes a GeometryNode different from its parent, SceneNode, is that it contains a BaseGeometry object. The BaseGeometry class is the parent class for all geometry in the engine. So you can make a BoxGeometry object, for example, that inherits from BaseGeometry and which defines the geometry for a box. I'm also working on classes like 'CaligariGeometry', or '3DSMaxGeometry' that will load geometry from disk. Similarly, I imagine there would be TerrainGeometry objects. Because all these Geometry classes inherit from BaseGeometry, they can all be passed to GeometryNodes in the scene graph. The scene graph doesn't really care what the geometry represents, it only knows about GeometryNodes, but what they contain is not of the scene graph's concern.
    • BaseGeometry BaseGeometry, as said, contains geometry data. I currently distinguish between many types of buffers. Think IndexBuffer, VertexBuffer, NormalBuffer, TexcoordBuffer etc. The reason why I did this is because I'm using the OpenGL buffer objects extension, and the framerate was higher when I used this system of many buffers for one object, rather than one large buffer with Vertex objects that contain everything from vertex position data to texture coordinates. I'm quite sure my tests are to be taken too seriously as I've never used any substantial number of vertices. Thus far all I'm rendering is a 100x100 grid with a couple of boxes on it, because I cannot load arbitrary 3D file formats yet that would allow me to more easily load more data.
    This is actually my first problem. With so many buffer objects for each (potentially small) geometric entity I foresee the rendering being horribly inefficient. Suppose you render 10,000 boxes with this scheme. Every box has 8 vertices, 8 colours, and 36 indices. That means this scene requires the use of 30,000 buffer objects whose contents all range from 8 to 36 indices. I thought it would be more efficient if the Renderer class kept several large buffers itself, but as far as I know that means you'd have to copy the geometry data into the renderer's buffers every frame (because with culling etc. going on I can't be certain if a geometric entity is still supposed to be in the renderer's buffers or not), which defeats the point of using vertex buffer objects altogether.
The second thing I have a hard time understanding how culling comes into play. It seems that using a scene graph for culling is not good; a scene graph is not meant for that. Reading through one of the threads on gamedev.net where Yann L. explains all about terrain and scene graphs, I got the impression that it may be good practise to let geometry cull itself. I can see the advantages of this in for example, a TerrainGeometry class, where the TerrainGeometry would implement a spatial partitioning scheme. This way the scene graph will still think of the terrain as a single "GeometryNode" entity, but the terrain itself could be a quadtree without the scene graph ever knowing. For terrain it'd work well, but other entities? Would it work to have an OctreeGeometry as a base class and have, for example, classes like 3DSMaxGeometry also inherit from OctreeGeometry (next to BaseGeometry)? I'm guessing this won't work as well... Animated models aren't really 'optimised' by octrees (from what I've read), and I can't see the benefit of putting small static geometry in an octree either. An other solution could be to work with several 'phases', first of which would be to update the scene graph, and then send all geometry to a culling phase. The drawback I can see to this is that all geometry will be treated equally, which I assume to not be desirable in all cases. Culling algorithms are going to be different for different types of geometry, so some distinction should still be made. These are just some of the questions I'm facing, and perhaps I shouldn't even bother with them and proceed making a horrible inefficient renderer that is to be improved upon later, but I'd still like to hear some opinions, as I find it nearly impossible to 'move on' without addressing any of these concerns. I constantly fear that I'm making my engine so inefficient that the second I'm done with it, I'll want to scratch half of it. Eventually, facing these questions will be inevitable so I thought I might as well do it now. Thanks in advance for any feedback :)

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I'm gonna do some shameless advertising first ;)

I've written a series of articles about engine architecture, intended for beginners here :
http://www.beyond3d.com/content/articles/98 (Part 1)
http://www.beyond3d.com/content/articles/102/ (Part 2)
(More written, but not published yet :( )

You might find something useful in there.


I tend to rename things, giving good names is difficult, but I find important to name things as accurately as possible in order to be understood easily.
So in my engine design (not necessarily the best, it's just one solution to the problem, probably not the worst at least ;) ), I have a SceneTree (a Tree is a Graph, but everyone gets what a tree is w/o any further specifications), a SpatialGraph (Directed Acyclic Graph), and I used to have a RenderGraph, but I replaced it by a kind of RenderQueue.

The SceneTree is solely used for hierarchical animation. (Be it skeletal animation or a sword held in a character's hand.) This node type has an update(...) function that allows animation, and the nodes make a tree (a single parent, any number of children).

The SpatialGraph is used for culling (in fact finding what's visible). It's its only purpose in life to make culling fast.

The RenderQueue, is filled during culling in the spatialgraph. It's not a single array though, it's a little more complex than that. I did write something directly inspired from Yann Lombard thread about his material system.
The only task of this RenderQueue is to render whatever has been found visible FAST. (ie means sorting sub arrays per key, each key holding data such as depth, shader ID...; whatever is appropriate.)



Examples:
Terrain
It has a single node in the SceneTree, that does nothing.
(Nothing happens based on time on this terrain.)
It's made of a number of nodes in that SpatialGraph, making a Quadtree, which root is the Terrain's SpatialNode, and Leaves would contain mesh or pointers to meshes.

Character
It has a number of nodes in the SceneTree, for Meshs and Bones, which both might be animated hierarchicaly. It might also have something attached to his hand(s), that would need hierarchical relationship.
It has one SpatialLeaf node per mesh.


I call Geometry a Mesh instead.
I might be a bit synthetic, ask if I'm not clear enough.

[Edited by - Ingenu on November 20, 2008 5:14:29 AM]

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I very much like the SceneTree, SpatialGraph and RenderView nomenclature. It says what it does and this is a huge advantage over the word SceneGraph. I will use Ingenu terms from now on.

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It's very hard to think of some kind of general 3D engine architecture which can handle everything in a nice object-oriented way and is also extremely fast and efficient. If something is for everything - it's for nothing.

Why not to keep all subsystems separately? Terrain could store all its nodes in quad-tree structure, characters could have its own hierarchy tree, and world geometry might be handled/occluded by its own BSP/PVS, or oct-tree structure (depending on what we need).
All subsystems write renderable items into render manager (render queue), which perform sorting by depth, material ID, priority, or whatever we want.

Using this approach we can optimize every single subsystem without touching the others.

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I suppose I'll throw my design into the discussion too, although it doesn't deal with the scene graph side of things as much as you are looking for:

Hieroglyph Design

You don't actually need to have many buffers to represent many objects. You can simply have a single buffer (or multiple large ones if you like) and each object indexes into that big buffer instead of each object having its own. Then you just need to centralize the access to the buffer, which can be coordinated through your renderer interface.

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      Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
      // Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
      // Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
      AntTweakBar sample is Diligent Engine’s “Hello World” example.

       
      Atmospheric scattering sample is a more advanced example. It demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.
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