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OpenGL Converting D3D vertex buffers to OpenGL vertex arrays...

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Hi, Firstly, apologies if this is in the wrong forum - moderators, feel free to move it if it is, but given that it mentions Direct3D and OpenGL, I didn't know which forum to put it in, and I didn't want to cross-post. Secondly, apologies if this seems like a really obvious question. It kinda does, to me, but I couldn't find any threads which answered when I searched the forums. So, I've got a bit of code I found online which I'm playing around with, which renders stuff using DirectX 8, and I'm trying to abstract the rendering calls to a common interface so I can have the same code run on an OpenGL renderer. I'm pretty comfortable with OpenGL, not so comfortable with DirectX, although I do know a little bit. The interface has methods, which do roughly the following. This isn't the exact or complete code because I've just tried to show the interesting stuff - I can post the full source if it'll help.
void CreateVertexBuffer(UINT nLength, UINT nVertexSize, DWORD FVF, D3DPOOL Pool)
    // m_pD3DDevice being an IDirect3DDevice8
		Pool, &m_pVertexBuffer);

    m_pD3DDevice->SetStreamSource(0, m_pVertexBuffer, nVertexSize);

void LockVertexBuffer(UINT nVertices, BYTE **ppVertices)
    m_pVertexBuffer->Lock(0, nVertices*m_nVertexSize, ppVertices, D3DLOCK_DISCARD);

void UnlockVertexBuffer()

// ... Similar functions for the index buffers - you get the picture ...

void DrawVertexBuffer(int numVerts, int numIndeces)
    m_pD3DDevice->DrawIndexedPrimitive(D3DPT_TRIANGLELIST, 0, numVerts, 0, numIndeces/3);
The program calls CreateVertexBuffer once on initialisation, and then several times a frame it calls LockVertexBuffer, fills the resulting buffer with triangle data, calls UnLockVertexBuffer then DrawVertexBuffer, then locks the buffers again to generate more vertices, and so on. So, how to port this to OpenGL? I've got CreateVertexBuffer just setting up an array of vertices, LockVertexBuffer trying to point *ppVertices at that array and calling glLockArraysEXT(0, nVertices). UnlockVertexBuffer does nothing but call glUnlockArraysEXT(), and DrawVertexBuffer sets up the client states, does a glDrawElements, and disables the client states again. This doesn't seem to work, it crashes in annoying ways which don't give me a callstack I understand or the source code for the point where the crash occurs. I suspect this is because I'm misunderstanding the differences between D3D vertex buffers and OpenGL vertex arrays. If I understand correctly, locking a vertex buffer in D3D with the D3DLOCK_DISCARD allocates memory for a new buffer to write to every time you call it (giving you a pointer to that memory), and then automagically cleaning that memory up somehow at a later date when it's not being used (or is it done in the m_pVertexBuffer->Unlock?). Whereas, my OpenGL implementation sets up one vertex array to rule them all, and just lets the program write to it, render it, write to it again, render it... I'm not sure what needs to change in my OpenGL implementation to get the same behaviour as the D3D version. What do Direct3D vertex buffers do that OpenGL vertex arrays don't? And how would I go about adding that functionality to my OpenGL implementation?

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Hi ElectroDruid,

I don't know much about DX so i'm shooting in the dark here. Here is how i think it should be done :

1) CreateVertexBuffer should create a VBO or allocate a chunk of system memory depending on the value of the pool variable. Also, usage (which is hard-coded in your case) can be given as a hint to the driver when creating the VBO.

2) LockVertexBuffer should map the VBO using glMapBuffer. glLockArrays() is from the GL_EXT_compiled_vertex_arrays extension which is deprecated iirc. In D3D when you lock a VB, you are requesting a pointer in order to write to it, as you described. The equivalent functionality in GL is given by glMapBuffer.

3) UnlockVertexBuffer should unmap the VBO in order to be able to use it for rendering.

4) DrawVertexBuffer should set all the glXXXPointer variables and enable all the required client state. I'm not really sure, but i suspect that in the D3D case you posted, this happens in the CreateVertexBuffer function. Maybe because the vertex format is needed to be known when locking the buffer? I'm not sure.
Finally you can safely call glDrawElements.

Hope that helps. As i said i don't know much about DX, so if anyone find any mistakes in the above descriptions, please correct me.


EDIT : I think, until now, there is no flag in GL equivalent to D3DLOCK_DISCARD, and other similar flags. I think this is because you can't specify the size of the memory you are going to modify when locking/mapping the VB. IIRC this will be added in Long Peaks. Check the latest OGL Pipeline article for more info.

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Original post by HellRaiZer
EDIT : I think, until now, there is no flag in GL equivalent to D3DLOCK_DISCARD, and other similar flags.
You can perform a discard lock in OGL by calling BufferData with a NULL pointer.

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The program calls CreateVertexBuffer once on initialisation, and then several times a frame it calls LockVertexBuffer, fills the resulting buffer with triangle data, calls UnLockVertexBuffer then DrawVertexBuffer, then locks the buffers again to generate more vertices, and so on.

I too know very little about Direct3D. But I wonder, are you rendering static objects? and if so then why must you keep filling the Vertex Buffer several times each frame? Do you use a single Vertex Buffer to render multiple objects?

Anyhows... with Vertex Buffer Objects in OpenGL, for static objects you only need to generate the Vertex Buffer Object, and load in the Vertex Data once, which is stored in Video Memory. You then can render them very much alike regular Vertex Arrays in OpenGL.

Note, you must use a separate, and different target/type of Vertex Buffer Object for Vertex Indices (ELEMENT_ARRAY_BUFFER, rather than ARRAY_BUFFER).

You allocate/load in the Vertex Data with glBufferData(), and at this time you specify the data usage, whether it be STATIC/DYNAMIC/STREAM along with READ/COPY/DRAW flags. You can then update the data via glBufferSubData().

You can also gain direct access to data via glMapBuffer()/glUnmapBuffer(), which returns a pointer to the Vertex Data.

BTW, it's recommended that you use glDrawRangeElements(), rather than glDrawElements() when drawing Vertex Buffer Objects, which you can read about in the article I link to below.

Finally, rather than try and learn/understand from my brief explainations, I recommend you checkout THIS.

To conclude, I made some pesudo code...


I think the official OpenGL API is HERE.

// init vertex buffer object...
// note, this would be done when you load your geometry

// first you generate your vertex buffer object via...
void glGenBuffers(GLsizei n, GLuint * buffers)

// then you bind this buffer via... void glBindBuffer(GLenum target, GLuint buffer)
void glBindBuffer(GLenum target, GLuint buffer)

// then you allocate/load data into this buffer via...
// note, "GLenum target" set to GL_ARRAY_BUFFER for vertex data, and GL_ELEMENT_ARRAY_BUFFER for vertex indices
void glBufferData(GLenum target, GLsizeiptr size, const GLvoid * data, GLenum usage)

// and remember to unbind your vertex buffer object via passing 0 for "uint buffer"...
void glBindBuffer(GLenum target, GLuint buffer)

// and later on when it comes to drawing...
// you draw alike regular opengl vertex arrays, except you specify offset relative to each vertex buffer object
// note, this would obviously be done in a separate function

// enable vertex arrays

// bind your vertex data VBO
void glBindBuffer(GL_ARRAY_BUFFER, GLuint buffer);

// set your vertex array pointers
// note, instead of pointer you use offset relative to origin of VBO data
glVertexPointer(3, GL_FLOAT, 0, (const GLvoid*)0);
glNormalPointer(GL_FLOAT, 0, (const GLvoid*)normalOffset);
glTexCoordPointer(2, GL_FLOAT, 0, (const GLvoid*)texCoordOffset);

// remember to unbind your VBO
glBindBuffer(GL_ARRAY_BUFFER_ARB, 0);

// now bind your vertex indices VBO
void glBindBuffer(GL_ELEMENT_ARRAY_BUFFER_ARB, GLuint buffer)

// draw vertex array
// note, again instead of pointer you use offset relative to origin of VBO data
// note, use glDrawRangeElements() for best performance
void glDrawRangeElements (GLenum mode, GLuint start, GLuint end, GLsizei count, GLenum type, (const GLvoid*)indexOffset);

// again, remember to unbind your VBO

// disable your vertex arrays

Please correct me if I have made any mistakes. I hope this helps :).


[Edited by - yosh64 on July 11, 2007 10:08:29 AM]

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Wow... Okay, I have to hold my hands up and admit that I'm a bit out of my depth here. I've not used any of the advanced (read: not immediate mode) stuff in OpenGL since version 1.2 (hence the use of the deprecated extensions in my code). I don't know what's new since then and I'm kinda lost.

VBOs, glMapBuffer, glBufferData, glBufferSubData, glMapBuffer, glUnmapBuffer... All of these things are new to me. I've been working as a gameplay coder on consoles for nearly 5 years now, and haven't done any graphics programming in that time, so I'm clearly pretty out of touch with how the APIs work nowadays. This is a pet project on the PC, so I need to play catch-up.

I looked on NeHe but didn't see anything which might be of use - can someone point me to any kind of resource/tutorial/example code/whatever which demonstrates this new (to me at least) OpenGL functionality in action, so I can see what makes it tick?

Oh, and:

But I wonder, are you rendering static objects? and if so then why must you keep filling the Vertex Buffer several times each frame? Do you use a single Vertex Buffer to render multiple objects?

You hit the nail on the head, kinda. I'm not rendering static objects, I'm rendering very dynamic ones (metaballs, specifically), so I'm needing to construct new meshes every frame. I mean meshes as a plural in the sense that I want to make sure that (for example) if two metaballs seperate, and form seperate unconnected meshes, I want to render both of them rather than have one of them suddenly vanish. I'm trying to use a single vertex buffer for everything, even though I'm aware that it's probably wrong, because that's the only way I know about how to do this kind of thing. It strikes me that the most efficient thing would be to do the maths to calculate a bunch of triangles (connected or not), and then just pipe all the triangles to get rendered at the same time. I'm aware that this might not work, and that I might need to render each mesh seperately, but right now I'm just trying to get *something* onscreen without the program crashing.

I guess what I really need (and I will thank my lucky stars if such a thing exists, although I suspect it doesn't) is something like a piece of source code (a "Rosetta Stone", if you will) which draws something very simple (a sphere, or a cube) using Direct3D vertex buffers, and can do the same thing with the same results in OpenGL using vertex arrays, or VBOs, or whatever the best equivalent is. Does such a thing exist?

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I am not sure what your OpenGL implementation looks like, but I have some comments about your original post:

If you allocate the vertex buffer in D3DPOOL_DEFAULT, Direct3D tries to put it in video memory or AGP memory (unless you're using software vertex processing, then it has to be in system memory). Either way, the memory is not directly accessible by you.

When you call Lock() on the buffer, Direct3D maps the vertex buffer to a special part of memory; when you perform a write, it doesn't go to system memory but is routed through an I/O bus and to the actual location of the buffer. When you call Unlock(), it unmaps the memory so that you no longer have direct access to it. I think that if you're getting a crash, it might be because you're keeping around this pointer.

With D3DLOCK_DISCARD, Direct3D discards the entire contents of the vertex buffer. The old vertex buffer could be floating around somewhere; your video card could be rendering from it right now. You would never know, because it's not in your control anymore. Instead, you get mapped a new vertex buffer that has nothing to do with your old buffer. This means all the data you uploaded before is gone, too.

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Original post by ElectroDruid
I looked on NeHe but didn't see anything which might be of use - can someone point me to any kind of resource/tutorial/example code/whatever which demonstrates this new (to me at least) OpenGL functionality in action, so I can see what makes it tick?

Can't point you to any direct turotials, but whenever i'm interested in seeing whats new or getting to grips with an opengl extension i usually check out Delphi3D first and its extension list each with full spec. Although i guess its no different to opengl.org list. They canbe a bit hard to get into, but after a while they begin to make sense.

I guess you could also search on thos sites for more specific information such as tutorials.

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I'm trying to use a single vertex buffer for everything, even though I'm aware that it's probably wrong, because that's the only way I know about how to do this kind of thing.

I don't see anything wrong with doing this, and are sure it's quite possible. When loading the scene, I would just allocate one Vertex Buffer Object, large enough to hold the maximum amount of vertex data you will need.

Ohh, if you would prefer to update your vertex data in system memory, then when it comes to rendering you can just bind your VBO, and use glBufferSubData() to copy and update your Vertex Buffer Object.

Remember you will also need to update your vertex indices, or maybe just change the number of elements to draw, if I make any sense?

Finally, remember for getting started you may prefer not even to use Vertex Buffer Objects, and/or even Vertex Arrays, and go back to the basics of glBegin()/glEnd(), hehe.

Here are a couple of tutorials I goog'd on Vertex Buffer Objects, THIS, and NeHe lesson 45. But I think you may need to refer back to that original link on Vertex Buffer Objects I posted, or the OpenGL API or something.

You can find an example of both Vertex Arrays, and Vertex Buffer Objects (refered to as AGP Memory) on page 5 of the OpenGL section of ultimategameprogramming.com. You can probably also find a simular example for DirectX at ultimategameprogramming.com also.


[Edited by - yosh64 on July 12, 2007 1:10:25 AM]

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Thanks for all the replies and links, I think I'm starting to get a handle on how VBOs work now - although I'm clearly not quite there yet :) I've got code which compiles and runs without crashing, but it's not drawing anything. I've tried the usual testing stuff (setting a non-black glClearColour, disabling culling, lighting, texturing and alpha blending, triple-checking my camera position, orientation, clip planes and FOV to make sure it matches what the Direct3D renderer is doing, and drawing an immediate mode test quad to prove that I can see it), and all of that seems to work fine. The rest of the program hasn't changed, and it seems to be filling the vertex and index arrays with sensible values, and telling the renderer to draw about the number of polygons I'd expect. I just don't see any polys, and I'm at a bit of a loss as to what could be going wrong.

I've ended up trying to use glDrawArrays for the rendering, because glDrawElements was crashing, and I can't get my compiler to accept glDrawRangedElements for some reason. There's also potentially some weirdness with the way I'm using glBindBufferARB in my Lock/Unlock functions, but I'm still not 100% sure how glBindBufferARB works. If someone could take a quick look at what my code is doing, and give me an idea about what might be going wrong, that would be great.

GLuint vboVerts = 0;
GLuint vboIndeces = 0;
SVertex* mpVertexBuffer;
UINT* mpIndexBuffer;

struct SVertex
Vector3f v; // Vertex
Vector3f n; // Normal
Vector2f t; // Texture coordinates (currently not used)

void Initialize()
// ... Set up the window, the openGL extensions, and set some default GL states
// (lighting, texturing and backface culling turned off for testing, etc) ...

CreateVertexBuffer(MAX_VERTICES, sizeof(SVertex));

void Uninitialize()
delete[] mpVertexBuffer;
delete[] mpIndexBuffer;

glDeleteBuffersARB(1, &vboVerts);
glDeleteBuffersARB(1, &vboIndeces);

void CreateVertexBuffer(UINT nLength, UINT nVertexSize)
mpVertexBuffer = new SVertex[nLength];
glGenBuffersARB(1, &vboVerts);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboVerts);
glBufferDataARB(GL_ARRAY_BUFFER_ARB, nLength * nVertexSize, mpVertexBuffer, GL_DYNAMIC_DRAW_ARB);

void LockVertexBuffer(UINT nVertices, BYTE **ppVertices)
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboVerts); // Not sure if this should be here
mpVertexBuffer = (SVertex*)(*ppVertices);

void UnlockVertexBuffer()
glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0); // Not sure if this should be here

void CreateIndexBuffer(UINT nLength)
mpIndexBuffer = new UINT[nLength];
glGenBuffersARB(1, &vboIndeces);
glBufferDataARB(GL_ELEMENT_ARRAY_BUFFER_ARB, nLength * sizeof(UINT), mpIndexBuffer, GL_DYNAMIC_DRAW_ARB);

void LockIndexBuffer(UINT nIndices, BYTE **ppIndices)
glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, vboIndeces); // Not sure if this should be here
mpIndexBuffer = (UINT*)(*ppIndices);

void UnlockIndexBuffer()
glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, 0); // Not sure if this should be here

void DrawVertexBuffer(int numVerts, int numIndeces)
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboVerts);
glNormalPointer(GL_FLOAT, sizeof(SVertex), &mpVertexBuffer[0].n);
glVertexPointer(3, GL_FLOAT, sizeof(SVertex), &mpVertexBuffer[0].v);
glIndexPointer(GL_UNSIGNED_SHORT, 0, 0);


// glDrawElements(GL_TRIANGLES, numIndeces, GL_UNSIGNED_INT, mpIndexBuffer); // This crashes the program
// glDrawRangeElements(GL_TRIANGLES, 0, numIndeces, numIndeces, GL_UNSIGNED_INT, mpIndexBuffer); // I couldn't get this to compile
glDrawArrays( GL_TRIANGLES, 0, numIndeces);


Usage: The rest of the code is doing stuff in the following order, about half a dozen times per frame...

... Generate a bunch of vertex/index data ...

Any ideas?

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I think there are a couple of problems with the code you posted.

1) You don't have to allocate memory for passing it to the glBufferData() function. You can pass NULL in there. This way, when you Lock the VB/IB you won't overwrite the pointer value in 'mpVertexBuffer' and 'mpIndexBuffer'.

2) About your comments/questions in the Lock functions. Yes you have to bind the buffer before mapping it. OTOH there is no need to bind a zero buffer when unmapping, but that's ok if you re-bind it later for using it.

3) What's the value of 'mpVertexBuffer' when specifying glXXXPointer in DrawVertexBuffer? If it holds the last value from the Lock function, then this is wrong. If it's NULL then it should work. The point is that when using VBOs you don't specify a true pointer in memory, but an offset in the VB.

4) glIndexPointer() isn't responsible for setting the triangle indices pointer. It is related to per-vertex indexed color, which you don't need. In order for the glDrawElements to work you have to bind the index buffer (as you do) and then pass an offset in the IBO as the last parameter in the glDrawElements() call.

The DrawVertexBuffer() function should look something like this :

void DrawVertexBuffer(int numVerts, int numIndeces)
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboVerts);
glNormalPointer(GL_FLOAT, sizeof(SVertex), (void*)(0 + sizeof(Vector3f)));
glVertexPointer(3, GL_FLOAT, sizeof(SVertex), (void*)(0));


glDrawElements(GL_TRIANGLES, numIndeces, GL_UNSIGNED_INT, 0);


I hope i haven't done any mistakes, and the above makes sense.


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Firstly, I'm not sure as to how you are using your functions... but, you must be sure to unbind each type of your bound VBO's after use. For the way I see your code, I think you just need to unbind each type of your VBO's in your CreateVertexBuffer, CreateVertexIndexBuffer, and your DrawVertexBuffer functions. That means in your DrawVertexBuffer function, you should unbind for each of your GL_ARRAY_BUFFER_ARB, and GL_ELEMENT_ARRAY_BUFFER_ARB VBO's.

Hmm, I quickly looked through THIS, and I don't think there is much written about when to unbind your VBO's and such, so my advise is just based on my experience, as I also ran into such problems when first using VBO's.

And yes, you should both bind, and unbind your VBO's in your Lock/Unlock functions.

Anyhows I hoped that helps :).


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@ HellRaiZer

I took your advice (passing NULL into the glBufferDataARB() calls in CreateVertexBuffer and CreateIndexBuffer, not directly allocating any memory for mpVertexBuffer or mpIndexBuffer, and I experimented with removing the glBindBufferDataARB() calls in my unlock functions although it doesn't seem to make much difference to anything), and rewriting the DrawVertexBuffer() function) as you suggested. I'm getting a few frame's worth of garbage, followed by a crash. With regards to your question:

What's the value of 'mpVertexBuffer' when specifying glXXXPointer in DrawVertexBuffer? If it holds the last value from the Lock function, then this is wrong. If it's NULL then it should work. The point is that when using VBOs you don't specify a true pointer in memory, but an offset in the VB.

mpVertexBuffer and mpIndexBuffer both get set the first time the respective Lock functions are called, and still have the same values when DrawVertexBuffer is called, since nowhere in my code resets those pointers to NULL. Given than none of my code explicity does anything through those pointers (they're not passed into glBufferDataARB when the buffers are created, and in your version of the draw code, they're not referenced in glNormalPointer or glVertexPointer either). So, do I even need to keep those pointers around inside my rendering code?


I'm not quite sure I follow you. Firstly, I assume that by unbinding, you mean calling glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0) or glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, 0)? Just checking...

I didn't quite follow what you said I should bind or unbind at which point, but I've added an unbind to CreateVertexBuffer, so it looks like this:
glGenBuffersARB(1, &vboVerts);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboVerts); // Bind
glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0); // Unbind

And similarly added an unbind to the end of CreateIndexBuffer. I've left the Lock and Unlock functions the same (ie Lock binds the buffer, Unlock unbinds it). And I'm also unbinding both buffers at the end of the draw function.

Still no joy :( Still getting garbage and crashes. I'll go through all of those tutorials and the white paper again with a fine tooth comb to see if there's anything I missed, I guess. It's just annoying that I can't debug any of the crashes. It seems like VBOs are pretty all-or-nothing; either they completely work, or they crash with no way of telling you why they crashed.

EDIT: although glDrawElements still crashes (as does glDrawRangedElements, which I figured out how to get compiling), using glDrawArrays actually gives something that looks a bit like what I expect. The vertices I can see all seem to be getting generated in the right place, but either the indeces are wrong or I'm missing about half of the vertex data - there are a lot of holes in my mesh, and a lot of weird triangles in places I don't expect. I can post the current version of my code, and screenshots about how the GL version and the D3D version look different if it'll help - although I suspect that it's wrong to be using glDrawArrays in this context anyway, so I might just be barking up the wrong tree.

[Edited by - ElectroDruid on July 15, 2007 3:23:54 PM]

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Just to let everyone know that I sorted the problem. I am a happy bunny.

I went back to the red book (I have an old copy) to read up on glDrawElements and glDrawArrays, and remembered that glDrawArrays doesn't use index data, so if I was getting crashes, it must have been because of the index data. Turns out I was using GL_UNSIGNED_INT when the program was storing indeces as unsigned shorts. Doh! The problem is fixed now, and the program renders beautifully under both APIs.

Many thanks for your help, it was much appreciated!


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    • By DiligentDev
      This article uses material originally posted on Diligent Graphics web site.
      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.
      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 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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