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OpenGL Advantage to OpenGL

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I saw that everyone here (that programs in 3d)seems to prefer OpenGL over Direct3D. Why is this? What exactly are the advantages to OpenGL? "Remember, I'm the monkey, and you're the cheese grater. So no messing around." -Grand Theft Auto, London

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That was just one poll and other polls have existed before. I think that most people here are using D3D, this is based from previous polls and that from my observations, a lot more of the questions asked in the various programming forums are D3D-specific. Also, a lot of people here are relatively new to computer graphics and nearly everyone starts on 2D graphics before they advance to 3D graphics, most tutorials use DDraw for the 2D graphics. Once one is familiar with DirectX and COM, learning something from the same family of APIs will generally be more appealing than learning a different standard.

I prefer OpenGL over D3D mainly because of idealistic values and a preference of coding style. I don''t particularly like the Hungarian coding style that goes along with COM and Windows programming and when I learned OpenGL (~2 years ago), it was clearly a superior API in terms of design and usability. Granted, D3D has made gigantic leaps in the past year, but I have not felt the need or pressure to change.

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I''ve never really had anything to do with DirectX until recently (I''m tentatively dipping my feet in the carnage that is DirectDraw at the moment), because I started straight into OpenGL and 3D programming (about 9-10 months ago).

OpenGL seems to have a much cleaner style to it than D3D, and I have certainly found it easier and more intuitive to learn. You also have the portability issue (with Linux becoming a bit of a force now), and OpenGL makes it easier to develop for multiple platforms.

A more immediate/recent advantage is the T&L accelerator support that OpenGL has (or does D3D use this too??).

Another bonus is that OpenGL doesn''t change spec every 23 seconds

I guess I''m just lazy, but I too do not feel inclined to learn D3D, and am quite happy with what can be done with OpenGL. Personal preference, I guess

-------------
squirrels are a remarkable source of protein...

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An obvious big advantage for OpenGL is that it is cross-platform, whereas D3D is Windows-only. Most people also feel that OpenGL is just a more elegantly designed API. The amount of setup code you need to write to get a simple polygon on screen has historically been much less under OpenGL than D3D, as D3D has, in the past, forced the programmer to handle every little detail of initialization, texture management, etc. In the early days of 3D hardware acceleration on PCs, this type of control is what game developers asked for, and Microsoft gave it to them.

Now, with so many different cards that implement so many different feature sets, it makes more sense to handle the details within the API driver (as the driver writers would, in theory, know best how to handle certain features for the card being used). OpenGL works well under this system, as it has always been a bit higher level than D3D, handling many details within the driver. To its credit, Microsoft has recognized this as a bit of a drawback to D3D and the past few versions have added functionality and utility libraries (like D3DX) to take away much of the common drudge work that used to be associated with D3D. Most people, myself included, no longer feel that D3D is a "horribly broken" (to invoke the words of John Carmack on this issue) API. However, many of the people who think it WAS a horrible broken API have so much experience with OpenGL at this point that there''s little reason for them to switch over.

Also, programming for OpenGL is not without its drawbacks.
For one, OpenGL and DirectDraw generally do not mix, and OpenGL''s framebuffer operations are amazing slow on many drivers. Also, Microsoft''s bindings for OpenGL on Windows -- wgl -- are horribly out of date (eg. multimon support? Heh) and its unlikely they will do much to fix the situation.

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The big advantage of OpenGL is that source codes of popular games are available like quake, doom,,,. People can get idea how popular games are doing. Direct3D does not have popular open souce games.

OpenGL is taught in many universaties. Lots of "brains" support OpenGL. This is why innovative work usually uses OpenGL, while Direct3D just uses results of OpenGL.

Kate

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Well the reason more people ask questions about D3D is ''cause it''s a damned sight harder than OpenGL, that''s not a representative statistic!

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Having read comments before on this board about GL/D3D, I decided to do something some might say is stupid, but I took the plunge and decided to try and learn a bit of both.

I can say, without doubt, I have come further with OpenGL than D3D, not because D3D is crap, it is a bit tough, and I agree with KeithV, thats probably why a lot of questions are asked about it.
I do feel that OpenGL has a much more clean and elegant design to it, and hence produces cleaner code (IMO, although any API can be misused to produce horrible source).
Although, OpenGL has been around for a long time now, and has had much time to develop/standardise, so that is to it''s advantage, D3D has made great leaps recently, and is not (anymore) a borribly broken API, and it seems more & more games are coming out with support for it.
I will continue in my conquest to learn both API''s, I hope it will stand me in good stead in the future. Also, I hope MS don''t re-vamp the frikkin'' thing before I''m done learning it.

All that aside, I do prefer GL.

Just my little addition to the on-going debate, take it as you will....

-Mezz

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I''m glad that for once someone hasn''t brought up silly things like "Carmack uses GL, so why shouldn''t everyone else in the whole world??? After all, he IS God!". I''ll just go ahead and add my two cents now.

First of all, I consider myself a very good graphics programmer, and I have done games in both D3D and OpenGL. In fact, I''m right now making a game that is both. Anyway, the thing that I''ve found is that they are two different API''s. Definitely different. Until you''re done setting them up. At that point, it just becomes a matter of function calls. Basically, the initialization where you tie it to your windows, you load some textures, and have a hell of a lot of fun is hecka different, but at least with D3D 7.0, we see not even a little difference from then on. Another nice thing that has happened to bring the two together is D3DX. I just discovered it a few months ago, and boy is it nice! Basically, your 20 line initialization from OpenGL becomes 5 in D3D, and they even made the calls more similar to draw! Personally, I am a D3D advocate, simply because I feel some taint on OpenGL that just doesn''t feel right. I think it''s the coding style, where everything is shrouded in mystery (everything being what''s actually happening down below the surface), as opposed to D3D giving you more control (and therefore more responsibility).

In addition, I think that at this point in time, most previously valid arguments have gone away. The fact that OpenGL is platform independent makes absolutely no difference for game companies. They still have to make a separate version of the game NO MATTER WHAT if they want it to work across the platform bounds. There are so many friggin API calls that require the programmer to know what OS is being used that the entire structure is often different. Also, there are no features in OpenGL that are not supported in D3D, though there are a few extra in D3D that make general coding nice. Nothing to write home about, but coding becomes easier as you find them. As for that "John Carmack is God, so if I use OpenGL, I must be a demi-god" argument, all I can say is get a life! He is one person who has been working on this type of stuff since before D3D was born, so OF COURSE HE''S GOING TO USE WHAT HE KNOWS!!! I also saw something about how some games that use OpenGL are open source, so you can look at the code. If you can seriously spend more than two minutes looking at their actual rendering routines, you probably have no idea what''s goin on. The three lines of code that are the rendering block can be just as easily converted to D3D. There is an entire game behind it that is really doing the graphics.

Back to the topic, I don''t see any advantages to OpenGL anymore, unless that''s all you know. If you don''t have time to learn D3D, by all means stay with GL, since it''ll probably be around for a few more years. If you do have time to learn D3D, though, I would recommend jumping on that bandwagon. It is developing in new directions, and with DX7, has made the leap past the single power that once was OpenGL.

Pythius

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Pythius, I think you''re wrong saying that it''s no longer significant that OpenGL is cross platform. In a world of unlimited timelines and unlimited budget it may be. However, I live in the real world. Quite frankly, If I develop something in Direct3D and want to run it on a Unix variant or, heaven forfend, a Macintosh. I''m going to have to port the graphics subsystem over to the new platform. Maybe I''ll have to port other parts of the application as well, but the less I have to port, the tighter my timeline is going to be. Additionally, if I port to a Unix variant, most likely the graphics subsystem is going to be in OpenGL anyway. So why waste the time programming with two different APIs when one of the APIs will run on both platforms? When I tell management that it''s going to be an additionaly 20 man-months for a DX version and a GL version, they''re going to say do only GL. And if I say that alternately we can modularlize out the graphics API calls, and get a graphics subsystem that isn''t optimized for either DX or GL, for an overall loss of quality, they''re going to say that quality matters so go GL. I''ve got nothing against DX for windows only apps, but cross-platform really does matter.

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Latest harware support of OpenGL is very impressive:

OpenGL hardware bump mapping is now supported by more cards than Direct3D: Geforce and ATI Rage128/Pro all support OpenGL hardware bump mapping, while Direct3D bump mapping does not work on these main stream boards.

Only Matrix400 supports Direct3D bump mapping, Direct3D bump mapping demo freezes with other video cards.

Virtual texturing: texture compression is now available by 3DLab.

Latest harware support of OpenGL is very nice.

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Pythius, I understand where you''re coming from, but unfortunately, you''re completely wrong.

I know both API''s reasonably well ( well actually I told Direct3D to bugger off after version 3 ).
I''m a halfway decent graphics programmer ( I''m doing a PhD in computer graphics ), and having used both I know the advantages.

DirectX: DirectDraw is great - nearly direct-to-memory access under protected mode windows.. there''s a reason even quake used it to have its OpenGL calls draw to. The reason it''s great? ''cause Microsoft made damn sure you couldn''t do it without DirectDraw. So it''s just great ''cause there is nothing else.
Direct3D: UGH! Initialisation is worse than OpenGL... code is convoluted. Com makes entry-level programmers cringe. And then there was that nice thing where Microsoft said: Windows NT doesn''t need a DirectX version larger than 3, we''ll implement that in NT5! ( now Win2k ).

That SINGULAR mistake has put me off using DirectX forever. If microsoft feels they can alienate the entire professional developing world by limiting their high-performance graphics API to win95 and win98 they can go suck a lemon. Sure, now with win2k there''s support for DirectX7, about 3 years too late!


You say OpenGL offers less control. Maybe, but that''s not a bad thing, and it has a very good reason. It gives hardware vendors the chance to do it "their way" under the hood. All that needs to happen is that on a standard OpenGL call you get OpenGL conformant output, and how it''s done doesn''t interest anyone. By keeping the standard tight, it allows for much better hardware integration for basic functionality, and if you wish to use the hardware to the hilt you''ll have to go to assembler anyway.

OpenGL and Direct3D were going to merge at one point, in Fahrenheit, but talks broke down, allegedly over Microsoft''s unwavering line on their own system. They would not respect any of OpenGL''s original design principles.

So now they''ve gone their separate ways. Microsoft has copied openGL with their directprimitive stuff, if I recall correctly, and OpenGL is heading towards general acceptance of the 1.2 specification, hopefully including the image manipulation parts.
I will continue to use OpenGL for as much of my basic 3D as possible, and for the rasterisation stage, because of conformance and ease of use.


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Just for the record, I largely prefer OpenGL -- but you might want to take a look at newer versions of Direct3D, Keith, as Microsoft has addressed a large number of issues since version 3. I understand how many people might have been put off of it forever though, based on the early versions....

The thing I dislike most about OpenGL that isn't an issue with Direct3D is that, under Windows/wgl at least, there's no really usable option for mixing pure 2D with 3D. You can't access DirectDraw and the restrictions on using GDI with OpenGL are so strict that its virtually useless for games. Add to that the fact that the gl*Pixel routines on most drivers totally destroy performance when used and overlay support with consumer hardware drivers has been a bit spotty. So you're stuck taking bitmaps and texturing them onto flat polygons to get 2D effects like HUDs, etc... Which is fine for some things, but there are many cases where I'd much rather just blit some pixels to the backbuffer and be done with it.

Also, in recent years the OpenGL ARB has been a bit lax in getting new features and extentions into the main spec -- and many OS vendors still don't support 1.2 anyway. The end result of which is that we'll end up with a crazy number of vendor-inserted extentions that may or may not (usually not) do the same things in different ways (ie. two or more different extention APIs to do hardware bumpmapping). I think now that Fahrenheit is dead in the water this might be changing -- I hope so.

So... I feel OpenGL is far more elegant, and the cross-platformness of it is great. But Microsoft has the benefit of being dictator of what DirectX is -- which is bad insomuch as it means DirectX will basically only ever be for Windows derivatives, but its good in that they can move the technology forward with the hardware makers directly and not have to worry about the overhead of dealing with a multi-company committee.

Oh, and Keith, Quake doesn't use DirectDraw to draw OpenGL calls -- you can't do that reliably (it might work on some hardware/drivers but not all). They did use DirectDraw for the software renderer of Quake1/2, but when running OpenGL mode, DirectDraw was not used, just DirectSound and DirectInput were.





Edited by - gmcbay on 3/16/00 9:54:04 AM

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File laoding code examples for various 3d files are available for OpenGL. dxf, 3ds, md2, half-life, cob, obj,lwo,,.

If people have a little patience, they can find them in the net. All free!

Joe

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OpenGL is crossplatform, in a time when most people don''t want to be under M$ rule anymore, I think it''s very good.

OpenGL is well designed and supported by people who thought.
That''s a big difference between OpenGL and Direct3D, while one is following(Direct3D), the other is planning and previewing the future(OpenGL).

Of course all is not bad with Direct3D, I used the 7.0 and I find it usefull, but OpenGL is best to me.
It fit best in my logic.

The only reason why I continue being interested by Direct3D is the Dreamcast.
Even if the PowerVR chip prefer OpenGL.
(It''s a matter of facts)

Something you must think of :
As a PC user do you accept M$ rule, are you happy to see your favorite games only available for windows ?
If not, then don''t support only their platform, use openGL, if yes, use DirectX.

-* Sounds, music and story makes the difference between good and great games *-

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Hmm, I''m sure you can get your OpenGL calls onto a DirectDraw surface.. I''ve never tried it though.

I was probably wrong about Quake''s GL calls then, but I know they never used Direct3D.

OpenGL''s 2D operation is indeed lacking so far. I was hoping that would change with the 1.2 spec because of support for convolution filters and such, but it seems they consider only 3D in their API. It''s hard to compare the two (OpenGL/DirectX) anyway, because they can do vastly dissimilar things. OpenGL and Direct3D perform just about the same functions though, and at more or less the same performance I suspect.

I think we need that new OpenAL, and an Open2D as well
But I''m going to stick with OpenGL, I''m never using Direct3D again.

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openGL's problems on PCs are largely caused by crap graphics card drivers, which are optimized for the most common routines, and not for the less common ones (such as glreadPixels etc). Hopefully the situation will improve now that all PC graphics cards (worth mentioning) support the full openGL 1.1 implementation.

BTW, Keith, openGL can be used as a 2D graphics api by using an orthogonal perspective matrix, although I take the point that its functionality is somewhat limited compared to direct draw.


Edited by - benjamin bunny on 3/16/00 10:43:46 AM

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      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.
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