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OpenGL Engine design questions

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Thus far I've found the gamedev forums, and this forum in particular, to provide very useful information on everything that has to do with game engines. I've read many 'classic' threads where Yann L. explains everything from shadow mapping to scene graph implementations, and they've proven very helpful. However, in the process of working on my own rendering, I'm finding it difficult to ty all these theories and 'best designs' together in a coherent design that makes sense for me. I'll list the issues I'm running into and my thoughts about them one by one below... Also note that I'm using OpenGL, and though the renderer is designed to easily allow support for other libraries such as Direct3D I see no need to this yet, so I may sometimes refer to OpenGL specific things.
  • The scenegraph Everything I've read about the subject has led me to believe that spatial ordering and other optimization techniques don't belong in a scene graph. Currently I have a very limited SceneGraph class that consists of GroupNode and GeometryNode objects only (both are derived from SceneNode). GroupNodes can have children, GeometryNodes can't have children, so all geometry nodes are leaves of the tree. This works exactly the way I thought a scene graph is supposed to work (I can add parent-child relations between objects in a scene, and transformations in each node are relative to the parent node). Though many threads I've read about scene graphs add more types of nodes, I probably won't be doing that in the near future because I want things to remain as simple as possible until I've grown more comfortable with the design. So far so good, but my understanding starts lacking when it comes to passing objects from the scene graph to the Renderer. This has to do with the way in which GeometryNodes are defined in my engine:
    • GeometryNode The only thing that makes a GeometryNode different from its parent, SceneNode, is that it contains a BaseGeometry object. The BaseGeometry class is the parent class for all geometry in the engine. So you can make a BoxGeometry object, for example, that inherits from BaseGeometry and which defines the geometry for a box. I'm also working on classes like 'CaligariGeometry', or '3DSMaxGeometry' that will load geometry from disk. Similarly, I imagine there would be TerrainGeometry objects. Because all these Geometry classes inherit from BaseGeometry, they can all be passed to GeometryNodes in the scene graph. The scene graph doesn't really care what the geometry represents, it only knows about GeometryNodes, but what they contain is not of the scene graph's concern.
    • BaseGeometry BaseGeometry, as said, contains geometry data. I currently distinguish between many types of buffers. Think IndexBuffer, VertexBuffer, NormalBuffer, TexcoordBuffer etc. The reason why I did this is because I'm using the OpenGL buffer objects extension, and the framerate was higher when I used this system of many buffers for one object, rather than one large buffer with Vertex objects that contain everything from vertex position data to texture coordinates. I'm quite sure my tests are to be taken too seriously as I've never used any substantial number of vertices. Thus far all I'm rendering is a 100x100 grid with a couple of boxes on it, because I cannot load arbitrary 3D file formats yet that would allow me to more easily load more data.
    This is actually my first problem. With so many buffer objects for each (potentially small) geometric entity I foresee the rendering being horribly inefficient. Suppose you render 10,000 boxes with this scheme. Every box has 8 vertices, 8 colours, and 36 indices. That means this scene requires the use of 30,000 buffer objects whose contents all range from 8 to 36 indices. I thought it would be more efficient if the Renderer class kept several large buffers itself, but as far as I know that means you'd have to copy the geometry data into the renderer's buffers every frame (because with culling etc. going on I can't be certain if a geometric entity is still supposed to be in the renderer's buffers or not), which defeats the point of using vertex buffer objects altogether.
The second thing I have a hard time understanding how culling comes into play. It seems that using a scene graph for culling is not good; a scene graph is not meant for that. Reading through one of the threads on gamedev.net where Yann L. explains all about terrain and scene graphs, I got the impression that it may be good practise to let geometry cull itself. I can see the advantages of this in for example, a TerrainGeometry class, where the TerrainGeometry would implement a spatial partitioning scheme. This way the scene graph will still think of the terrain as a single "GeometryNode" entity, but the terrain itself could be a quadtree without the scene graph ever knowing. For terrain it'd work well, but other entities? Would it work to have an OctreeGeometry as a base class and have, for example, classes like 3DSMaxGeometry also inherit from OctreeGeometry (next to BaseGeometry)? I'm guessing this won't work as well... Animated models aren't really 'optimised' by octrees (from what I've read), and I can't see the benefit of putting small static geometry in an octree either. An other solution could be to work with several 'phases', first of which would be to update the scene graph, and then send all geometry to a culling phase. The drawback I can see to this is that all geometry will be treated equally, which I assume to not be desirable in all cases. Culling algorithms are going to be different for different types of geometry, so some distinction should still be made. These are just some of the questions I'm facing, and perhaps I shouldn't even bother with them and proceed making a horrible inefficient renderer that is to be improved upon later, but I'd still like to hear some opinions, as I find it nearly impossible to 'move on' without addressing any of these concerns. I constantly fear that I'm making my engine so inefficient that the second I'm done with it, I'll want to scratch half of it. Eventually, facing these questions will be inevitable so I thought I might as well do it now. Thanks in advance for any feedback :)

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I'm gonna do some shameless advertising first ;)

I've written a series of articles about engine architecture, intended for beginners here :
http://www.beyond3d.com/content/articles/98 (Part 1)
http://www.beyond3d.com/content/articles/102/ (Part 2)
(More written, but not published yet :( )

You might find something useful in there.


I tend to rename things, giving good names is difficult, but I find important to name things as accurately as possible in order to be understood easily.
So in my engine design (not necessarily the best, it's just one solution to the problem, probably not the worst at least ;) ), I have a SceneTree (a Tree is a Graph, but everyone gets what a tree is w/o any further specifications), a SpatialGraph (Directed Acyclic Graph), and I used to have a RenderGraph, but I replaced it by a kind of RenderQueue.

The SceneTree is solely used for hierarchical animation. (Be it skeletal animation or a sword held in a character's hand.) This node type has an update(...) function that allows animation, and the nodes make a tree (a single parent, any number of children).

The SpatialGraph is used for culling (in fact finding what's visible). It's its only purpose in life to make culling fast.

The RenderQueue, is filled during culling in the spatialgraph. It's not a single array though, it's a little more complex than that. I did write something directly inspired from Yann Lombard thread about his material system.
The only task of this RenderQueue is to render whatever has been found visible FAST. (ie means sorting sub arrays per key, each key holding data such as depth, shader ID...; whatever is appropriate.)



Examples:
Terrain
It has a single node in the SceneTree, that does nothing.
(Nothing happens based on time on this terrain.)
It's made of a number of nodes in that SpatialGraph, making a Quadtree, which root is the Terrain's SpatialNode, and Leaves would contain mesh or pointers to meshes.

Character
It has a number of nodes in the SceneTree, for Meshs and Bones, which both might be animated hierarchicaly. It might also have something attached to his hand(s), that would need hierarchical relationship.
It has one SpatialLeaf node per mesh.


I call Geometry a Mesh instead.
I might be a bit synthetic, ask if I'm not clear enough.

[Edited by - Ingenu on November 20, 2008 5:14:29 AM]

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I very much like the SceneTree, SpatialGraph and RenderView nomenclature. It says what it does and this is a huge advantage over the word SceneGraph. I will use Ingenu terms from now on.

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If all of the boxes are the same, then why would you need so many vertex buffers? Can't you reuse them?

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It's very hard to think of some kind of general 3D engine architecture which can handle everything in a nice object-oriented way and is also extremely fast and efficient. If something is for everything - it's for nothing.

Why not to keep all subsystems separately? Terrain could store all its nodes in quad-tree structure, characters could have its own hierarchy tree, and world geometry might be handled/occluded by its own BSP/PVS, or oct-tree structure (depending on what we need).
All subsystems write renderable items into render manager (render queue), which perform sorting by depth, material ID, priority, or whatever we want.

Using this approach we can optimize every single subsystem without touching the others.

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I suppose I'll throw my design into the discussion too, although it doesn't deal with the scene graph side of things as much as you are looking for:

Hieroglyph Design

You don't actually need to have many buffers to represent many objects. You can simply have a single buffer (or multiple large ones if you like) and each object indexes into that big buffer instead of each object having its own. Then you just need to centralize the access to the buffer, which can be coordinated through your renderer interface.

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      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.
    • By reenigne
      For those that don't know me. I am the individual who's two videos are listed here under setup for https://wiki.libsdl.org/Tutorials
      I also run grhmedia.com where I host the projects and code for the tutorials I have online.
      Recently, I received a notice from youtube they will be implementing their new policy in protecting video content as of which I won't be monetized till I meat there required number of viewers and views each month.

      Frankly, I'm pretty sick of youtube. I put up a video and someone else learns from it and puts up another video and because of the way youtube does their placement they end up with more views.
      Even guys that clearly post false information such as one individual who said GLEW 2.0 was broken because he didn't know how to compile it. He in short didn't know how to modify the script he used because he didn't understand make files and how the requirements of the compiler and library changes needed some different flags.

      At the end of the month when they implement this I will take down the content and host on my own server purely and it will be a paid system and or patreon. 

      I get my videos may be a bit dry, I generally figure people are there to learn how to do something and I rather not waste their time. 
      I used to also help people for free even those coming from the other videos. That won't be the case any more. I used to just take anyone emails and work with them my email is posted on the site.

      I don't expect to get the required number of subscribers in that time or increased views. Even if I did well it wouldn't take care of each reoccurring month.
      I figure this is simpler and I don't plan on putting some sort of exorbitant fee for a monthly subscription or the like.
      I was thinking on the lines of a few dollars 1,2, and 3 and the larger subscription gets you assistance with the content in the tutorials if needed that month.
      Maybe another fee if it is related but not directly in the content. 
      The fees would serve to cut down on the number of people who ask for help and maybe encourage some of the people to actually pay attention to what is said rather than do their own thing. That actually turns out to be 90% of the issues. I spent 6 hours helping one individual last week I must have asked him 20 times did you do exactly like I said in the video even pointed directly to the section. When he finally sent me a copy of the what he entered I knew then and there he had not. I circled it and I pointed out that wasn't what I said to do in the video. I didn't tell him what was wrong and how I knew that way he would go back and actually follow what it said to do. He then reported it worked. Yea, no kidding following directions works. But hey isn't alone and well its part of the learning process.

      So the point of this isn't to be a gripe session. I'm just looking for a bit of feed back. Do you think the fees are unreasonable?
      Should I keep the youtube channel and do just the fees with patreon or do you think locking the content to my site and require a subscription is an idea.

      I'm just looking at the fact it is unrealistic to think youtube/google will actually get stuff right or that youtube viewers will actually bother to start looking for more accurate videos. 
    • By Balma Alparisi
      i got error 1282 in my code.
      sf::ContextSettings settings; settings.majorVersion = 4; settings.minorVersion = 5; settings.attributeFlags = settings.Core; sf::Window window; window.create(sf::VideoMode(1600, 900), "Texture Unit Rectangle", sf::Style::Close, settings); window.setActive(true); window.setVerticalSyncEnabled(true); glewInit(); GLuint shaderProgram = createShaderProgram("FX/Rectangle.vss", "FX/Rectangle.fss"); float vertex[] = { -0.5f,0.5f,0.0f, 0.0f,0.0f, -0.5f,-0.5f,0.0f, 0.0f,1.0f, 0.5f,0.5f,0.0f, 1.0f,0.0f, 0.5,-0.5f,0.0f, 1.0f,1.0f, }; GLuint indices[] = { 0,1,2, 1,2,3, }; GLuint vao; glGenVertexArrays(1, &vao); glBindVertexArray(vao); GLuint vbo; glGenBuffers(1, &vbo); glBindBuffer(GL_ARRAY_BUFFER, vbo); glBufferData(GL_ARRAY_BUFFER, sizeof(vertex), vertex, GL_STATIC_DRAW); GLuint ebo; glGenBuffers(1, &ebo); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices,GL_STATIC_DRAW); glVertexAttribPointer(0, 3, GL_FLOAT, false, sizeof(float) * 5, (void*)0); glEnableVertexAttribArray(0); glVertexAttribPointer(1, 2, GL_FLOAT, false, sizeof(float) * 5, (void*)(sizeof(float) * 3)); glEnableVertexAttribArray(1); GLuint texture[2]; glGenTextures(2, texture); glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageOne = new sf::Image; bool isImageOneLoaded = imageOne->loadFromFile("Texture/container.jpg"); if (isImageOneLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageOne->getSize().x, imageOne->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageOne->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageOne; glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageTwo = new sf::Image; bool isImageTwoLoaded = imageTwo->loadFromFile("Texture/awesomeface.png"); if (isImageTwoLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageTwo->getSize().x, imageTwo->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageTwo->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageTwo; glUniform1i(glGetUniformLocation(shaderProgram, "inTextureOne"), 0); glUniform1i(glGetUniformLocation(shaderProgram, "inTextureTwo"), 1); GLenum error = glGetError(); std::cout << error << std::endl; sf::Event event; bool isRunning = true; while (isRunning) { while (window.pollEvent(event)) { if (event.type == event.Closed) { isRunning = false; } } glClear(GL_COLOR_BUFFER_BIT); if (isImageOneLoaded && isImageTwoLoaded) { glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glUseProgram(shaderProgram); } glBindVertexArray(vao); glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, nullptr); glBindVertexArray(0); window.display(); } glDeleteVertexArrays(1, &vao); glDeleteBuffers(1, &vbo); glDeleteBuffers(1, &ebo); glDeleteProgram(shaderProgram); glDeleteTextures(2,texture); return 0; } and this is the vertex shader
      #version 450 core layout(location=0) in vec3 inPos; layout(location=1) in vec2 inTexCoord; out vec2 TexCoord; void main() { gl_Position=vec4(inPos,1.0); TexCoord=inTexCoord; } and the fragment shader
      #version 450 core in vec2 TexCoord; uniform sampler2D inTextureOne; uniform sampler2D inTextureTwo; out vec4 FragmentColor; void main() { FragmentColor=mix(texture(inTextureOne,TexCoord),texture(inTextureTwo,TexCoord),0.2); } I was expecting awesomeface.png on top of container.jpg

    • By khawk
      We've just released all of the source code for the NeHe OpenGL lessons on our Github page at https://github.com/gamedev-net/nehe-opengl. code - 43 total platforms, configurations, and languages are included.
      Now operated by GameDev.net, NeHe is located at http://nehe.gamedev.net where it has been a valuable resource for developers wanting to learn OpenGL and graphics programming.

      View full story
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