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OpenGL From OpenGL 2.1 to DirectX 10

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Hi, I asked these question on the DirectX/XNA forum, but I wanted to get a range of opinions, so I am posting here as well. After reading the OpenGL3 spec I have decided that DirectX is the best choice for the long-term future of our company. I intend to start learning it so the next iteration of our technology can be written using DirectX. This will be at least a year from now, and will probably be longer, but I want to start planning for our long-term future. I have a few basic questions. -What are the key differences between the D3D way of doing things and the OpenGL way? -How far apart are DirectX versions usually released, and how much difference will there be between DX10 and 11? I have heard the transition from 9 to 10 involved significant changes. -Does a program have to be written in C# to run on the XBox? -Does the XBox use DirectX 9 or 10, or some special version? I don't have time to support two APIs, so which is the best version to use? Can I use the same exact code for the XBox and the PC versions? -What is the GPU used in the XBox? Is geometry instancing supported? Is a deferred renderer possible? I have heard Crysis cannot run on the XBox because the console is not powerful enough. C# seems to be the new trend, but C++ provides slightly faster code. Code execution time for a 3D engine isn't that crucial, since the rendering is all on the GPU and physics usually use a third-party library. Would you recommend C++ or C#? I prefer a small manageable source I can quickly edit and test different techniques with. Is there a DX SDK for C#, or is that what XNA is? Is XNA considered the "kiddie" version of DirectX? Is it a disadvantage for writing a high-end renderer? Here is some of our work, by the way. I expect our current technology to be good for a long while, but that doesn't mean I don't need to plan ahead. http://www.youtube.com/watch?v=cmN7OLy6yUY Is there anything special in DirectX 10 that is friendlier towards deferred renderers? I heard something about a way to sample pixels that are using MSAA or FSAA. Thanks for your answers.

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-What are the key differences between the D3D way of doing things and the OpenGL way?

I can only speak for D3D9, but from the API standpoint there really isn't a big difference. Since most of the heavy graphics work is done in shaders these days, the API differences become less important. For everything else, such as texture management, offscreen rendering, etc .. both API ares equally competent. D3D has the added benefit of Effects, which makes it easier in managing your shaders. To me, the real benefit of D3D over OpenGL is in the driver support/performance. Hardware vendors seem to be on top of their game in D3D drivers, but OpenGL drivers are usually a lot more buggy or are missing support for certain features (even in the core spec).

-Does a program have to be written in C# to run on the XBox?

No, you only have to use C# if you are creating an XNA game. Otherwise, you can write a native c++ application with the XDA (name of the official SDK), but that requires an official devkit from MS.

-Does the XBox use DirectX 9 or 10, or some special version? I don't have time to support two APIs, so which is the best version to use? Can I use the same exact code for the XBox and the PC versions?

I last did Xbox dev a couple of years ago, but at the time they had a special version of the D3D that was somewhere between D3D 8 & 9. I'm assuming that they constantly update their API as the SDK matures. My best guess is that its now some sort of D3D9 incarnation with extra features.

-What is the GPU used in the XBox? Is geometry instancing supported? Is a deferred renderer possible? I have heard Crysis cannot run on the XBox because the console is not powerful enough.

No sure if the current XBox SDK supports geometry instancing, but I don't see why deferred rendering wouldn't be supported, since it can be done as long as rendering to offscreen buffers is possible (which it is).

C# seems to be the new trend, but C++ provides slightly faster code. Code execution time for a 3D engine isn't that crucial, since the rendering is all on the GPU and physics usually use a third-party library. Would you recommend C++ or C#? I prefer a small manageable source I can quickly edit and test different techniques with. Is there a DX SDK for C#, or is that what XNA is? Is XNA considered the "kiddie" version of DirectX? Is it a disadvantage for writing a high-end renderer?

From the functionality point of view, XNA is just as feature rich as the native D3D versions, and it also provides a lot of convenience functions that will save time. In my opinion, its perfect for hobbyists and independent devs that are under more time constraints. For commercial level games/engines, C++ is definately the way to go as managed code is almost always slower than properly optimized native code. For console development, you will likely want to manage your limited amount of memory quite closely, and native C++ is the only way to do this.

Hope this helps!

-Clapfoot

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On a side note, don't buy into the current hate thats being thrown around about OpenGL 3.0. Most people on this forum haven't even done enough graphics work to make a credible comment on the spec and I question how closely they've studied the spec. To be honest, for most hobbyist/independent game development work, you wouldn't really even need an API more powerful than OpenGL 2.1 so the fact that OpenGL 3.0 is mostly comprised of promoted extensions isn't really a big deal. While I understand that everyone was expecting a revolutionary "reboot" of the API, I seriously think the current trend of wanting to switch APIs is more of a bandwagon thing than anything else.

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My two cents:

If you want to support Windows and Xbox 360, you can use C# and XNA Game Studio. This will allow the same program to run on both, but will limit you to D3D9 level functionality. If that's not enough, then you'll have to use C++, and write special code for Xbox. It's not D3D10 nor D3D9.

Direct3D 10 API is different in some ways from OpenGL and D3D9 (though I haven't used modern OpenGL, so I'm not fluent in it). The API treats resources separately from views, and most things need to be defined in advance. You can't just expect to set states and have things work efficiently. Conceptually it's of course similar to the others, but constructing an engine around it can be a little different than other API's, in some respects.

D3D11 will use an interface similar to D3D10, but will add some abilities. Some of them will be available on D3D10 level hardware, for example a display list sort of functionality (allowing for better multithreading).

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Hi,

I asked these question on the DirectX/XNA forum, but I wanted to get a range of opinions, so I am posting here as well.

After reading the OpenGL3 spec I have decided that DirectX is the best choice for the long-term future of our company. I intend to start learning it so the next iteration of our technology can be written using DirectX. This will be at least a year from now, and will probably be longer, but I want to start planning for our long-term future. I have a few basic questions.


Hello, and welcome to DirectX land. We hope you enjoy your stay.

Quote:
What are the key differences between the D3D way of doing things and the OpenGL way?


All of the concepts behind 3D rendering remain the same. The way in which you go about applying those concepts can look radically different, however, since DirectX makes heavy use of COM and object based (in the C++ sense of the word) interfaces to provide the functionality, while OpenGL presents a more procedural, C-style interface.

Quote:
How far apart are DirectX versions usually released, and how much difference will there be between DX10 and 11? I have heard the transition from 9 to 10 involved significant changes.


In the past, DirectX versions were updated quite rarely. The Direct3D9 model presented a capabilities system that allowed the API to be used across a vast range of hardware, greatly prolonging its life. With the switch to Direct3D10, the capabilities system has almost completely vanished in favor of specifying a base line for all Direct3D10 capable hardware. In order to keep up with hardware growth, newer versions will need to be released more frequently in order to raise the base line. However, I don't expect the newer versions to be as large a jump as D3D10 was.

Quote:
Does a program have to be written in C# to run on the XBox?


Only if you are a hobbyist. If you go through official channels, you can purchase an official XBox development kit, although I've heard that this is very difficult and very expensive.

Quote:
Does the XBox use DirectX 9 or 10, or some special version? I don't have time to support two APIs, so which is the best version to use? Can I use the same exact code for the XBox and the PC versions?


The XBox supports Direct3D9, but not 10. No matter whether you decide to use C# or C++, your code will still not be able to run on both the XBox and the PC without very careful attention to cross platform issues. It's no different from the way OpenGL can be used on multiple platforms; you need to be careful and provide extra effort to ensure that you all platform specific code is set up to conditionally compile correctly.

Quote:
What is the GPU used in the XBox? Is geometry instancing supported? Is a deferred renderer possible? I have heard Crysis cannot run on the XBox because the console is not powerful enough.


The XBox360 has a custom ATI GPU; there are several articles on the internet where you can read about it, such as this one. You are certainly able to use hardware instancing, as well as deferred rendering.

Quote:
C# seems to be the new trend, but C++ provides slightly faster code. Code execution time for a 3D engine isn't that crucial, since the rendering is all on the GPU and physics usually use a third-party library. Would you recommend C++ or C#? I prefer a small manageable source I can quickly edit and test different techniques with. Is there a DX SDK for C#, or is that what XNA is? Is XNA considered the "kiddie" version of DirectX? Is it a disadvantage for writing a high-end renderer?


Your choices boil down to this:
- If you want to release an official game for the XBox360, you are going to want to use C++ along with Direct3D9. In order to keep your code simpler, you would most likely take the same path for a PC release.

- If you want to release to XBox Live Arcade, you can make use of XNA with C#, which will let you release to both the XBox and the PC with minimal extra effort.

- If you want to make use of Direct3D10, you have no choice but to target the PC only. You can make use of either C# or C++ for this task. Personally I would choose C#, since it is a much nicer language, easier to use, and will get you moving much faster.

- If you are targeting Direct3D9 for the PC only, you can choose from any of the technology options presented above.

If you are going to be targeting the PC only and are interested in C#, you may prefer using SlimDX, which is a direct translation of DirectX to C#, over XNA, which is a higher level framework that occasionally leaves out features in order to ensure compatibility with its lowest common denominator (ie. the XBox).

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Mike has good points and opinions.

As a somewhat "insider" in D3D11 development, I would like to add that converting D3D10 code to D3D11 is going to be very easy, with the basic interface hierarchy being mostly same. There is a minor difference in basic render state setting, but the impact is only about one extra row of code.

Only when you access D3D11-specific features such as compute shaders, tessellation and SM5.0 stuff, there will be major differences in code since there are no equivalent D3D10 interfaces.

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Original post by V-man
Is there a middleware that allows you to use C++, call some routines in a link library which routes to XNA?


Why would you want to?

That aside, you could always use C++/CLI. But if you're going to go that route, you might as well use C#. And if you mix unmanaged code in, you won't be able to deploy to the XBox360.

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Original post by Mike.Popoloski
Quote:
Original post by V-man
Is there a middleware that allows you to use C++, call some routines in a link library which routes to XNA?


Why would you want to?

That aside, you could always use C++/CLI. But if you're going to go that route, you might as well use C#. And if you mix unmanaged code in, you won't be able to deploy to the XBox360.


Because I have a lot of code in C++ and don't want to translate. Reduced productivity.

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      // 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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