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OpenGL Indexed arrays.. pointless?

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Maybe im missing something here, but i recently went ahead and put my geometry into indexed arrays, thinking cutting my total verts from 14k to 2k would be a huge performance increase. Then i find out or, am led to believe that OpenGL dosn't support mutliple indexed arrays for the different arrays of information. For example, i have an array of unique verts, uv's, normals and tangents. I also have unique index arrays to access each of these, all the index arrays are the same size. I go ahead and try to implement this into opengl only to find out that i can't do this, and im forced to have ONE index array that indexes all of my arrays. I've looked through all of the functions i can trying to figure out a way around this, and while some seem like they may solve my problem (IE: Interleaved arrays) the way they work leads me to believe they don't really do what i think they do, or i just don't understand how they work. So, if i need one index, to index the vert,normal,uv and tangent arrays, i basically have the same amount of verts, normals, uvs, and tangents as before. Maybe im missing something, but it seems pointless to even use an indexed array in this sense. This puts me back at one vertex, per point on the triangle, which wouldn't be to bad, but then when i reuse that point, i cant make a index represent to spots in two different arrays. I don't see what's so hard about just specifiing multiple index arrays for each type of data, especially if the arrays at least are the same size.

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I feel the same way, actually... multiple indicies per vertex attribute do help in lowering memory requirements. But it's not OpenGL's fault -- video hardware simply doesn't support that functionality. This includes DirectX as well.

Supposedly, you can do it with Shader Model 3.0. I haven't looked into the details on how it is done, however.

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it's the age odl problem. Use a single index per vertex/normal/uv combination. There are good reasons for doing this though (so all is not lost). Compared to unindexed arrays, this is a fairly big memory saving. The other advantage is that using shared indices reduces the amount of vertex processing required by the GPU (unlike glBegin/glEnd and display lists), so it will be considerably faster in say a static VBO.

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You are right, there is no way to use multiple index arrays to access different streams at the same time. The hardware simply doesn't work that way.

It is not pointless though. If you have two vertices that share the same attributes (position, normal, tex coords, etc.. ), you can only store one vertex and use an index to access it from different triangles, rather than repeating that vertex for each triangle that references it. Depending on your type of models, this can be a massive gain. The worst case being something like a cube (you can hardly reuse any vertex since they all have different normals), and best case being something like a heightmap.

Video cards have a vertex transform cache, and that cache uses the index to know if previous vertices have already been transformed or not. Using multiple indices would likely disable that cache, killing all the performance. And obviously using a single index is better than using none. It's a matter of balance..

Y.

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alright, first thanks for the quick replys guys.

Now, i went ahead and tried implementing another method of getting the one index array idea. Im still having a hard time comprehending how its supposed to work, but this is what im understanding right now.

I need one number that indexs into an array of verts, normals, uvs, tangents and returns the correct value for the current vertex.

To do this, i wrote a function that takes in the current vert,normal,uv and tangent of the vertex as well as a vector for each property that starts empty for each one. If i can find within each vector the vert,normal,uv and tangent, all at the same index and equal to the ones i passed in, then i return that index, otherwise i push them all on to their individual vectors and return the last index.

I ended up saving 8 verts out of 14,228. Now, im thinking what you guys are saying is only remove dupe vertex positions, and go ahead an just render the duped normals, tangents and uvs?

It just dosnt seem possible for me to be able to have one index that correctly indexes into all four at the right time. I think im missing something important, but like i said, i'm having a hard time understand how this is really supposed to work. thx again.

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if you have hard edges like a cube has, don't bother using an indexed array. Simpy use an unidexed. Smooth objects work way better

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Just a thought. How do you compare positions, uv, normals and tangents? I'd suggest using a threshold when comparing.

E.g.
(pos1 - pos2).MagnitudeSqr() < 1e-4 and
dot(normal1, normal2) > cos(5.0 degrees).

Of course you can play with the thresholds, in case to not reject too many vertices. But by using a direct comparison of floats (pos1.x == pos2.x) may not give correct results.

Hope that helps a little.

HellRaiZer

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Well I'd say that you have too many different factors, of course, if you need hard edged lighting between polygons then there is no other way.

If you would for example just take only the position and uv, and then do average for normal and tangents you would end up with much less vertices.

Of course some normals and tangents shouldn't be blended together, but that's just one more parameter.

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Quote:
Original post by nefthy
if you have hard edges like a cube has, don't bother using an indexed array. Simpy use an unidexed. Smooth objects work way better


Well, the problem is looping through 14k verts for vertex animation sucks, and would be a lot better if i was only looping through the non-duped (2k). My problem is speed here, and im sure i would get a speed increase looping through 2k and not 14k ;)

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Quote:
Original post by HellRaiZer
Just a thought. How do you compare positions, uv, normals and tangents? I'd suggest using a threshold when comparing.

E.g.
(pos1 - pos2).MagnitudeSqr() < 1e-4 and
dot(normal1, normal2) > cos(5.0 degrees).

Of course you can play with the thresholds, in case to not reject too many vertices. But by using a direct comparison of floats (pos1.x == pos2.x) may not give correct results.

Hope that helps a little.

HellRaiZer


The way i compare, i don't think this would be a problem. I have all the dupes, and to remove dupes, i start with an empty stl vector of positions. I check the vector to see if the position exists, if it does return the index, if not push it on and return the last index.

This works great, i can remove over 10k verts. I can even index the verts, normals, tangents and uvs all correctly.

Now i have the problem of making the indices match up, which is where i get lost. I may try taking tangents out of the equation and see what i end up with.

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well if the you have 14k different vertex, normal, uvs combinations your stuck. Indexed arrays wont help with that and they will add aproximately 14k indexes to the data you have to pass to the gpu. What kind of model are you trying to draw, that has 14k different vertexes, normals, uvs combinations?

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OK, just a final update for those of you interested in this, or having similar issues.

The tips you guys provided did help, when i went and made that one function to check all together and get one index, where it only found 8 dupes, that was the way to go.

Apparently, just about every single vertex has it's own tangent. So trying to remove dupe tangents didnt work when there were only 8 ;)

I dropped down from 14k verts/normals/uvs to 3.2k. Im happy with this and will allow me to use the old method of passing in tangents to opengl. All i really wanted was less iterations for the animation system.

Thanks for the help all, and if anyone else runs into this problem, don't remove tangents.

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Quote:
Original post by nefthy
well if the you have 14k different vertex, normal, uvs combinations your stuck. Indexed arrays wont help with that and they will add aproximately 14k indexes to the data you have to pass to the gpu. What kind of model are you trying to draw, that has 14k different vertexes, normals, uvs combinations?


it was the tangents. i had 14k different tangents, which made everything else so massive :p

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nef,

regarding your comparison speed when looping through ton's of vertices: implement a hashlist on this, it will cut down the time needed to compare this stuff massivly (i had the same problem with our 3DSMAX exporter - with a hashlist the time cut down was by 70-80%).
Imagine a model with 80-90k vertices - doing this without the hashlist you can go for lunch in the while :-)

regarding the problem with normals: don't bother with computing the normals yourself in your code - just export the normals from your modelling package; this way will save you the headaches with normals smoothing and all this stuff, additionally you can be sure that you will have the normals exactly as the artists wants it.

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      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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