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OpenGL OpenGL clear screen much slower than DirectX

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Hello! I was trying to make an app which renders both in OpenGL and Direct3D. When I got to the point to clear the screen I noticed that OpenGL is much slower than Direct3D. So I made an iteration to clear the screen 100 times. The DirectX version always performs 2 times faster. The framebuffer format is 32bit color / 24bit depth / 8bit stencil. Is this normal? Or I did something wrong in my code? This is the code:
#include <windows.h>
#include <string.h>
#include <sstream>
#include <gl/gl.h>
#include <gl/glu.h>
#include <d3d9.h> 
#include <d3dx9.h>

//Windows Specific

HWND            hWnd=NULL;							
HINSTANCE       hInstance;							
MSG				msg;

//OpenGL specific

HGLRC           hRC=NULL;							
HDC             hDC=NULL;							
PIXELFORMATDESCRIPTOR pfd;

//DirectX Specific

LPDIRECT3D9				dx_Object;
D3DPRESENT_PARAMETERS	dx_Params;
LPDIRECT3DDEVICE9		dx_Device;




LPCSTR errmsg;

bool AppRunning=true;


bool OpenGL=false;
bool DirectX=false;

int frame_count=0;

int frame_start;
unsigned int curr_time;
unsigned int start_time;
int fps;
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////


LRESULT CALLBACK WinProc(HWND han_Wind,UINT uint_Message,WPARAM parameter1,LPARAM parameter2)
{
   switch(uint_Message)
     {
         case WM_KEYDOWN:
		 case WM_DESTROY:
         {
             AppRunning = false;
             break;
         }
		 
		 break;
     }

return DefWindowProc(han_Wind,uint_Message,parameter1,parameter2);    	
}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////


bool Init()
{


		WNDCLASSEX wnd_Structure;

		wnd_Structure.cbSize = sizeof(WNDCLASSEX);
		wnd_Structure.style = CS_HREDRAW | CS_VREDRAW;


		wnd_Structure.lpfnWndProc = WinProc;

		wnd_Structure.cbClsExtra = 0;
		wnd_Structure.cbWndExtra = 0;
		wnd_Structure.hInstance = GetModuleHandle(NULL);
		wnd_Structure.hIcon = NULL;
		wnd_Structure.hCursor = NULL;
		wnd_Structure.hbrBackground = GetSysColorBrush(COLOR_BTNFACE);
		wnd_Structure.lpszMenuName = NULL;
		wnd_Structure.lpszClassName = "CLASS";
		wnd_Structure.hIconSm = LoadIcon(NULL,IDI_APPLICATION);
	 
		RegisterClassEx(&wnd_Structure);
	    
		LPCSTR title;
		
		if (OpenGL)  title="Test (OpenGL)";
        if (DirectX) title="Test (DirectX)";

		hWnd=CreateWindowEx(WS_EX_TOPMOST,
							"CLASS",
							title,
		                    WS_VISIBLE
							| WS_OVERLAPPED
							| WS_CAPTION
							| WS_SYSMENU							
							| WS_MINIMIZEBOX
							| WS_POPUPWINDOW 
							,0, 0, 
							1024, 768, 
							NULL, 
							NULL, 
							NULL, 
							NULL);

		if (hWnd==0) 
			{
			 errmsg="Error creating window!";
 			 return false;
			}


		//Initialize OpenGL

		if (OpenGL)
		{

			hDC=GetDC(hWnd); 
			
			ZeroMemory( &pfd, sizeof( pfd ) );
			pfd.nSize = sizeof(pfd);
			pfd.nVersion = 1;
			pfd.dwFlags = PFD_SUPPORT_OPENGL | PFD_DRAW_TO_WINDOW | PFD_DOUBLEBUFFER;
			pfd.iPixelType = PFD_TYPE_RGBA;
			pfd.cColorBits = 32;
			pfd.cDepthBits = 24;
			pfd.cStencilBits = 8;
			

			int pf=ChoosePixelFormat(hDC,&pfd);

			if (pf==0) 
				{
				 errmsg="Error setting pixel format!";
 				 return false;
				}

			SetPixelFormat( hDC, pf, &pfd );

			hRC = wglCreateContext( hDC );

			if (hRC==0) 
				{
				 errmsg="Error creating render context!";
 				 return false;
				}

			wglMakeCurrent( hDC, hRC );
		}

                //Initialize DirectX

		if (DirectX)
		{
			dx_Object = Direct3DCreate9(D3D_SDK_VERSION);
			if (dx_Object==NULL)
			{
				errmsg="DirectX 9 not supported!";
				return false;
			}
			 
			ZeroMemory( &dx_Params,sizeof(dx_Params));
			dx_Params.Windowed = TRUE;
			dx_Params.SwapEffect = D3DSWAPEFFECT_DISCARD;
			dx_Params.BackBufferFormat = D3DFMT_UNKNOWN;
			//dx_Params.BackBufferFormat = D3DFMT_A8R8G8B8;
			
			dx_Params.PresentationInterval = D3DPRESENT_INTERVAL_IMMEDIATE;
			dx_Params.EnableAutoDepthStencil = true;
			dx_Params.AutoDepthStencilFormat = D3DFMT_D24S8;
			
			
			dx_Object->CreateDevice(D3DADAPTER_DEFAULT, D3DDEVTYPE_HAL, hWnd, D3DCREATE_HARDWARE_VERTEXPROCESSING, &dx_Params, &dx_Device);
			if (dx_Device==NULL)
			{
				errmsg="No hardware support for DirectX 9!";
				return false;
			}
						
		}
			  
				



return true;
		
}
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
void ShutDown()
{
	if (OpenGL)
	{
		ReleaseDC(hWnd,hDC);
	}

	if (DirectX)
	{
		dx_Device->Release();
	}

	
	DestroyWindow(hWnd);
}
 

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
void Render()
{
	if (OpenGL)
	{
		for (int i=1;i<=100;i++)	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);		
	}

	if (DirectX)
	{
		for (int i=1;i<=100;i++)	dx_Device->Clear(0, NULL, D3DCLEAR_TARGET | D3DCLEAR_ZBUFFER | D3DCLEAR_STENCIL, D3DCOLOR_XRGB(0,0,0), 1.0f, 0);
	}
}

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////


int WINAPI WinMain(HINSTANCE hInstance,
                   HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine,
                   int nCmdShow)
{
	int res=MessageBox(0,"","OpenGL or DirectX",MB_YESNO);

	if (res==IDYES) OpenGL=true;
    if (res==IDNO) DirectX=true;

	if (!Init())
	{
		MessageBox(0,errmsg,"Error",MB_OK);
		return 0;
	}

	

	while(AppRunning)
	 {
	     Render();
		 
		 if (OpenGL) SwapBuffers(hDC);
		 if (DirectX) dx_Device->Present(NULL, NULL, NULL, NULL);

         frame_count++;
		 
		 curr_time=GetTickCount();

		 if ((curr_time-start_time)>=1000)
		 {
			 fps=frame_count-frame_start;
			 frame_start=frame_count;
			 start_time=curr_time;
			 
			 std::ostringstream myStream;
			 myStream << "fps:" << fps << ";";
			 
			 SetWindowText(hWnd,LPCSTR(myStream.str().c_str()));
		 }


		 if(PeekMessage(&msg,hWnd,0,0,PM_REMOVE))
		 {
			 if(!IsDialogMessage(hWnd,&msg))
			 {
				 DispatchMessage(&msg);
			 }
		 }
	 }


	ShutDown();
  return 0;
}


My specs: GeForce 6600 (latest 64 bit drivers) AthlonXP 64 3000+ 512RAM WinXP 64 bit

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Does it really matter? It's just one call. After the thousands of calls it takes to render any normal scene, I'm sure the differences will be negligible.

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Quote:
Original post by Vampyre_Dark
Does it really matter? It's just one call. After the thousands of calls it takes to render any normal scene, I'm sure the differences will be negligible.


Well, I'm just surprised about the big difference (2X). It doesn't seem normal to me...

I only got to the point to clear the screen. I wonder what will happen when I render something.

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It's probably due to vsync, with your directx initialization you have the presentation interval set to immediate which will cause directx to ignore the vsync. This would in turn cause directx to be faster then openGL.

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It's a bogus measurement. Render something worthwhile. Something that brings the FPS down to around 20 to 100 FPS. This will tell if the driver is good at dealing with your GL or D3D calls.

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Quote:
Original post by Dancin_Fool
It's probably due to vsync, with your directx initialization you have the presentation interval set to immediate which will cause directx to ignore the vsync. This would in turn cause directx to be faster then openGL.


I already checked that, vsync is forced off...

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Quote:
Original post by V-man
It's a bogus measurement. Render something worthwhile. Something that brings the FPS down to around 20 to 100 FPS. This will tell if the driver is good at dealing with your GL or D3D calls.


As I told I clear the screen 100 times. OpenGL renders 20 fps. DirectX renders 39fps.

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Quote:
Original post by jsg007
Quote:
Original post by V-man
It's a bogus measurement. Render something worthwhile. Something that brings the FPS down to around 20 to 100 FPS. This will tell if the driver is good at dealing with your GL or D3D calls.


As I told I clear the screen 100 times. OpenGL renders 20 fps. DirectX renders 39fps.
So? Your test is idiotic. It fabricates a situation that never exists in reality and turns out useless benchmark information.

Come back after you've written something productive.

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For an empty loop you should be looking at more like a few thousand iterations in 1 second. Sorry I cant be more helpful but there's something wrong with the measure.

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If you're rendering full environments, should you even be clearing the screen at all?

And although a little cold, Promit has a point. Render a scene that brings your FPS down to, say 50, and THEN perform your 100x test. Simply clearing an already blank scene will produce very inaccurate results.

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The driver could just have an un-optimized clear function, or maybe the d3d driver is "cheating" and notices you're not drawing anything in between the clears and just skips it.


Like everyone else is saying though, this really doesn't mean anything, most games don't clear the screen every frame anyways...

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Quote:
Original post by Promit
Quote:
Original post by jsg007
Quote:
Original post by V-man
It's a bogus measurement. Render something worthwhile. Something that brings the FPS down to around 20 to 100 FPS. This will tell if the driver is good at dealing with your GL or D3D calls.


As I told I clear the screen 100 times. OpenGL renders 20 fps. DirectX renders 39fps.
So? Your test is idiotic. It fabricates a situation that never exists in reality and turns out useless benchmark information.

Come back after you've written something productive.


ok,ok, jesus, i was just asking if this is normal, what's you fuckin problem?

and on a high resolution the effect is noticable even with a single call, and I never said it's a fuckin benchmark for fuck's sake...

come back when you can reply something "productive"...

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Quote:
Original post by deadstar
If you're rendering full environments, should you even be clearing the screen at all?
Absolutely 100% yes. The driver does a lot more than simply wiping out a buffer when you call clear. (A lot more. I've seen the code.)
Quote:
and on a high resolution the effect is noticable even with a single call, and I never said it's a fuckin benchmark for fuck's sake...
That setup can probably clear a 1600x1200 screen in the vicinity of a thousand times per second anyway. At that speed, a 50% drop is negligible and insignificant.

Like I said, come back when you're actually doing something useful. The driver is not sitting around going "gee, I'd better be on my toes in case he decides to clear the buffer over and over for no reason".

Oh, and cursing at a moderator is usually ill advised.

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Benchmarks only tell you something about the benchmark context. Your context is pathologically flawed. Try something more realistic and see what you get; "clearing" is not as simple as you might initially think.

Put some stuff in the scene and let us know.

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Quote:
Original post by jsg007
ok,ok, jesus, i was just asking if this is normal

Well the scenario you're testing for is not normal, so yeah I would expect the results to be quirky.

Quote:
and I never said it's a fuckin benchmark for fuck's sake...

None the less it's still a benchmark, just not one of any practical benefit.

If you render several nice large polygon batches and clear the buffers between each frame, then you should find that either OGL and D3D stabilise to about the same performance, or that your OGL implementation will crash and burn indicating that you need to update the driver.

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Quote:
Original post by jsg007
** stuff **


Cool it, there was no need to fly off the handle like that so if you think you are going to start swearing again at ANYONE take a moment and step away from the computer.

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Quote:
Original post by jsg007
Quote:
Original post by Dancin_Fool
It's probably due to vsync, with your directx initialization you have the presentation interval set to immediate which will cause directx to ignore the vsync. This would in turn cause directx to be faster then openGL.

I already checked that, vsync is forced off...


You did something wrong in your code. As Dancin_Fool have already mentioned, it is due to vsync. You set DirectX to ignore vsync while leaving OpenGL vsync status to its default value. For a NVidiia display card, the default behaviour of OpenGL is synchronizing with vsync.

To get a more reasonable result, you should set DirectX and OpenGL to either "both ignoring vsync" or "both synchronizeing with vsync". If you doing it correctly. You should endup with identical frame rates.

However, even after the correction, the frame rates are still not a valid performance measurement. The GPU have a command queue. Your commands are catched in the command queue first. They are not executed until the queue is full or being told to execute explicitly. Therefore, more than likely, your frame rate measuring program is just measuring the time of uploading commands to GPU without execution if ignoring vsync (or the screen refresh rate if synchronized with vsync).

In either cases, the values are meaningless.

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What code did you use to disable V-synch in GL?


Quote:
Original post by Promit
Oh, and cursing at a moderator is usually ill advised.

He shouldn't have bit back at you, but you still laid the bait for him by basically calling him an idiot. Pulling out the moderator card doesn't change that.

It also shouldn't mean that you can break a "will not be tolerated" rule and then be so uptight and sensitive about a "should be avoided" rule.

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Quote:
Original post by Hodgman
Quote:
Original post by Promit
Oh, and cursing at a moderator is usually ill advised.

He shouldn't have bit back at you, but you still laid the bait for him by basically calling him an idiot. Pulling out the moderator card doesn't change that.


Promit said the program is idiotic. He never said the one wrote it is an idiot (which are words I really wish to say).

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Quote:
Original post by ma_hty
Promit said the program is idiotic. He never said the one wrote it is an idiot (which are words I really wish to say).

Yes - he said the program is idiotic, as in, the program is characterized by idiocy, as in, the program's author is also characterized by idiocy, as in, the program's author suffers from extreme mental retardation.

Put yourself in jsg007's shoes and it's easy to see how he took it offensively...

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Hey Hodgman,

I don't think here is the correct venue to discuss discipline. If you are enthusiastic about the discipline discussion, start a new thread in a discussion board for general discussion and ask me to join it (though I'm not interested at all).

Gary

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No no no!

Wait. You are using GetTickCount() which runs by default at 60HZ so I'm wondering how do you get 100 FPS ? Okay. Let's put this aside.

If you would like to get the maximum time accurancy you will have to use QueryPerformanenceCounter. This will give you more accurate time results.

Also you have to be careful, that you not clear only the color buffer in DirectX where on the other side you might clean color,stencil and depth buffer as well. I didn't check the code yet, but for such time critical measures GetTickCount is too slow!

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I just think the moderator over reacted calling me an idiot and maybe I over reacted to that.

And interestingly my user rating dropped considerably.

This was just a shitty sample since I never used DirectX and was making a simple program to test basic functionality, and I noticed that DirectX clears the buffer much faster, ok? No reason for jumping around.

It's not a benchmark, it does not prove anything.

I know about QueryPerformanenceCounter, and believe it or not this simple Gettickcount counter works fine, but that's not the point anyway.


And because I'm not that experienced programmer is not a reason for you to call me an idiot. Specially I would not expect something like that from the moderator.


As a forum moderator, Mr.Promit, you fail!

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and the thread gets closed.

@Hodgman : You didn't remotely help the situation here, and your convoluted "reasoning" behind why he might take offense... well, it was dumb. Pease don't be doing things like that again, more so when a mod (ie ME) has already told the person in question to cool it.

@jsg007: I asked you to cool it, you apprently haven't.
a) he didn't call YOU and idiot, he called your program idiotic. Please learn to read what has been written not what you think has been written. Too many people take offense at percived slights and, believe me, if Promit thought you were an idiot he would have called you on it.
b) Regardless of what you think your program IS a benchmark as it is timing the differences between to things.
c) Cheap parting shots are not tolerated, more so when you've been asked to cool it and frankly they are just childish; don't do it again.

And before you make a thread complaining about this one being closed I will "remind" you that doing such is against the forum rules.

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      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 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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