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OpenGL VBO questions

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I'm learning VBOs for us in my vertex arrays. I found this article on the subject but I have a few questions. As seen in the artice, "data" is loaded into the VBO tagged bufferObject. That buffer is then bound to then "selected" using pglBindBufferARB(). Then the vertex pointer is set to point to...0? This is the part that confuses me. Does OpenGL just know to revert to the most recently bound VBO when passed null for a vertex array pointer? If this is indeed the case, would I do the exact same thing for my normals, materials, etc.? Just create the VA as normal but pass null (0) in as the pointer and just ensure I bind the correct VBO first? Assuming I am correct up to this point, can I use any of the VA rendering functions? glDrawElements, for example? And if I can, does it make sense to pass the indexed array as a VBO? Can you just pass anything into a VBO? I'd imagine you would only want to put stuff in there used by the GPU (that's the point, right? to avoid AGP transfers and use speedy VRAM), but is it possible to put any list of real numbers in there to be used however the hell you want? How do you get them out, then? It seems to me they're only available in situations like the one above, where you just select the VBO and hope what you're using next can use it? If anyone could answer some/all of these questions, I would be very appreciative. Thanks in advance! [Edited by - CyberSlag5k on March 14, 2005 9:27:06 AM]

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When you setup a VBO as the source for data the 'address' param. of the gl*Pointer() functions changes from meaning a memory address to meaning an offset in bytes from the start of the currently bound VBO (remember, OpenGL is a state machine so it knows the current state of bound buffers etc).

So, if you just have a buffer with vertex data in and you want to draw it all then yes, you'd supply an off set of 0. If the data is interleaved around other data in the same VBO (say vertex pos, vertex normal and texture coords) then you'd bind each pointer with an offset to that data. (glVertexPointer(...., 0), glNormalPointer(....., 12), glTexCoordPointer(....., 24) for example).

VBOs are basically VAs which can exist in video ram, so all VA calls apply to them when they are bound as the source for the data.

You can put indices in VBOs (there is a special token for it, check the VBO spec for details) which can help the speed as the gfx card is just reading from VRam and not VRam and System ram.
You can put practically anything in them (for example, if you are doing normal mapping you could put the various tangent space vectors in them), but the infomation will only be usefull if access and used by the GPU (again, see normal mapping as an example of this).

Its not so much a case of 'hope' you should know what you are binding next and what you are using it for, remember you are in complete control of the operations which go on.

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Thanks, _the_phantom_. I thought you'd be the one to answer my question :P. I think understand what is going on now. Plus I never really thought to check the official VBO spec, so now I have that as a resource too.

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heh, its a pretty safe bet that i'll turn up in a VBO or GLSL thread [wink]

the VBO spec is a pretty good source of infomation for VBOs, pretty much the best spec around for an extension (framebuffer_object is also pretty good from what i can remember)

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Share on other sites doesn't like it when I pass an int to the last argument of glVertexPointer. The compiler says it can't convert from an int to a const GLvoid. Do I have to do anything special? Right now it's just:

glNormalPointer(GL_FLOAT, sizeof(struct myStruct), 0);
glVertexPointer(3, GL_FLOAT, sizeof(struct myStruct), 12);

It makes it past the call to glNormalPointer (must just see 0 as NULL), but it blows up on the second line. Am I making the call wrong?


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Original post by _the_phantom_
if you check the spec there is a little macro at the top of the examples which they use for converting ints to GLvoid type for use with the function call.

I was afraid you'd say something like that. The spec has been down all day :(. Perhaps there is a mirror site? Er...we're talking about the extension registery on, right?

EDIT: Haha, thank you google cache! I'm in. Thanks phantom.

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ah, that would be a problem indeed, however via the magic of having used it in a project I happen to have it to hand [grin]

#define BUFFER_OFFSET(i) ((char *)NULL + (i))

Its as simple as that.
Used thusly;

glVertexPointer(3, GL_FLOAT, sizeof(terraindata), BUFFER_OFFSET(vertoffset_));

Where vertoffset_ is the offset in bytes from the start of the buffer to the first vertices position data [smile]

(coz I didnt want to recalculate the positions of various parts of my terraindata structure if I moved it around/added bits I calculatate the offsets and store them)

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Will my call to glDrawRangeElements need to be altered at all to use the VBO? Will the final argument still be the indexed array I'm using? I have it like that now and my object just...disappears.

Here's the generation code:

pglGenBuffersARB(1, &buffList[0]);
pglBindBufferARB(GL_ARRAY_BUFFER_ARB, buffList[0]);
pglBufferDataARB(GL_ARRAY_BUFFER_ARB, sizeof(*myStruct), vertList, GL_STATIC_DRAW);

glNormalPointer(GL_FLOAT, sizeof(myStruct), BUFFER_OFFSET(0));
glVertexPointer(3, GL_FLOAT, sizeof(myStruct), BUFFER_OFFSET(12));

And here's the rendering call:

//display list code
pglBindBufferARB(GL_ARRAY_BUFFER_ARB, buffList[0]);
pglDrawRangeElements(GL_TRIANGLES, 0, numVert-1, numIndex, GL_UNSIGNED_INT, index);

Am I using this wrong? I don't think I am...

EDIT: Wait a minute. Since I'm using glDraw*Elements(), I shouldn't have to bind the VBO as I render, right? I mean it won't make a difference as I only have 1 VBO anyway, but no VBO is being used in the call to pglDrawRangeElements (at least not until I move my indexed array into a VBO itself), so I don't need to bind anything. Right? That still doesn't explain why the object doesn't draw right.

To clarify, I have a key bound to switch the drawing mode from display lists to the VA (I want to note the performance changes). When I go from the DL to the VA, the object disppears.

[Edited by - CyberSlag5k on March 11, 2005 2:25:54 PM]

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I still can't seem to find out what I've done wrong with the VBOs. Perhaps the silence to mean everybody concures, that I've made no mistakes and that my record of life-long infallibility continues!

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heh, i'm sorry to say you have done something wrong [wink]

You are using VBOs as a source for the data, which means you need to tell OpenGL that it needs to look in the currently bound VBO for the data, this applies as much to the index as it does to the vertex data.

So, when you us a VBO for vertex data you use;

glbindBuffer(GL_ARRAY_BUFFER_ARB, somebuffer);
glVertexPointer(3, GL_FLOAT, sizeof(myStruct), BUFFER_OFFSET(0));

where the offset tells it how far into the VBO to find the first peice of data.

The same applies for the final drawing call, you have to tell OpenGL that you are wanting it to draw using a VBO as the source for the indices...

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER_ARB, someindexbuffer);
glDrawRangeElements(GL_TRIANGLES, 0, numVert-1, numIndex, GL_UNSIGNED_INT,BUFFER_OFFSET(0));

where again, the bufferoffset(0) tells OGL how far into the VBO to look for the first index.

An important point here;
Make sure you use GL_ELEMENT_ARRAY_BUFFER_ARB as the target. This is important as its a different binding point to GL_ARRAY_BUFFER_ARB. Meaning that you can do this;

glbindBuffer(GL_ARRAY_BUFFER_ARB, somebuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER_ARB, someindexbuffer);
glVertexPointer(3, GL_FLOAT, sizeof(myStruct), BUFFER_OFFSET(0));
glDrawRangeElements(GL_TRIANGLES, 0, numVert-1, numIndex, GL_UNSIGNED_INT,BUFFER_OFFSET(0));

and the glVertexPointer() will still be using somebuffer and the glDrawRangeElements() will be using someindexbuffer.

If you dont get it from that, think of it like this;
You have two varibles, foo and bar.
Now, when you bind a buffer to the GL_ARRAY_BUFFER_ARB attachment point its number is stored in foo and any gl*Pointer() calls which happen afterwards will look in foo for the VBO to use.
When you bind a buffer to GL_ELEMENT_ARRAY_BUFFER_ARB attachment point its number is stored in bar and only drawing operations read from bar, so you can set bar when you like and it wont be changed until you try to bind something to that attachment point again and it wont be read until you issue a draw command.

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Thank you for your response, _the_phantom_.


heh, i'm sorry to say you have done something wrong

Damn! Oh well, it had to happen some day ;).

Ok, I think I'm closer. I do this to create the VBOs:

pglGenBuffersARB(2, buffList);

pglBindBufferARB(GL_ARRAY_BUFFER_ARB, buffList[0]);
pglBufferDataARB(GL_ARRAY_BUFFER_ARB, sizeof(*vertList), vertList, GL_STATIC_DRAW);

pglBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, buffList[1]);
pglBufferDataARB(GL_ARRAY_BUFFER_ARB, sizeof(*indexList), indexList, GL_STATIC_READ);

and this to render:

pglBindBufferARB(GL_ARRAY_BUFFER_ARB, buffList[0]);
glNormalPointer(GL_TRIANGLES, sizeof(struct _Vertex), BUFFER_OFFSET(0));
glVertexPointer(3, GL_TRIANGLES, sizeof(struct _Vertex), BUFFER_OFFSET(12));

pglBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, buffList[1]);
pglDrawRangeElements(GL_TRIANGLES, 0, numVerts-1, indexLength, GL_UNSIGNED_INT, BUFFER_OFFSET(0));

It looks good to me, but I must be doing something wrong as it blows up on the call to pglDrawRangeElements(). I set the index VBO as a GL_STATIC_READ because I figured it was only being used for drawing indirectly. Using GL_STATIC_DRAW didn't help, though.

Thanks in advance!

[Edited by - CyberSlag5k on March 14, 2005 10:19:16 AM]

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Original post by _the_phantom_
define 'blows up'?

Program ignites in a ball of flaming code and cinder.

Or, if you'd like a more technical definition, my debugger tells me this:

Unhandled exception at 0x04e4642a in project3.exe: 0xC0000005: Access violation reading location 0x00000000.

Made a few changes but it's still not quite there. I now multiply the size of each piece of data by the number of elements in each array (I was before only passing the VBO the size of a single element), and I noticed that the call to pglBufferDataARB() had GL_ARRAY_BUFFER_ARB for the index VBO. I changed it to the appropriate GL_ELEMENT_ARRAY_BUFFER_ARB. It now looks like this:

pglGenBuffersARB(2, buffList);

pglBindBufferARB(GL_ARRAY_BUFFER_ARB, buffList[0]);
pglBufferDataARB(GL_ARRAY_BUFFER_ARB, sizeof(*vertList) * numVerts, vertList, GL_STATIC_DRAW);

pglBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, buffList[1]);
pglBufferDataARB(GL_ELEMENT_ARRAY_BUFFER_ARB, sizeof(*indexList) * numIndex, indexList, GL_STATIC_READ);

It still crashes on the call to pglDrawRangeElements :(. Oh well, back to stare at the code for a while longer before one of you says "Silly CyberSlag, you forgot to..."

[Edited by - CyberSlag5k on March 14, 2005 4:47:26 PM]

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      // Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
      AntTweakBar sample is Diligent Engine’s “Hello World” example.

      Atmospheric scattering sample is a more advanced example. It demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.
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