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OpenGL DirectX versus OpenGL - A very newbie question...

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Hello there. I was wondering, is the only difference between the DirectX API and the OpenGL API is that the OpenGL API is being supported by more OSs and game stations than the DirectX API does (while DirectX runs only under the Windows OS and maybe on some game stations, I'm not totally sure - OpenGL runs under the Linux, Mac OS X and Windows OSs AND under the Game-cube, X-box and Playstation game stations)? As far as my understanding goes, they both capable of the same abilities (that is, performance wise and functionality wise). So, is the aforementioned correct? The only difference between the two APIs is that the OpenGL API is being supported by more OSs and game stations than the DirectX API does? IMO there must be another difference, because an API that is being supported by more OSs and game stations than another API AND that has the same abilities as the other API would have been used by every programmer (more OSs and game stations that support the API you are using = more copies of your software being sold). Am I correct? I'm asking this because I've just finished my C++ course and I'm ready to start and learn some API, I just don't know which one to choose (well, it must be one of the two mentioned above - DirectX\OpenGL. SDL and such are not "sophisticated" enough for a decent programmer such as I [wink]). I got all the foundations needed (that is, math, physics, high-level programming language, etcetera...), all I need to know is the differences between the two (advantages\disadvantages). Thanks a lot in advance, Arie. P.S. - I'm sorry if this question has been asked before (and I'm sure it has [rolleyes]), I just couldn't seem to find the answers I'm looking for anywhere. Thanks a lot again. [smile]

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While OpenGL is an API for rendering graphics, DirectX is an API for graphics, sound, video, input and networking. There are still other APIs like OpenAL.

Anyways, if you create a game engine, provide enough abstraction to let the user select between OpenGL and DirectX. Sometimes the graphics drivers for one of them are better than for the other. I think it will also lead to a better design of your engine.

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AP: I know they are, as you said, "almost the same". My question is how they differ from one another? does one give you more control? more efficiency? what ARE the differences between them?
Thanks a lot [smile].

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I believe we have a fairly fresh topic about this, here. Perhaps you can find some information there. Actually, there is an article about it, too, right here.

To answer your question specifically, yes, OpenGL supports multiple platforms. However, in my experience, I've found that not many gamers play on those other platforms, anyways. Most stay with Windows.

However, the exception to this is with the Sony line on consoles. The PS2 uses OpenGL for rendering (IIRC), and with the advent of OpenGL-ES, only more are going to use it. On the other hand, Direct3D is also used on both the XBOX and the XBOX2 (aka Xenon).

If you spend the time to learn the fundamentals of graphics programming, then learning both really isn't that hard at all. However, it's important that you understand the concepts behind them, first.

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Guest Anonymous Poster
Quote:
Original post by Phillipe
My question is how they differ from one another? does one give you more control?

No/

Quote:
Original post by Phillipe
more efficiency?

No.

Quote:
Original post by Phillipe
what ARE the differences between them?


Besides that DX provides input, sound, network and only runs on Windows and XBox, both are the same. That includes control and efficiency.

It is good to know both.

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So I see... Well - learning both and letting the user choose which API would he like to run the software under seems like the ultimate answer for all my questions.
But yet, as said above (or somewhere else I jsut can't seem to recall where), OpenGL is a Graphic API (that is, it is only taking care of the aspect of displaying objects on the screen) while DirectX is a Multimedia API (that is, it is taking care of the same thing that the OpenGL API does plus it is talking care of the networking, sound, etcetera...).
So, assuming I only know OpenGL, how am I supposed to control the sound and networking, for example?
Thanks a lot in advance [smile].

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Direct3D is more frequently and, IMO, coherently updated, but new functionality provided by certain hardware can not be used until officially "rolled in." OpenGL API updates are less frequent, but new functionality can be accessed through vendor extensions, which eventually migrate into ARB extensions.

Quote:
Original post by Phillipe
So, assuming I only know OpenGL, how am I supposed to control the sound and networking, for example?
There are multiplatform abstraction libraries for many of these aspects, such as OpenAL or FMOD for sound and HawkNL for networking. We have an Alternative Game Libraries forum dedicated to these various libraries, for instance.

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It's important to understand that you CAN use the individual components of DirectX separately. For instance you CAN use DirectSound with OpenGL. Or Direct3D with some other networking library, etc... So for Microsoft-based platforms, you might as well just use DirectX for the non-graphics components until something better pops up. If you design your software carefully, it should, theoretically, be very easy to replace any platform specific APIs.

For other platforms, I really can't say since I don't have experience with any. I'm sure their are several, free, libraries out their. FMOD for sound has come up plenty of times. It's supposed to be platform independent and free.

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Guest Anonymous Poster
Quote:
Original post by Oluseyi
Direct3D is more frequently and, IMO, coherently updated, but new functionality provided by certain hardware can not be used until officially "rolled in." OpenGL API updates are less frequent, but new functionality can be accessed through vendor extensions, which eventually migrate into ARB extensions.


Oluseyi, sorry for this stupid question, but how exactly does OpenGL get updated? DirectX has the dowloadable SDK from Microsoft but for the life of me I cannot find downloadable OpenGL libs. I know that there are OpenGL libs implemented by Microsoft and packaged in VC++ but how do you get to update that?

Thanks.

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Quote:
Original post by circlesoft
The PS2 uses OpenGL for rendering (IIRC)


Not natively, you need to do something special to get it to work (maybe as drastic as installing linux).

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Assuming you mean Direct3D (since as mentioned by others, DirectX is a collection of APIs), there really isn't much difference. Both APIs abstract the same hardware functionality, mostly differing due to the philosophy of the API (OpenGL being C and extension-based, while D3D uses COM/OO and periodic updates).

Use either based on your goals (ie. cross-platform you'll pick OpenGL) and preferences (like OO and don't need cross-platform, use D3D).

If you understand the functionality offered and how it works, you'll have no problem switching at a later point. I hadn't used OpenGL in years, but when I decided to go cross-platform, switching was a snap. Toss in a extension handling lib like GLEW, browse the docs, and you're set.

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Quote:
Original post by Promit
Quote:
Original post by circlesoft
The PS2 uses OpenGL for rendering (IIRC)

Not natively, you need to do something special to get it to work (maybe as drastic as installing linux).


Aye, okay. I am assuming it just uses an API designed by Sony, then. I heard rumors that the PS3 was supporting OpenGL or something - do you know anything about that?

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Quote:
Original post by Anonymous Poster
Oluseyi, sorry for this stupid question, but how exactly does OpenGL get updated? DirectX has the dowloadable SDK from Microsoft but for the life of me I cannot find downloadable OpenGL libs. I know that there are OpenGL libs implemented by Microsoft and packaged in VC++ but how do you get to update that?

Thanks.
Microsoft has not updated the OpenGL libs for Windows in a long time, but they promise that Longhorn will have up-to-date Windows libs (OpenGL 2.0, for now). To access functionality beyond the 1.1 version provided for Windows by Microsoft, Dave Astle (yes, our very own Myopic Rhino) wrote Moving Beyond OpenGL 1.1 for Windows. It only applies to 1.2 and 1.3, but I believe it should be intuitable to 1.4.

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Quote:
Original post by circlesoft
I am assuming [the PS2] just uses an API designed by Sony, then.
From what I hear, while there is an available API from Sony, it's hardly performant. Most developers end up writing their own pedal-to-the-metal libraries for the PS2, which is why it took a while for the full graphical potential of the system to be tapped. That the hardware architecture was ... "challenging" ... didn't make anyone's life easoer [smile].

Quote:
I heard rumors that the PS3 was supporting OpenGL or something - do you know anything about that?
I have no information, but it'd be a good thing. Now if only Microsoft could find a way to interface OpenGL with its proposed XNA, demonstrating that it truly isn't about establishing D3D as the only viable option...

*sigh*

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Quote:
Original post by Anonymous Poster
Oluseyi, sorry for this stupid question, but how exactly does OpenGL get updated? DirectX has the dowloadable SDK from Microsoft but for the life of me I cannot find downloadable OpenGL libs. I know that there are OpenGL libs implemented by Microsoft and packaged in VC++ but how do you get to update that?


The answer to this question is why we love MS so much. You can't! Because Microsoft has DX it refuses to update the OpenGL libs on windows from the 1.1 versions you get standard (pretty sure its 1.1, but its quite an old version anyway). The drivers to your graphics card will give you access to all the avaliable OpenGL functions and extensions supported but as there is no Windows lib or .h files so you can't compile them into your app directly. The work around to this is to use function pointers. Basically you request a pointer to the extension you want and then your graphics driver gives it to you if supported. Its a bit annoying at first but once you get your head around it, its not a big deal. Still its very much a hack, and its a pitty there is no better way round it but not much you can do about it. MS has no reason to support/help out an API thats in competition with them so they've made it as hard as possible to use any new features.

Or at least thats how I understand it :P

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A point worth noting is that the PC (windows) is home based and therefore
has games being played on it. Many of the other platforms are for buisness
and they kind of frown on game playing. So I am beginning to see
the cross platform of OPENGl is not that big a deal for games.
I could be wrong but that is my opinion.

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      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 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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