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OpenGL Fastest and most efficient rendering technique? (solved)

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I have an OpenGL game that renders a full 3D world. I have noticed a considerable slowdown when having a world of > 10k polygons. Here is what I do: 1. Check distance to all objects in the world. Those that are within a certain range are rendered and taken to the next stage. 2. Check which polygon vertices fall inside the view frustum. Those that do are rendered, all other are ignored. 3. Apply volumetric fog, texturing, lighting (custom pre-lighting system is used) and detail texturing. 4. Render the stuff that needs to be rendered. I get an FPS of about 30-40. It's noticeable that the game runs a bit slow when you walk around the world. I dunno which of all possible techniques is the most efficient but here are those I know exist: Display lists Vertex lists Octree Do I need any of those, will they help me? [Edited by - azjerei on January 1, 2005 4:14:59 PM]

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First you need to find out if you are GPU- or CPU limited. 10K polys and 40 FPS means only 400K poly per second, which is really low. If you are using immediate mode, however, you can't expect much higher frame reates with some gfx drivers.
You can subsequently try using display lists (rendering itself should be magnitudes faster, depending on the driver), vertex arrays (could be a little faster than display lists) and VBOs (should be the fastest solution, as much as rendering goes).
Octree's and friends have little to do with the actual rendering. These are tools for scene management that are used to determine, which part of the scene is to be rendered.

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OpenGL does step one for you. You should at least be using vertex arrays for models. Vertex buffer objects are an extension which allow you to load the vertex arrays into the card's high performance memory. Check them out too.

An octree can help for culling out stuff and can also speed up collision detection.

Anyway, profile your application and find out what the major slow down is. Is it rendering? If you aren't using vertex arrays or display lists probably. But it could also be your math code (like unoptimized math functions), AI, physics, or anything else. You really have to profile to know.

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After re-reading your post, it's pretty obvious that you are CPU limited. A brute force distance testing each frame is a huge overhead if you have a big world with many objects (at least I hope that you do that on a per-object basis and not per-poly; otherwise I'd be very impressed if you still get 30 - 40 fps[smile]).
Depending on your scene geometry, a simple quadtree might be enough to speed up step 1) significantly.

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Quote:
Original post by Puzzler183
OpenGL does step one for you.

Not really. It is definately not possible to get reasonable frame rates if you throw several million polys at the card each frame, even if only a few ten thousand are actually being rendered especially in IM.

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Quote:
A brute force distance testing each frame is a huge overhead if you have a big world with many objects


Yes, but I have not clue as to how to limit the amount of drawn objects (yes, we have many objects, each with many polys). Is it wise to actually render offscreen objects for the sake of keeping track of NPCs and projectiles?

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Quote:

1. Check distance to all objects in the world. Those that are within a certain range are rendered and taken to the next stage.

2. Check which polygon vertices fall inside the view frustum. Those that do are rendered, all other are ignored.


Are you serious? Especially with (2). You check every vertex against the frustum? (1) is also expensive, but (2) is a complete killer, and unnecessary, since OpenGL does the exact same thing anyway. If you remove (2), I'm sure your performance will raise.

For starters, check out space partioning techniques(BSP,Octrees...), which you use to perform frustum culling, and reject big chunks of geometry with as much little tests as possible.

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Quote:
Original post by azjerei
Quote:
A brute force distance testing each frame is a huge overhead if you have a big world with many objects


Yes, but I have not clue as to how to limit the amount of drawn objects (yes, we have many objects, each with many polys). Is it wise to actually render offscreen objects for the sake of keeping track of NPCs and projectiles?

As you have no experience with space partitioning techniques, I'd suggest taking a step-by-step approach (you may hum this New Kids On The Block-song now [grin]).

First devide your world into sectors of reasonable size. For all objets in the world, assign a sector id to them. If the camera moves, find out which sector it is in and render all objects in that sector. Next check the 6 neighbouring sectors and render each if the viewing frustum crosses one of their borders.
This reduces the overhead significantly as you only test at the most 3 sectors (since the camera is facing in one direction only and thus never more than 3 adjacent sectors can be crossed by the frustum provided their size is well chosen). You can place the objects in the rendered sectors in a rendering queue.

Step 1) Implement that and render the object list. Observe the frame rate.

Step 2)
Once you got that working, take all the objects in the render queue you obtained from step 1) and test if their bounding sphere is inside the viewing frustum (e.g. squared distance between bounding sphere centre and one of the frustum egdes is less than squared bounding sphere radius → at most 6 tests per object). Remove all objects that are outside the frustum from the render queue.
Don't care about single polys!

Step 3)
After successfull implementation of step 2) apply front to back sorting to the render queue. Sub-sort the objects by texture/material. Render the final queue.

Step 4a)
Your high level render optimisation is nearly completed. You can now apply LOD techniques to the objects if you want/need to.

Step 4b)
Replace step 1) with a more sophisticated technique like quadtrees for outdoor scenes or BSP/ABT/Loose Octrees for indoor or mixed scenes.

The above is high level optimisation. The actual rendering performance can be improved by leaving IM and using:

a) Display lists.
b) vertex arrays.
c) VBOs.

Good luck with that and keep asking if something is unclear [smile]
Pat.

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Practical algorithms are

1) Octree
2) Portals
3) Hybrid solutions

(google, google, google...:)


Your polygon-vertices against view frustum test is useless because

1) You test too much vertices and this can be slower that send them directly to OpenGL
2) OpenGL repeats (faster) what you have done :) so you do the same thing twice!
3) Your test is inaccurate because you can have vertices outside view frustum but the polygon is visible...you should discard vertices that lies outside the same plane but this is too conservative and slow!

And display lists, vertex array are optimizations...space subdivision and polygon set selection are the core.

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mikeman is correct,
what u should be doing is finding out the 6 planes of the viewfrustum (check the faq at www.opengl.org)
and then for each object, see if its bounding shape is ouside any of the 6 planes if so discard it otherwise add it on the to be rendered list.

also concerning
1) Octree
2) Portals
etc, it depends totally what scene youre drawing (eg indoors scenes with a lot of rooms usually benifit, less so with outdoors stuff), eg there is overhead accosiated with implementing one of these, for maybe no (or even worse) performance, my advice just do the coarse culling first if thats still not good enuf, then look into so sort of scene partitioning

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If you have not an editor that describes portals and sectors...the simple solution is to use a view frustum culling based on octree partition.
You will have still overdraw but the number of rendered polygons will be independent from the total number.

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Quote:
Original post by darookie
Step 3)
After successfull implementation of step 2) apply front to back sorting to the render queue. Sub-sort the objects by texture/material. Render the final queue.


I reckon everything else but this.. It´s just that you don´t need to sort objects or polygons now-a-days. If you´re after zero-overwrite, then you should render the world twice. First render to z-buffer only with z-buffering and second time draw the pixels with z-buffering again. Sorting the objects by texture is big optimization though.

ch.

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Puzzler: But you are still throwing lots of unnecessary vertices into the pipeline. Sure they are not rendered, but doesn't that still mean lots of avoidable AGP bus traffic? I'm just being curious here.

Cheers,
Pat.

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Quote:
Original post by christian h
I reckon everything else but this.. It´s just that you don´t need to sort objects or polygons now-a-days. If you´re after zero-overwrite, then you should render the world twice. First render to z-buffer only with z-buffering and second time draw the pixels with z-buffering again. Sorting the objects by texture is big optimization though.


Even with a z-only pass you might well gain some speed benifit, if i had a complex scene to hand I'd test however I dont [wink], but in a rough front-to-back order should elimate 99% of overdraw even on the first pass (infact, if you first pass is z + ambient term its even better).

The sort routine wants to be quick however, something like radix sort should work well as you can even reuse the data from frame to frame (as between frames things very rarely change by huge degrees).

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I have checked out the two tutorials on Octrees at Game Tutorials, plus read three articles on them. I get everything about them, but I cannot really figure out one thing:

In my world, I have several objects. The world is not one big piece, but many small objects. When creating an Octree, you supply the vertice data in the world as well as the amount of vertices. This is very confusing since I have several objects. I gave this a shot, just to see how well the tutorials and articles had explained them. Apparently not good enough since it did not work at all (got an error in the CVector3 class, and I never have time to start dissecting the code looking for an answer). Anyways.. I did this:

I defined a COctree octree and a CDebug nodeDisplay for EACH object in the game world, and then created one octree per object. Is this a correct thing to do? What should I do otherwise? I cannot really have the world as one large piece.

Also, I checked my rendering code one additional time, here is how I have it:


FOR each object
IF CubeInFrustum(center point of object, radius of object) returns FALSE
THEN go to next object
ELSE
FOR each face on the object
IF PointInFrustum(vert[0]) AND PointInFrustum(vert[1]) AND
PointInFrustum(vert[2]) all return FALSE
THEN go to next object
ELSE
glVertex... etc. etc.
collision detection: player vs. poly
collision detection: NPCs vs. poly
collision detection: projectiles vs. poly
END
END
END
END



I tried removing the PointInFrustum checks, but that made the FPS even lower. If I can, I would rather avoid having to try and get Octrees or whatever into the engine since they do not seem as easy as people tend to make of them, and neither tutorial shows them in action with multiple objects.

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Quote:
Original post by _the_phantom_
Even with a z-only pass you might well gain some speed benifit, if i had a complex scene to hand I'd test however I dont [wink], but in a rough front-to-back order should elimate 99% of overdraw even on the first pass (infact, if you first pass is z + ambient term its even better).

The sort routine wants to be quick however, something like radix sort should work well as you can even reuse the data from frame to frame (as between frames things very rarely change by huge degrees).


But...if you are able to z sort polygons (for example with BSP) you cannot eliminate overdrawing!
In other words: you can say that a polygon is in front or behind another but you cannot say if it is visible so you must send it to the rendering pipeline (and you have no benefit).
Z sorting was good when computing z buffer was expensive...but today z test and texture mapping are 'free' (in terms of speed).

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I have a small update... noticed that I had not defined the center point and radius for the objects in the scene, so it rendered them all(!), which was clumsy of me. After having added the needed data, it only drew as many objects as it should, though with slight anomalies.

Since I am using a CubeInFrustum function similar to that seen in some frustum tutorial, where you specify a cube (in thise case it is created from the center point of each object and goes the radius units in each direction) and compare it to the frustum, I wonder if someone else have gotten the same problems as I, mainly that some objects near the lower portion of the screen get clipped, while objects for off in the horizon still gets drawn. I am using a simulated isometric view, I look upon the world with a FOV of 90 degrees with an isometric concept in mind (I would not call it third person.. it's more like a bird's eye view of sorts). Could this be causing these problems?

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Quote:
Original post by blizzard999
But...if you are able to z sort polygons (for example with BSP) you cannot eliminate overdrawing!
In other words: you can say that a polygon is in front or behind another but you cannot say if it is visible so you must send it to the rendering pipeline (and you have no benefit).
Z sorting was good when computing z buffer was expensive...but today z test and texture mapping are 'free' (in terms of speed).


I never said anything about polygons, its all about objects.
Determin which objects are visable, do a rougth front-to-back sort on them and then draw a z-only pass (with or without ambient term).
The rough front to back order means you'll draw to a fragment the min number of times possible and can reject fragments that much faster (early z-rejection works much better under those conditions), hust throwing a poly soup at the gfx card wont help it at all.

So z-sorting IS good on a per-object level and while z-testing might be cheap texturing certainly isnt free, doubly so with shaders.

Look at it this way, if we didnt do a rought object sort and we had one object behind another but the one at the back was the first to be drawn then we'd do ALL the fragments for that object (on that pass) and then overwrite anywhere up to 100% of the fragments written for the closer object, which is a total waste of time.
If it had been sorted first then we'd draw the closer object and then only draw the fragments which are visable for the object behind it as the rest would get rejected by the z-buffer, saving a whole load of writes to the various buffers (z,color and any others active).

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Made a simple distance calculation that works well. The frustum culling takes into account even the objects that are very far off in the distance, so I had to scrap that one. Well, anyways, thanks everyone :)

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      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 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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