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• By elect
Hi,
ok, so, we are having problems with our current mirror reflection implementation.
At the moment we are doing it very simple, so for the i-th frame, we calculate the reflection vectors given the viewPoint and some predefined points on the mirror surface (position and normal).
Then, using the least squared algorithm, we find the point that has the minimum distance from all these reflections vectors. This is going to be our virtual viewPoint (with the right orientation).
After that, we render offscreen to a texture by setting the OpenGL camera on the virtual viewPoint.
And finally we use the rendered texture on the mirror surface.
So far this has always been fine, but now we are having some more strong constraints on accuracy.
What are our best options given that:
- we have a dynamic scene, the mirror and parts of the scene can change continuously from frame to frame
- we have about 3k points (with normals) per mirror, calculated offline using some cad program (such as Catia)
- all the mirror are always perfectly spherical (with different radius vertically and horizontally) and they are always convex
- a scene can have up to 10 mirror
- it should be fast enough also for vr (Htc Vive) on fastest gpus (only desktops)

Looking around, some papers talk about calculating some caustic surface derivation offline, but I don't know if this suits my case
Also, another paper, used some acceleration structures to detect the intersection between the reflection vectors and the scene, and then adjust the corresponding texture coordinate. This looks the most accurate but also very heavy from a computational point of view.

Other than that, I couldn't find anything updated/exhaustive around, can you help me?

• Hello all,
I am currently working on a game engine for use with my game development that I would like to be as flexible as possible.  As such the exact requirements for how things should work can't be nailed down to a specific implementation and I am looking for, at least now, a default good average case scenario design.
Here is what I have implemented:
Deferred rendering using OpenGL Arbitrary number of lights and shadow mapping Each rendered object, as defined by a set of geometry, textures, animation data, and a model matrix is rendered with its own draw call Skeletal animations implemented on the GPU.   Model matrix transformation implemented on the GPU Frustum and octree culling for optimization Here are my questions and concerns:
Doing the skeletal animation on the GPU, currently, requires doing the skinning for each object multiple times per frame: once for the initial geometry rendering and once for the shadow map rendering for each light for which it is not culled.  This seems very inefficient.  Is there a way to do skeletal animation on the GPU only once across these render calls? Without doing the model matrix transformation on the CPU, I fail to see how I can easily batch objects with the same textures and shaders in a single draw call without passing a ton of matrix data to the GPU (an array of model matrices then an index for each vertex into that array for transformation purposes?) If I do the matrix transformations on the CPU, It seems I can't really do the skinning on the GPU as the pre-transformed vertexes will wreck havoc with the calculations, so this seems not viable unless I am missing something Overall it seems like simplest solution is to just do all of the vertex manipulation on the CPU and pass the pre-transformed data to the GPU, using vertex shaders that do basically nothing.  This doesn't seem the most efficient use of the graphics hardware, but could potentially reduce the number of draw calls needed.

Really, I am looking for some advice on how to proceed with this, how something like this is typically handled.  Are the multiple draw calls and skinning calculations not a huge deal?  I would LIKE to save as much of the CPU's time per frame so it can be tasked with other things, as to keep CPU resources open to the implementation of the engine.  However, that becomes a moot point if the GPU becomes a bottleneck.

• Hello!
I would like to introduce Diligent Engine, a project that I've been recently working on. Diligent Engine is a light-weight cross-platform abstraction layer between the application and the platform-specific graphics API. 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 front-end for all supported platforms and provides interoperability with underlying native API. Shader source code converter allows shaders authored in HLSL to be translated to GLSL and used on all platforms. Diligent Engine supports integration with Unity and is designed to be used as a graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. It is distributed under Apache 2.0 license and is free to use. Full source code is available for download on GitHub.
Features:
True cross-platform Exact same client code for all supported platforms and rendering backends No #if defined(_WIN32) ... #elif defined(LINUX) ... #elif defined(ANDROID) ... No #if defined(D3D11) ... #elif defined(D3D12) ... #elif defined(OPENGL) ... Exact same HLSL shaders run on all platforms and all backends Modular design Components are clearly separated logically and physically and can be used as needed Only take what you need for your project (do not want to keep samples and tutorials in your codebase? Simply remove Samples submodule. Only need core functionality? Use only Core submodule) No 15000 lines-of-code files Clear object-based interface No global states Key graphics features: Automatic shader resource binding designed to leverage the next-generation rendering APIs Multithreaded command buffer generation 50,000 draw calls at 300 fps with D3D12 backend Descriptor, memory and resource state management Modern c++ features to make code fast and reliable The following platforms and low-level APIs are currently supported:
Windows Desktop: Direct3D11, Direct3D12, OpenGL Universal Windows: Direct3D11, Direct3D12 Linux: OpenGL Android: OpenGLES MacOS: OpenGL iOS: OpenGLES API Basics
Initialization
The engine can perform initialization of the API or attach to already existing D3D11/D3D12 device or OpenGL/GLES context. For instance, the following code shows how the engine can be initialized in D3D12 mode:
#include "RenderDeviceFactoryD3D12.h" using namespace Diligent; // ...  GetEngineFactoryD3D12Type GetEngineFactoryD3D12 = nullptr; // Load the dll and import GetEngineFactoryD3D12() function LoadGraphicsEngineD3D12(GetEngineFactoryD3D12); auto *pFactoryD3D11 = GetEngineFactoryD3D12(); EngineD3D12Attribs EngD3D12Attribs; EngD3D12Attribs.CPUDescriptorHeapAllocationSize[0] = 1024; EngD3D12Attribs.CPUDescriptorHeapAllocationSize[1] = 32; EngD3D12Attribs.CPUDescriptorHeapAllocationSize[2] = 16; EngD3D12Attribs.CPUDescriptorHeapAllocationSize[3] = 16; EngD3D12Attribs.NumCommandsToFlushCmdList = 64; RefCntAutoPtr<IRenderDevice> pRenderDevice; RefCntAutoPtr<IDeviceContext> pImmediateContext; SwapChainDesc SwapChainDesc; RefCntAutoPtr<ISwapChain> pSwapChain; pFactoryD3D11->CreateDeviceAndContextsD3D12( EngD3D12Attribs, &pRenderDevice, &pImmediateContext, 0 ); pFactoryD3D11->CreateSwapChainD3D12( pRenderDevice, pImmediateContext, SwapChainDesc, hWnd, &pSwapChain ); 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. To create a buffer, you need to populate BufferDesc structure and call IRenderDevice::CreateBuffer(). The following code creates a uniform (constant) buffer:
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 ); Similar, to create a texture, populate TextureDesc structure and call IRenderDevice::CreateTexture() as in the following example:
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 ); Initializing Pipeline State
Diligent Engine follows Direct3D12 style 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.)
To create a shader, populate ShaderCreationAttribs structure. An important member is ShaderCreationAttribs::SourceLanguage. The following are valid values for this member:
SHADER_SOURCE_LANGUAGE_DEFAULT  - The shader source format matches the underlying graphics API: HLSL for D3D11 or D3D12 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. See shader converter for details. SHADER_SOURCE_LANGUAGE_GLSL  - The shader source is in GLSL. There is currently no GLSL to HLSL converter. To allow grouping of resources based on the frequency of expected change, Diligent Engine introduces classification of shader variables:
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. This post describes the resource binding model in Diligent Engine.
The following is an example of shader initialization:
To create a pipeline state object, define instance of PipelineStateDesc structure. The structure defines the pipeline specifics such as if the pipeline is a compute pipeline, number and format of render targets as well as depth-stencil format:
// This is a graphics pipeline PSODesc.IsComputePipeline = false; PSODesc.GraphicsPipeline.NumRenderTargets = 1; PSODesc.GraphicsPipeline.RTVFormats[0] = TEX_FORMAT_RGBA8_UNORM_SRGB; PSODesc.GraphicsPipeline.DSVFormat = TEX_FORMAT_D32_FLOAT; The structure also defines depth-stencil, rasterizer, blend state, input layout and other parameters. For instance, rasterizer state can be defined as in the code snippet below:
// Init rasterizer state RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; //RSDesc.MultisampleEnable = false; // do not allow msaa (fonts would be degraded) RasterizerDesc.AntialiasedLineEnable = False; When all fields are populated, call IRenderDevice::CreatePipelineState() to create the PSO:
Shader resource binding in Diligent Engine is based on grouping variables in 3 different groups (static, mutable and dynamic). Static variables 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. They are bound directly to the shader object:

m_pPSO->CreateShaderResourceBinding(&m_pSRB); Dynamic and mutable resources are then bound through SRB object:
m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "tex2DDiffuse")->Set(pDiffuseTexSRV); m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); The difference between mutable and dynamic resources is that mutable ones can only be set once for every instance of a shader resource binding. Dynamic resources can be set multiple times. It is important to properly set the variable type as this may affect performance. Static variables are generally most efficient, followed by mutable. Dynamic variables are most expensive from performance point of view. This post explains shader resource binding in more details.
Setting the Pipeline State and Invoking Draw Command
Before any draw command can be invoked, all required vertex and index buffers as well as the pipeline state should be bound to the device context:
// Clear render target const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); // 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); m_pContext->SetPipelineState(m_pPSO); Also, all shader resources must be committed to the device context:
m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); When all required states and resources are bound, IDeviceContext::Draw() can be used to execute draw command or IDeviceContext::DispatchCompute() can be used to execute compute command. Note that for a draw command, graphics pipeline must be bound, and for dispatch command, compute pipeline must be bound. Draw() 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); Tutorials and Samples
The GitHub repository contains a number of tutorials and sample applications that demonstrate the API usage.

AntTweakBar sample demonstrates how to use AntTweakBar library to create simple user interface.

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 textures, using compute shaders and unordered access views, etc.

The repository includes Asteroids performance benchmark based on this demo developed by Intel. It renders 50,000 unique textured asteroids and lets compare performance of D3D11 and D3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

Integration with Unity
Diligent Engine supports integration with Unity through Unity low-level native plugin interface. The engine relies on Native API Interoperability to attach to the graphics API initialized by Unity. After Diligent Engine device and context are created, they can be used us usual to create resources and issue rendering commands. GhostCubePlugin shows an example how Diligent Engine can be used to render a ghost cube only visible as a reflection in a mirror.

• By Yxjmir
I'm trying to load data from a .gltf file into a struct to use to load a .bin file. I don't think there is a problem with how the vertex positions are loaded, but with the indices. This is what I get when drawing with glDrawArrays(GL_LINES, ...):

Also, using glDrawElements gives a similar result. Since it looks like its drawing triangles using the wrong vertices for each face, I'm assuming it needs an index buffer/element buffer. (I'm not sure why there is a line going through part of it, it doesn't look like it belongs to a side, re-exported it without texture coordinates checked, and its not there)
I'm using jsoncpp to load the GLTF file, its format is based on JSON. Here is the gltf struct I'm using, and how I parse the file:
glBindVertexArray(g_pGame->m_VAO);
glDrawElements(GL_LINES, g_pGame->m_indices.size(), GL_UNSIGNED_BYTE, (void*)0); // Only shows with GL_UNSIGNED_BYTE
glDrawArrays(GL_LINES, 0, g_pGame->m_vertexCount);
So, I'm asking what type should I use for the indices? it doesn't seem to be unsigned short, which is what I selected with the Khronos Group Exporter for blender. Also, am I reading part or all of the .bin file wrong?
Test.gltf
Test.bin

• That means how do I use base DirectX or OpenGL api's to make a physics based destruction simulation?
Will it be just smart rendering or something else is required?

# OpenGL Game thread synchronization (display & game logic)

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I am wondering how others handle this situation:

You have an application (e.g. a game) that calculates a state (e.g. the game logic) and also displays this one. From what I read it seems that the game logic is most of the time running in a different thread than the display. Which brings me to the question:

How is the display synchronized with the game logic? If there is no synchronization, we can have following situations:

- game is stepped forward, displayed, stepped forward twice, displayed, etc. --> the display will appear shacking!

- while the game is stepped forward a frame is displayed --> the display can appear "strange" (e.g. the bullet can appear as hitting the game character, but since its state was not yet updated, it will actually not hit it)

- A game character can be removed from the scene during the game logic calculation. If the display happens at that time, there might be a crash.

We can synchronize to some extent the 2 threads by locking resources (the last example above can be handled by deferring object destruction). But in order to avoid all above mentionned problems, one should run the 2 threads in alternance (or similar, e.g.  step the game twice, render, step the game twice, render, etc.). Doing so makes the use of 2 threads not interesting anymore, since a single thread would be running at the same speed (more or less) and all the resource locking synchronization would not have to be taken care of: a single thread would be easier and have the same result. No?

I guess that the game state must be demultiplied in some way (e.g. all positions would be stored as "current" and "forDisplay", and at the end of a game step, "current" would be copied to "forDisplay", so that the rendering thread would be able to run concurrently).

And what happens if the game logic needs to use some OpenGL commands? e.g. to render to FBO and do some simple image processing on that? Then there will be again the need to synchronize the 2 threads in order to be able to correctly switch the OpenGL contexts!

Just curious how things are done usually ;)

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From what I read it seems that the game logic is most of the time running in a different thread than the display.

In my experience, it doesn't make much sense to dedicate a whole thread to one small task, such as communicating with the graphics API.

In the engine's I've used, threading is not done in this way; their usage of threads is rotated 90 degrees to this design ;) There is one thread for each CPU core, and every thread contributes to task #1, then they all contribute to task #2, and so on... Usually this is achieved via a shared queue of "jobs" that need to be executed. The threads simply consume work out of this queue, and add collections of work back into it.

- while the game is stepped forward a frame is displayed --> the display can appear "strange" (e.g. the bullet can appear as hitting the game character, but since its state was not yet updated, it will actually not hit it)
- A game character can be removed from the scene during the game logic calculation. If the display happens at that time, there might be a crash.

These things should obviously not happen -- they're symptoms from two threads both using the same data set at the same time, which is a race condition!

Seeing that your update and render threads solving completely different problems, they don't even need to share much state because the data required by each is different -- render functions don't need "hitpoints" and update functions don't need "triangle count"s. There's nothing wrong with having a "NPC instance" with a position member used by the update thread, which owns a "model instance" that also owns a duplicated position member used by the render thread -- two different problems are best solved with two different data layouts. Don't try and represent everything in one big ball of sphagetti, and then have two completely different processes try and weave their way through it.

The update thread should produce a big blob of data that is consumed by the render thread, containing just the information required for rendering. The update thread should not have access to any data that is only used for rendering, and the render thread should not have access to any data that is only used for updating.

And what happens if the game logic needs to use some OpenGL commands?

Then it should ask the render thread to issue those commands, in the same way that it asks it to issue all the other rendering GL commands. There shouldn't be any real difference in this use case vs 'normal' rendering.

- game is stepped forward, displayed, stepped forward twice, displayed, etc. --> the display will appear shacking!

Often your rendering tasks are designed to run at some fixed display rate, e.g. 30Hz, 60Hz, etc. If so, you've got a time-based target for your updates -- a 60Hz game should try and advance the simulation by 16.6ms worth of 'ticks' before each render. If you're using vsync, then you can make a pretty accurate guess as to when each image will be displayed to the user (1/refresh-rate seconds after the last one), so you want to advance the simulation that far into the future, reliably, to avoid jitters.

This is one reason why I see absolutely no point on putting update/render on their own threads and leaving it up to the OS to make sure each one runs for an appropriate amount of time... You can determine how many updates are optimal for each render, and then perform them serially -- N-cores performing your updates, and then N-cores performing your rendering.

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game is stepped forward, displayed, stepped forward twice, displayed, etc. --> the display will appear shacking!

If you read the preferred way of updating the simulation and rendering in Fix your timestep, even in single threaded scenarios, it is possible that if the rendering took too long, the physics will start updating more often than rendering.
In other words this is a problem that appears in single threaded programs as well. It's not shacking, it's frame skipping.

However, I agree that without proper care, the update order can be pretty chaotic, and indeed it will look like it's shacking; which is exclusively a multithreading problem. However, let's see it in more detail:

First, Rendering only needs 4 elements from Logic, if you need more, you should rethink the design:

• Transformation state of every object: Position, Quaternion and scale. That's 40 bytes (64 bytes if you choose a matrix 4x4 representation)
• The playback state of the animation (if animation needs to be sync'ed from Logic). That's anywhere from 0 to 32 bytes
• A list of Entities created in a frame
• A list of Entities destroyed in a frame

Second, forget the idea that you need to render exactly what the simulation has. If your game can avoid that restriction (99% chance it can), you can relax the synchronization.

Third, locks aren't expensive, lock contention is.

Now, creation can be handled without invasive locks: Logic creates a list of entities and at the end of the frame, it locks a lightweight mutex, updates Graphic's list and releases the lock. Chances are, Graphic thread wasn't accessing that list because it has a lot to do. At the end of Graphic's update... it locks, clones the list, and releases the lock.
In both cases, it takes almost no time to work inside the locked resource and it consists of a little fraction of all the work they both have to do, so lock contention is extremely low. (Furthermore you can avoid mutexes entirely using a preallocated space and interlocked instructions, and only lock if the preallocated space got full, but I won't go there)

There's a catch here, and remember my second advise. You don't care that you're rendering exactly what is in the simulation. Suppose Frame A is simulated, and created 3 objects, but Graphics was too fast and looked into the list. Then loops again, uses renders frame A but without those 3 new objects. Do you really care? those 3 will get added in the next frame. It's a 16ms difference. And not a big difference because the user doesn't even know those 3 objects should've been there.

Same happens when destroying objects. Note that a pointer shouldn't be deleted until Graphics has marked that object as "I know you killed the foe, I'm done rendering it"; so that you're sure both threads aren't using the pointer. Only then you can delete the pointer. In other words, you've retired the object from scene, but delayed deleting the ptr.
Otherwise, as you say, a crash will happen.
So in this case, an object may be rendered one more frame that it should. Big deal (sarcasm).

Now we're left into updating position & animation data. You have two choices:

• You do care about consistency. Use a lightweight mutex when copying the physics transform to another place inside Logic thread, and from Graphics Thread do the same. In other words, is the same as above but with locks. Lock contention is again very low.

I've tried both and #1 really works ok if done properly (don't take my word, try for yourself! it's easy to switch between both, just disable/reenabled the mutexes!).
Note that #1 isn't a holy grail of scalability, because it can still slowdown your loop a lot due to cache sharing and forcing them to flush too often (which only happens if both threads are accessing the same data at the same time with and one of them writes to it).

Same happens with animation, but it's a bit more complex because you really don't want time going backwards in some cases (i.e. when spawning a particle effect at a given keyframe it could spawn twice), I won't go into detail. Getting that one right and scalable is actually hard (but again, solutions rely on the assumption that lock contention will be minimum).

Remember, you don't care that you're rendering the exact thing, but 99% of the time you will, and when it screws up it often gets unnoticed and fixes itself in the next frame.

And remember, synchronizing points 1 to 4 should only be a tiny fraction of what your threads do. Logic thread spends most of it's time integrating physics, a lesser part updating the logic side, and only then syncing.
Graphics spends most of it's time issuing culling, updating derived transform of complex node setups, sorting the render queue, and sending commands to the GPU; and only then syncing.

Note if you read transform state directly from the the Physics engine data you'll have terrible cache miss rates or will have to put a mutex to protect the physics integration step, and that does have a lot of lock contention.

All of this works if there are at least two cores. If the threads struggle for CPU time, then the "quirckiness" when rendering becomes awfully noticeable. Personally, I just switch to a single threaded loop when I detect single core machines. If you've designed your system well, creating both loops shouldn't give you any effort, at all. Just a couple of lines.

And last but not least there's the case where you really care about consistency, and you absolutely care that you're rendering exactly what is in the simulation.
In that case you can only resort to using a barrier at the end of the frame, clone the state from logic to graphics, and continue. If both threads take a similar amount of time to finish the frame, multi-threading will really improve your game's performance, otherwise one thread will stall and must wait for the other one to reach the barrier, and hence the difference between single-threaded version of your game and a multithreaded one will be minimal.

You asked how this is dealt with, and there is no simple answer, because multithreading can be quite complex and there's no simple answer. There are numerous way to deal with it.
A game may put locks for adding & removing objects & updating the transform, another game may not use locks for transforms. Another engine may use interlocked functions to add & remove objects without mutexes.
Another game may just use barriers. Another game may not use this render-split model at all and rely on tasks instead*. There's a lot of ways to address the problem; and it boils down to trading "visual correctness" vs scalability.

*Edit: And it can be like Hodgman described (all cores contribute to task #1, then to task #2, etc) or they may issue commands using older data and process independently (i.e. process physics in one task, process AI independently in another task using results from a previous frame, etc)

Edited by Matias Goldberg

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Hodgman and Matias,

Thanks a lot for the very clear and exhaustive explanations. The links you mentionned were also helpful.

To give a little bit more background: I used to get inspired a lot on the gamedev forums, I however am more in the field of simulation. There, it is of importance if something gets rendered wrongly, or if a frame gets skipped without specific reason. Interpolation between two states could work, but might lead to some unrealistic renderings too and confusion, specially when stepping generated videos later on (usually there is one frame per simulation step, which helps debugging certain set-ups). Finally, the simulation (or game logic) uses the openGL functionality, in order to generate virtual images, operate on them (e.g. image processing) and create an output. The time at which the "internal" or FBO rendering occures depends on the simulation loop and how it is programmed. So there I get another heavy restriction regarding multithreading: the rendering thread and "game logic" thread can both generate OpenGL commands, and need to switch contexts every time. In that case, locking (or rather blocking) the other thread is the only option.

Given the many constraints and limitations, I concluded that having an additional thread in charge of rendering would not give me much speed increase, but would complicate drastically the architecture.

My application uses basically one single thread (of course it also uses worker threads for specific tasks) that handles the "game logic" and the visualization. But I wanted to evaluate the benefits of spliting the task into 2 different threads, and maybe even offering the 2 alternatives.

Again thanks for the insightful replies!

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I see you intent issueing OpenGL calls from multiple threads. As you said, this is a very bad idea, and I personally avoid them due to lots of issues in the past; unless you keep 100% independent GL context for each thread.

Otherwise switching contexts is so error-prone (and driver bugs may appear, to be honest) that any performance gain you intend to get from going multithreading is going to be nullified, or worse; just leave it single threaded.

If your logic needs to issue rendering calls, you're not abstracting enough rendering from logic. If you're on a tight schedule, well ok; but if you've got the time, take your time to rethink how the systems relate; and whenever logic needs something from OpenGL, it requests to the render thread, and periodically check for results arrived.

For what you describe, your project appears to involve a lot of image processing from what has been rendered already (am I right?) in that case, since your game logic is rendering, and could not be decoupled, you should go single-threaded and rely on a method more like what Hodgman described (map buffer from GPU to CPU, then issue N threads to work on the received image, wait for all of them to finish) for multithreaded approaches.

Edited by Matias Goldberg