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DX11 DX11 multithreading - why bother?

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DX11 allows a graphics pipeline to be multithreaded. But, why do I want to use my available CPU parallelism on feeding the graphics pipeline? Games have got other jobs to do, like physics simulation and AI. Coarse parallelism would seem to do fine here, and the app would be easier to write, debug, and port to other platforms. Maybe you say you want to use the GPU for physics simulation and AI, and so you need a tighter coupling between producer threads and the consuming graphics pipeline. Fine, but then you've locked yourself into DX11 HW. My 2.5 year old laptop is DX10 class HW, for instance. Also, your physics and AI code would be API specific. Not only does this limit you to Microsoft platforms, but GPUs do not have the nicest set of programming languages and tools available. We put up with GPUs when we want things to be fast; they're pretty much a detriment to programmer productivity. What am I missing here? Does anyone have a compelling rationale for bothering with more tightly coupled multithreading? Cynically, this seems like a way for Microsoft / NVIDIA / ATI to push perceived bells and whistles and sell HW "upgrades". Maybe they really can show a pure graphics benefit on high end HW with a lot of CPU cores. But most consumers don't have high end HW, and there's more to games than pure graphics. DX11 is way ahead of the installed base. Last I checked, consumers are only just now getting around to Vista / Windows 7 and DX10 class HW, and that took ~3 years. Do you want to waste all your time chasing around the top tier of game players? Some games have lost a lot of money doing that, like Crysis. Also, the performance results I've seen on my midrange consumer HW are not compelling: MultiThreadedRendering11 demo D3D11 Vsync off (640x480), R8G8B8A*_UNORM_SRGB (MS1, Q0) NVIDIA 8600M GT laptop, 256MB dedicated memory, driver 195.62 (this is DX10 class HW) windowed, with mouse focus in window ~22 fps Immediate ~20 fps Single Threaded, Deferred per Scene ~21 fps Multi Threaded, Deferred per Scene ~20 fps Single Threaded, Deferred per Chunk ~18 fps Multi Threaded, Deferred per Chunk Methodology: I manually observed the demo window. I picked fps values that seem to occur most frequently. I went through all the settings twice, just in case some system process happened to slow something down. These values seem reasonably stable. I didn't worry much about fractions. I wouldn't regard a difference of ~1 fps as significant, as it's probably a 0.5 fps difference. ~2 fps is observable, however. To the extent that multithreading matters at all, it seems to slow things down slightly. This demo does not make a compelling case for bothering with DX11 multithreading on midrange consumer HW. Does anyone have some code that demonstrates an actual benefit?

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DX11 multithreading needs to be supported by the hardware, otherwise it's just a software fallback and it's slower that way than immediate mode, obviously. AFAIK no pre-DX11 card supports it.

http://msdn.microsoft.com/en-us/library/ff476893%28VS.85%29.aspx

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The other point is: FPS is an old value. It is ok for single threaded games. But for multi-threading, this only shows you how many frames the graphics device can render a scene. In background, the game can run the speed it wants and perform much complex things. The more complex a scene becomes, the more important multi-threading will be.

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Multithreading the graphics pipeline is nothing new to directx. DX9 had some parallel ability that many game companies made use of.

Why go to all of the dev and test effort to make a parallel API if no one wants it? Well, people do want it, game companies want it. DX10 had no multithreading abilities and many many requests came in asking for it. So lets look at some of the reasons why.

Object creation is slow. It can stall your rendering thread any time your app discovers that it needs to create a new object. These calls are slow enough that MS, ATI, NVIDIA all wrote white papers telling developers to avoid creating and destroying resources during the application runtime. The API supports multithreaded creates so that you can defer to the driver to pick the best times to create objects -- for instance when it has a few spare cycles -- which allows your rendering thread to continue its work -- which is to get stuff drawn to the screen.

Next, DX11 supports deffered contexts. These allow multiple threads to build command lists at the same time and for the DX runtime to preform validation on separate threads in advance. DX10 was an API redesign where one of the many goals was to reduce CPU overhead of the API calls. CPU overhead was a huge problem for DX9 -- and many many game companies were limited in what they could get on screen just because the API ate too much CPU. So DX10 reduce that cost significantly, in some places by an order of 10-100. However there are some calls that were difficult to trim down because the validation was necessary, or perhaps the driver had a lot of work to do. Being able to build command lists on separate CPU threads allows some of that work to take place in parallel and in advance of trying to actually draw the data. Several game studios are already taking advantage of deffered contexts and are seeing improvements in performance, even when using the CPU fallback for lack of driver support.

So, DX9 would allow roughly 2000 API calls per frame before the API would become a bottle neck, DX10 is around 12000, DX11 should be even higher when using deffered contexts. These are call limits based on using the whole API to do actual work, not just calling some API like SetPrimitiveTopology() x number of times. The trouble is that studios are trying to put more and more stuff on the screen and would surely take advantage of anything that could be provided performance wise.

Plus your engine has to do a lot of CPU work anyway to make draw calls. It has to build matrix transformations, sort objects and draw calls, make all sorts of decisions on what to draw and how to draw it. All of this could be done in parallel with big wins -- provided that your app actually has enough work to do that these things become bottle necks.

A consumer won't need high end hardware to take advantage of multithreading. It's all about preventing the GPU from being starved of data to crunch.

I don't think that there are many drivers out yet that fully support the multithreading APIs yet. This feature requires a lot of effort to get right and is a huge test burden -- but they will come out eventually.

The DX10.1 feature level supports hardware multithreading. This means that there is a reasonable sized slice of hardware out there already that can support this stuff once drivers arrive.

AAA Games take 2-4 years to develop - about the time span you pointed out required to adopt a new technology. Interesting how that works.

Vendor lock in is not an insurmountably problem for developers. The reality is that there are lots of game engines that wrap the graphics system into a layer so that they can run on xbox, or PC, or Playstation. These problems have been solved over and over again and are just part of reality. These same game engines have multithreaded deffered contexts built in because it makes a difference. DX11 gives them a way to map their engine API more closely to the hardware which results in a bigger win. There's no reason why an API should really lock you to any vendor if you layer your software. You want to support someone else, then target them too.

GPU tools and languages have been getting better and better over the years. Sure it's not as ideal as native tools, but it's getting there. With the spreading use of DX, compute, CUDA, opencl, etc. more and more people invest in GPU technologies which means that the whole infrastructure continues to improve. Lack of perfect tools shouldn't stop you from leveraging the amazing power of the GPU -- though I admit there are areas of debugging that are still frustrating but they will get better. People with a lot of practice writing shaders can actually get a lot done. It's not python, but it's also not asm.

A new API or hardware rev will always be ahead of the install base at launch time. This is not new.

Not all of the available API's are needed by every developer. Multithreading probably falls into one of those categories of optimization -- why do it if you don't have a problem. Granted multithreading normally requires a lot more forethought in code design, but I guarantee that if you're not seeing a win, it's because you're not running a scenario that it was designed to fix -- which is CPU and DX API bottle-necking.

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Original post by bvanevery
why do I want to use my available CPU parallelism on...
Why *don't* you want to use available parallelism on *everything*?
The game I'm writing at the moment is based on a SPMD (single program multiple data) type architecture, where essentially the same code is executed on every thread, with each thread processing a different range of the data. Every thread does physics together, then they all do AI together, then they all do rendering together, etc...

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Original post by darkelf2k5
DX11 multithreading needs to be supported by the hardware, otherwise it's just a software fallback and it's slower that way than immediate mode, obviously. AFAIK no pre-DX11 card supports it.

http://msdn.microsoft.com/en-us/library/ff476893%28VS.85%29.aspx


That's not a HW support issue, that's a driver support issue. Theoretically, a DX11 multithreading application architecture should benefit a DX10 class card, if the drivers have been updated. In practice, I don't know if IHVs have updated their drivers, or will update them. It's quite possible that they'll be cheap bastards and expect people to just buy DX11 HW. If that happens in practice, then DX11 multithreading will have no benefit whatsoever on older HW.

I suppose I'll have to check my own driver. NVIDIA's support of older laptop HW has been notoriously poor. They dumped the problem in OEM's laps for some silly reason. For quite some time, their stock drivers refused to install on laptops; you had to get your driver from the OEM. Of course, the OEMs don't care about updating their drivers very often so you end up with really old drivers that don't have current features and fixes. Only recently did NVIDIA start to offer a stock driver that will work on laptops. There is still a disconnect as far as their most current drivers; for instance, the recently released OpenGL 3.3 driver will not install by default on my laptop. I have been getting around these problems using laptopvideo2go.com, a website that adds .inf files to enable the drivers on laptops. This doesn't help the general deployment situation however.

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Original post by Pyrogame
The other point is: FPS is an old value. It is ok for single threaded games. But for multi-threading, this only shows you how many frames the graphics device can render a scene.


There is no readout for "CPU load" in the MultiThreadedRendering11 demo. This is unfortunate as it would be useful diagnostic information. That's part of why I asked if anyone had code that demonstrates an actual benefit.

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In background, the game can run the speed it wants and perform much complex things. The more complex a scene becomes, the more important multi-threading will be.


I think you may have missed the point. You don't need DX11 multithreading to do multithreading in your app. You can have an AI thread, a physics thread, or whatever. Your multithreading architecture will be simpler to write and debug, and it will not be tied to DX11.

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Original post by Hodgman
Quote:
Original post by bvanevery
why do I want to use my available CPU parallelism on...
Why *don't* you want to use available parallelism on *everything*?


Because the debugging will drive you nuts.

Because it can easily become premature optimization.

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A current high end desktop CPU has 8 hardware threads, and that number is only going to rise in the future. What possible reason could MS have for not improving multithreaded support? Coarse parallelism in games is okay up to 4 threads, maybe 6. Moving past that will require us to move beyond the rather naive approach of one graphics thread.
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Original post by bvanevery
why do I want to use my available CPU parallelism on...
Why *don't* you want to use available parallelism on *everything*?


Because the debugging will drive you nuts.
Jeez, it's not like these are problems never tackled before. People in other segments of software have been dealing with these issues for ages.

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Original post by DieterVW
AAA Games take 2-4 years to develop - about the time span you pointed out required to adopt a new technology. Interesting how that works.


For an indie working on shorter development cycles, these adoption timelines make no sense. Yes, the way it works is whatever "heavyweight" development wants. NVIDIA / ATI / Microsoft / EA all pushing their core product, using lots of programmer worker bees to do it. It's mainly for selling more HW, more OSes, and more AAA games. Except that it clearly doesn't sell AAA games if you get on the tech bandwagon too early, as what happened to Crysis. So it's mainly about selling more HW and more OSes... except that most consumers have wised up.

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Original post by Promit
A current high end desktop CPU has 8 hardware threads, and that number is only going to rise in the future. What possible reason could MS have for not improving multithreaded support? Coarse parallelism in games is okay up to 4 threads, maybe 6. Moving past that will require us to move beyond the rather naive approach of one graphics thread.
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Original post by bvanevery
why do I want to use my available CPU parallelism on...
Why *don't* you want to use available parallelism on *everything*?


Because the debugging will drive you nuts.
Jeez, it's not like these are problems never tackled before. People in other segments of software have been dealing with these issues for ages.


It's been tackled before, it will be tacked over and over again forever. It will still drive you nuts. As in, make development costs more expensive and time consuming.


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Everything makes development more expensive and time consuming. That's why major projects now are 30M-50M budgets. What on earth does any of it have to do with DX11 multithreading? Or indies?

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Original post by Promit
Everything makes development more expensive and time consuming. That's why major projects now are 30M-50M budgets. What on earth does any of it have to do with DX11 multithreading? Or indies?


Indies don't spend 30M..50M, DUH. It's about what API investments make sense from a money standpoint, and what's a trap / treadmill.

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Original post by bvanevery
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Original post by Pyrogame
The other point is: FPS is an old value. It is ok for single threaded games. But for multi-threading, this only shows you how many frames the graphics device can render a scene.


There is no readout for "CPU load" in the MultiThreadedRendering11 demo. This is unfortunate as it would be useful diagnostic information. That's part of why I asked if anyone had code that demonstrates an actual benefit.


My current engine runs a test scene on a single HT (Hardware Thread) with ~80 FPS with 16% global CPU-load. If I enable all the 8 threads, then this boosts the engine to ~3k FPS with near to 50% global CPU-load. Because the CPU uses Hyprethreading, the 50% is a very good value. With only 2 HT's enabled on different cores, I get 2.5k FPS with 24% load.

Ofcourse my engine does not render only things, but it calculates some other stuff (zero-gravity w/o collision detection physics, no AI). It renders a GUI, which renders the world on a window. The entire engine is based on a job manager, which creates at least 4 job workers (for example on a single HT). If the system has more then 4 HT's, more job workers will be created. Then all the work is done by jobs. If the engine want to calculate something, a job is created an in realtime attached to a job worker. Every camera (which is the world camera, gui camera, shadow camera, etc.) has its own rendering job. Each job can have a state machine, which can pause the calculation, if the job has dependency to another job. Because of this, I do not have any job or thread, that is called "the main renderer". All the rendering jobs can use the immediate context to calculate their prepared deffered contexts. But you have to synchronize your device to do this, because the device itself has not a real multithreding API (the device driver itself blocks the calls, so you get an exception, but not a self destructing graphics card ^^).

DX11 delivers the support for multithreading meaning the deffered contexts, that is in my opinion a very nice feature.

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Original post by Promit
So you're saying that DX11 is a terrible choice for indies because the optional multithreading support doesn't work well on your laptop?


Not just my laptop, probably 90% of the installed base.

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So, what you are saying is that because 90% (which sounds like a bullstat to me) of people currently can't then we shouldnt come up with technology to use in the future now?

So, what, in your world do we wait until everyone has at least 8 cores then dump this tech on people and say 'hey! get good at it now!'.. that's just madness and doing so would just stop progress.

No one says 'because you are using DX11 you must use Multi-threading' yet at the same time if you are targetting high end systems (my current target hardware is DX11 cards, 4 core/8thread systems) then it gives you a wonderful chunk of flexibility.

Oh, and if you are careful then frankly MT code is easy to write; hell back when I was 21 and pretty green I wrote an application which would query 50K game servers in less than 3 mins using a multi-threaded app. At the time I had practically zero experiance with networking and threads and wrote the whole thing in about 8 weeks; when it went live it never once crashed or gave the wrong output despite running every 3mins 24h a day.

And that was with raw threads, with task based systems it is even easier these days to do MT with existing libraries (be it MS's Concurrency Runtime, Intel's Threading Building Blocks or the .Net concurreny stuff).

Sure, you'll get bugs but if you are careful at what you write they won't be that hard to figure out... so either it's easier than people make out or I'm some sort of coding/design/multithreading god... come to think of it I'm good with either answer [grin]

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Original post by phantom
So, what you are saying is that because 90% (which sounds like a bullstat to me)


Indeed. I was being too kind.

Quote:
of people currently can't then we shouldnt come up with technology to use in the future now?


I've watched the DX10 API impasse for ~3 years. Have fun watching the paint dry with DX11 for ~3 as well. The reality is that most games start life on consoles and they have DX9 HW specs.

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Yes, your link just proved my point somewhat; if you drill down into the numbers then you'll see that 26.54% of people have 4 core CPUs, which is an increase of 3.5% over Jan's numbers.

Now, maybe my maths isn't too hot so remind me; what is 100 - 26.54? Is it 90? I can't recall?

As for DX10, it as an API strangled by the FUD thrown at Vista; DX11 on the other hand has had games AT LAUNCH which support it.

And you also didn't answer my question; how are we, as game programmers, meant to test out multi-threaded designs without the API support there? Because I'd lay money on MS's next console supporting DX11 style multi-threaded submission and more cores in general so by learning how to do things NOW means we'll be better positioned in the future.

But hey, if you want to stay with single threaded stuff here is the scoop; no one is going to stop you. Carry on as you were and all that.. the rest of us will be over here, trying to advance the state of the art instead of holding back advancement...

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Original post by bvanevery
The reality is that most games start life on consoles and they have DX9 HW specs.


On consoles you can multithread your command buffer generation. PC was the the odd man out in this regard until D3D11 came along.

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Original post by phantom
Yes, your link just proved my point somewhat; if you drill down into the numbers then you'll see that 26.54% of people have 4 core CPUs, which is an increase of 3.5% over Jan's numbers.


3.29% are DX11 systems. Read my original post. I'm not against multithreading, I'm against multithreading that's tied to the DX11 API. Most of the installed base does not have enough cores to waste them on 3D graphics. Games have got other things they need to do.

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the rest of us will be over here, trying to advance the state of the art instead of holding back advancement...


You mean like Crysis? You learn slowly.

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      This article uses material originally posted on Diligent Graphics web site.
      Introduction
      Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
      There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
      Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. Its main goal is to take advantages of the next-generation APIs such as Direct3D12 and Vulkan, but at the same time provide support for older platforms via Direct3D11, OpenGL and OpenGLES. Diligent Engine exposes common C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
      Overview
      Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
      Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
      Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
      An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
      The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
      In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
      Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
      Render device, device contexts and swap chain are created during the engine initialization.
      Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
      Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
      Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
      Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
      API Basics
      Creating Resources
      Device resources are created by the render device. The two main resource types are buffers, which represent linear memory, and textures, which use memory layouts optimized for fast filtering. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
      BufferDesc BuffDesc; BufferDesc.Name = "Uniform buffer"; BuffDesc.BindFlags = BIND_UNIFORM_BUFFER; BuffDesc.Usage = USAGE_DYNAMIC; BuffDesc.uiSizeInBytes = sizeof(ShaderConstants); BuffDesc.CPUAccessFlags = CPU_ACCESS_WRITE; m_pDevice->CreateBuffer( BuffDesc, BufferData(), &m_pConstantBuffer ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
      TextureDesc TexDesc; TexDesc.Name = "My texture 2D"; TexDesc.Type = TEXTURE_TYPE_2D; TexDesc.Width = 1024; TexDesc.Height = 1024; TexDesc.Format = TEX_FORMAT_RGBA8_UNORM; TexDesc.Usage = USAGE_DEFAULT; TexDesc.BindFlags = BIND_SHADER_RESOURCE | BIND_RENDER_TARGET | BIND_UNORDERED_ACCESS; TexDesc.Name = "Sample 2D Texture"; m_pRenderDevice->CreateTexture( TexDesc, TextureData(), &m_pTestTex ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
      Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
      Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
      Initializing the Pipeline State
      As it was mentioned earlier, Diligent Engine follows next-gen APIs to configure the graphics/compute pipeline. One big Pipelines State Object (PSO) encompasses all required states (all shader stages, input layout description, depth stencil, rasterizer and blend state descriptions etc.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
      Creating Shaders
      While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 mode, and GLSL for OpenGL and OpenGLES modes. SHADER_SOURCE_LANGUAGE_HLSL - The shader source is in HLSL. For OpenGL and OpenGLES modes, the source code will be converted to GLSL. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
      The following is an example of shader initialization:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      Creating the Pipeline State Object
      After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
      PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
      Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
      // Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
      // Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains 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.
    • By noodleBowl
      I am currently working on my first iteration of my sprite renderer and I'm trying to draw 2 sprites. They both use the same texture and are placed into the same buffer, but unfortunately only the second sprite is shown on the the screen. I assume I messed something up when I place them into the buffer and that I am overwriting the data of the first sprite.

      So how should I be mapping my buffer with an offset?
      /* Code that sets up the sprite vertices and etc */ D3D11_MAPPED_SUBRESOURCE resource = vertexBuffer->map(vertexBufferMapType); memcpy(resource.pData, verts, sizeof(SpriteVertex) * VERTEX_PER_QUAD); vertexBuffer->unmap(); vertexCount += VERTEX_PER_QUAD; I feel like I should be doing something like:
      /* Code that sets up the sprite vertices and etc */ D3D11_MAPPED_SUBRESOURCE resource = vertexBuffer->map(vertexBufferMapType); //Place the sprite vertex data into the pData using the current vertex count as offset //The code resource.pData[vertexCount] is syntatically wrong though :( Not sure how it should look since pData is void pointer memcpy(resource.pData[vertexCount], verts, sizeof(SpriteVertex) * VERTEX_PER_QUAD); vertexBuffer->unmap(); vertexCount += VERTEX_PER_QUAD;  
      Also speaking of offsets can someone give an example of when the pOffsets param for the IASetVertexBuffers call would not be 0
       
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