• Advertisement
Sign in to follow this  

OpenGL Scene Graph Resources

This topic is 4469 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

As scene graphs are a such an important and frequently questioned topic here (and else where) i decided to begin compiling a comprehensive listing of resource links about them for those interested in implementing/using them. Hopefully this will become a sticky for all to find of great use. To those who have not done so already and are new to the topic of scene graphs i recommend reading up on some/all design patterns in particular:
  • Composite - reason being that scene graphs are typically implementated with this pattern
  • Decorator - reason being it's related to Composite and is in fact a degenerate form/case of Composite where decorators only have a single or fixed number of children. It is also frequently used with the Composite pattern. In the case of scene graphs implementated via the Composite pattern we have Grouping node types (they are the Composites) typically there will be an instantiable base Group node type who's sole purpose is child containment and child management, this instantiable base is just a plain instance of the composite pattern and the most general grouping node type of all in a scene graph. Then we add more specific kinds of grouping nodes, like transforms, switch nodes, LOD nodes etc these are sub-types of the base group node type thus inherit child containment and child management (code-reuse) but also adding new behaviours/state to internal nodes of a tree/DAG (the reason why i emphasis this is it does not make sense to have something like a Switch/LOD node who's sole purpose is to select/switch between other nodes as terminating/leaf nodes of a tree/DAG). The specific kinds of grouping node types are Composites aswell they are also instances of the decorator pattern too!.
  • Interpreter - reason being it's related to composite but the main point is the idea of a "global" context i.e SG could have global state or rendering context.
  • Observer - Scene Graphs are/use the Observer pattern implicitly without the developer realizing it but it can be used explicitly when scene graphs are DAGs because there maybe occasions where leaf nodes need to notify state changes to all parents (and there parent's parents and so on all the way upto the root/roots if necessary). Therefore leaf node types are subjects and grouping node types are the observers (they are also subjects when viewed in a different context since grouping node types typically share a common base with leaf node types).
  • flyweight - reason being you can permit node sharing in an n-ary tree without resorting to use a directed acyclic graph (DAG) which are harder to deal with but more flexible.
  • Visitor - reason being seperating the operation and data of scene graphs, it will beome more apparent when this pattern is read

Caveat: The main point/advantage of the composite pattern is the distinction between internal (non-terminal) and leaf (terminal) node types (yet they have a common super-type), Composite intances are internal nodes of a tree/DAG, concreate component instances are terminating, leaf nodes of a tree/DAG. The advantage of this there is no need todo a run-time check whether a node is a leaf/terminal, there is no redundant data of child containment in leaf node types (aswell as other state/behaviour that should only ever be in internal nodes of a tree/DAG). Here is the Caveat, Composite pattern is generally applied to OO designs indeed Gang Of Four (GOF) design patterns are concise & general designs/concepts to recurring solutions that arise in OO systems/languages. As such Composite pattern is implementated in-terms/via sub-type polymorphism (the type of polymorphism most widely known). However there is another method to achieve virtually the same (that is making a distinction between leafs and internal nodes) thing without sub-type polymorphism, without the need of a common polymorphic base, even no need for pointers & heap allocation by using something called recursive variants. Virtually all Functional programming languages (i don't mean imperative but some of them do also) natively support variants AKA disjoint disjoint (discriminated) unions, one value - finite set of types, they also can also be recursive (this is the key here). For example in SML we can describe a binary tree as such:
datatype 'a tree = Leaf of 'a | Node of (('a tree) * ('a tree));
This just basically says that a "tree" is either leaf of some type or an internal node. It's what we want. We can do this in C/C++, in C++ the best method is by using boost::variant, we an use boost::variant to achieve what the composite does and implement the basic structure of a typical scene graph interms of it:
#include <deque>
#include <boost/variant.hpp>

typedef boost::variant<
    struct leaf,
    struct leaf2,
    struct leaf3,
    boost::recursive_wrapper<struct group>,
> scene_node;

//...

struct leaf { /*...*/ };
struct leaf2 { /*...*/ };
struct leaf3 { /*...*/ };

struct group {
  
  typedef std::deque<scene_node> child_list; // notice recursive composition

private:  

  child_list kids;

public:

   void add_child(const scene_node& sn) { kids.push_back(sn); }
};

int main() {

    group root;

    root.add_child(leaf());
    root.add_child(leaf2());
    root.add_child(leaf3());
    root.add_child(group());

    group& inner_grp = boost::get<group>(grp.kids.back());

    inner_grp.add_child(leaf());
    inner_grp.add_child(leaf2());

}
You get the idea, read boost::variant docs on how to use it properly, use boost::static/apply_visitor etc. The interesting thing about this method you will still end up using inheritance but not for sub-type polymorphism or interfaces but purely for code reuse, no need for virtual destructors, not only that you will end up replacing sub-type polymorphism with static polymorphism using Curiously Recurring Template Pattern (CRTP). This idea is even possible in standard C (with some extra work) because C/C++ have union user-defined types but it is not a discriminated union (like boost::variant is) though, in C you can simulate discriminated unions using a simple technique called a tagged unions and enforcing state invariants through an interface. All tagged unions is is a union plus a type tag identifier, in pure standard C it will look like this:
typedef enum tag_type { FOO, BAR, .... } tag_type; // enumrated set of types

typedef struct my_variant_ {

    tag_type type_id;

    union { foo f, bar b, ... } var;

} my_variant;
Many C compilers have a non-standard extension that make this slightly less verbose:
typedef enum tag_type { FOO, BAR, .... } tag_type; // enumrated set of types

typedef struct my_variant_ {

    tag_type type_id;

    union { foo f, bar b, ... };

} my_variant;
Now we have variants in C we can apply this again to implement the composite pattern in C with recursive variants:
#include <stddef.h> // size_t
typedef enum tag_type { LEAF, LEAF2, GROUP };

typedef struct scene_node {
    tag_type type_id;
     union {
        struct leaf*   l1;
        struct leaf2*  l2;
        struct group*  grp;
     };
} scene_node;

typedef struct group {
     scene_node* kids; // pointer to an array of scene_node,
                       // notice recursive composition again
     size_t num_of_kids;
} group;

typedef struct leaf {} leaf;
typedef struct leaf2 {} leaf2;
Another advantage of this specifically to C it's slightly more type-safe, you can (almost) completely avoid void pointer syndrome. The obvious disadvantage to using recursive variants in C/C++ is the lack of pattern matching that statically typed functional languages enjoy aswell as the method completely automated, and type-checked. Saying that though its not to bad with boost::variant's static/apply_visitor. Lastly i'm not suggesting this method to be best/better than the traditional OO method of the Composite. i'm just stating it's not the only method. You could probably get the best of both worlds in OCaml.
I'll continuously add/update this, please PM me if you have any more links and i'll stick up here for all to see [smile]. [Edited by - snk_kid on November 4, 2005 6:29:34 AM]

Share this post


Link to post
Share on other sites
Advertisement
Excellent. I'm not a big fan of stickies however and would suggest moving it to the FAQ right away (as in my view it belongs there).

Tom

Share this post


Link to post
Share on other sites
Yay, I wrote most of that Wikipedia entry only the other week! - nice to see that its getting some publicity, the article only just hinges on some of the things that make up a good scene-graph but I think it is quite good as read-first article - thanks snk_kid [smile].

Share this post


Link to post
Share on other sites
Thumbs up to the #graphicsdev logs! (except for the Iain one simply because I was such a newbie back then :-P)

Share this post


Link to post
Share on other sites
I'll sticky this until I get around to integrating it into the FAQ. Nice work.

Share this post


Link to post
Share on other sites
New! added a "Caveat" section + stuff [wink].

[Edited by - snk_kid on October 10, 2005 10:47:12 AM]

Share this post


Link to post
Share on other sites
Quote:
Original post by snk_kid
New! added a "Caveat" section + stuff [wink].


Man, you should write an article. If you continue in this rate, it will be way too big for the FAQ :)

Tom

Share this post


Link to post
Share on other sites
THANK YOU! This is possibly one of the most valuable posts I've seen on gamedev. I can't thank you enough ^_^

Share this post


Link to post
Share on other sites
Quote:
Original post by acid2
THANK YOU! This is possibly one of the most valuable posts I've seen on gamedev. I can't thank you enough ^_^


and I would move for "my most wanted" post in any forum! thank you so much this will certainly come in handy!

Share this post


Link to post
Share on other sites
I once wrote a scene graph when I worked at a VR company (based on some articles I read on GameDev). When I left the company I wanted to write my own from scratch but couldn't seem to wrap my head around the various design concepts (and i couldnt use things I did for a company). None the less thank you for all of the topics you've compiled I'm pretty sure I'll be able to tackle this problem once and for all.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • By too_many_stars
      Hello Everyone,
      I have been going over a number of books and examples that deal with GLSL. It's common after viewing the source code to have something like this...
      class Model{ public: Model(); void render(); private: GLSL glsl_program; }; ////// .cpp Model::Model(){ glsl_program.compileAndLinkShaders() } void Model::render(){ glsl_program.use() //render something glsl_program.unUse(); } Is this how a shader program should be used in real time applications? For example, if I have a particle class, for every particle that's created, do I want to compiling and linking a vertex, frag shader? It seems to a noob such as myself this might not be the best approach to real time applications.
      If I am correct, what is the best work around?
      Thanks so much for all the help,
       
      Mike
       
    • By getoutofmycar
      I'm having some difficulty understanding how data would flow or get inserted into a multi-threaded opengl renderer where there is a thread pool and a render thread and an update thread (possibly main). My understanding is that the threadpool will continually execute jobs, assemble these and when done send them off to be rendered where I can further sort these and achieve some cheap form of statelessness. I don't want anything overly complicated or too fine grained,  fibers,  job stealing etc. My end goal is to simply have my renderer isolated in its own thread and only concerned with drawing and swapping buffers. 
      My questions are:
      1. At what point in this pipeline are resources created?
      Say I have a
      class CCommandList { void SetVertexBuffer(...); void SetIndexBuffer(...); void SetVertexShader(...); void SetPixelShader(...); } borrowed from an existing post here. I would need to generate a VAO at some point and call glGenBuffers etc especially if I start with an empty scene. If my context lives on another thread, how do I call these commands if the command list is only supposed to be a collection of state and what command to use. I don't think that the render thread should do this and somehow add a task to the queue or am I wrong?
      Or could I do some variation where I do the loading in a thread with shared context and from there generate a command that has the handle to the resources needed.
       
      2. How do I know all my jobs are done.
      I'm working with C++, is this as simple as knowing how many objects there are in the scene, for every task that gets added increment a counter and when it matches aforementioned count I signal the renderer that the command list is ready? I was thinking a condition_variable or something would suffice to alert the renderthread that work is ready.
       
      3. Does all work come from a singular queue that the thread pool constantly cycles over?
      With the notion of jobs, we are basically sending the same work repeatedly right? Do all jobs need to be added to a single persistent queue to be submitted over and over again?
       
      4. Are resources destroyed with commands?
      Likewise with initializing and assuming #3 is correct, removing an item from the scene would mean removing it from the job queue, no? Would I need to send a onetime command to the renderer to cleanup?
    • By Finalspace
      I am starting to get into linux X11/GLX programming, but from every C example i found - there is this XVisualInfo thing parameter passed to XCreateWindow always.
      Can i control this parameter later on - when the window is already created? What i want it to change my own non GLX window to be a GLX window - without recreating. Is that possible?
       
      On win32 this works just fine to create a rendering context later on, i simply find and setup the pixel format from a pixel format descriptor and create the context and are ready to go.
       
      I am asking, because if that doesent work - i need to change a few things to support both worlds (Create a context from a existing window, create a context for a new window).
    • By DiligentDev
      This article uses material originally posted on Diligent Graphics web site.
      Introduction
      Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
      There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
      Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. Its main goal is to take advantages of the next-generation APIs such as Direct3D12 and Vulkan, but at the same time provide support for older platforms via Direct3D11, OpenGL and OpenGLES. Diligent Engine exposes common C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
      Overview
      Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
      Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
      Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
      An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
      The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
      In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
      Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
      Render device, device contexts and swap chain are created during the engine initialization.
      Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
      Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
      Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
      Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
      API Basics
      Creating Resources
      Device resources are created by the render device. The two main resource types are buffers, which represent linear memory, and textures, which use memory layouts optimized for fast filtering. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
      BufferDesc BuffDesc; BufferDesc.Name = "Uniform buffer"; BuffDesc.BindFlags = BIND_UNIFORM_BUFFER; BuffDesc.Usage = USAGE_DYNAMIC; BuffDesc.uiSizeInBytes = sizeof(ShaderConstants); BuffDesc.CPUAccessFlags = CPU_ACCESS_WRITE; m_pDevice->CreateBuffer( BuffDesc, BufferData(), &m_pConstantBuffer ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
      TextureDesc TexDesc; TexDesc.Name = "My texture 2D"; TexDesc.Type = TEXTURE_TYPE_2D; TexDesc.Width = 1024; TexDesc.Height = 1024; TexDesc.Format = TEX_FORMAT_RGBA8_UNORM; TexDesc.Usage = USAGE_DEFAULT; TexDesc.BindFlags = BIND_SHADER_RESOURCE | BIND_RENDER_TARGET | BIND_UNORDERED_ACCESS; TexDesc.Name = "Sample 2D Texture"; m_pRenderDevice->CreateTexture( TexDesc, TextureData(), &m_pTestTex ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
      Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
      Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
      Initializing the Pipeline State
      As it was mentioned earlier, Diligent Engine follows next-gen APIs to configure the graphics/compute pipeline. One big Pipelines State Object (PSO) encompasses all required states (all shader stages, input layout description, depth stencil, rasterizer and blend state descriptions etc.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
      Creating Shaders
      While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 mode, and GLSL for OpenGL and OpenGLES modes. SHADER_SOURCE_LANGUAGE_HLSL - The shader source is in HLSL. For OpenGL and OpenGLES modes, the source code will be converted to GLSL. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
      The following is an example of shader initialization:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      Creating the Pipeline State Object
      After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
      PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
      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 tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
      Atmospheric scattering sample 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, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.
    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
      Regards,
      LifeArtist
  • Advertisement