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OpenGL How to build a "renderer"

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I render with OpenGL and I wonder how common engines build their Renderer class.

 

1. Does this class hold data members like: vec4 m_clearColor; bool m_isWireframe, or they they just contain functions that abstract the actual opengl routines? Something like

void MyRenderer::setWireframe(bool flag)
{
    glPolygonMode( GL_FRONT_AND_BACK, flag ? GL_FILL : GL_FILL );
}

2. How does the whole framework use this class? Does it even have to be a class?

 

I know I asked really broad questions so if you can link me to other sources it would be nice too.

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Just a question for you rendering architects: Do you have some central place that does all the drawing, regardless of what is actually drawn, or do you have something like custom render modules where each implements their own rendering, including setting up buffers, fixed function state, shaders, submitting draw commands etc.?

 

I have been thinking... it should be enough when those custom renderCmds gather the data that is required to render somewhere and submit it to a central renderer (which buffers do I need? which shaders do I need? etc.). Would get rid of a lot of boiler plate (only a couple hundred lines of rendering code for each API backend? one can dream), and that central renderer would have all the info available to batch stuff efficiently.  But I'm not sure how feasible this is in practice until you end up with lots of ifs() that handle special cases anyways, or if there's something else that prevents it from being as clean as it sounds.

 

edit: Having read it now, the presentation by Hodgman above seems to implement exactly this. Does it work for everything? No custom drawing code needed anymore?

Edited by agleed

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Most renderers are all the same.

Typically you have an abstraction interface for all the API's that it supports. Then because Directx's Effect files are no longer useable, they will typically build their own passes. Which to be fair, is more efficient this way anyways.

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I prefer generic design where graphic engine only consist of a RenderQue, RenderParamTracker and TokenProcessor (meshes, materials and vertex definitions have those). INI file has definition of render modes, render targets, render params, token rules, etc...

 

It's tiny and efficient and you don't have to touch engine code ever (if implemented properly)

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I prefer generic design where graphic engine only consist of a RenderQue, RenderParamTracker and TokenProcessor (meshes, materials and vertex definitions have those). INI file has definition of render modes, render targets, render params, token rules, etc...

 

It's tiny and efficient and you don't have to touch engine code ever (if implemented properly)

 

 

Essentially something like this.

My current design seperates the engine's data from it's own.When The engine calls the renderer, the rendering logic does not need to know about the particulars of the actual engine's data. Instead, it's api recieves copies and transforms the data to a state that is needed.

 

When it comes time to render, the culling system will work independently of the current game state, it'll display a latent frame instead, and cull data based on what it has. This also means that the Renderer uses it's own octree for it's own processes. Primarily culling, But also as a way of determining some broader spectrum of LOD.

 

The engine's logic has it's own octree for logical processes. RayCasting, Scripts that effect certain regions of land, Navmesh Collisions, etc.

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I prefer generic design where graphic engine only consist of a RenderQue, RenderParamTracker and TokenProcessor (meshes, materials and vertex definitions have those). INI file has definition of render modes, render targets, render params, token rules, etc...

 

It's tiny and efficient and you don't have to touch engine code ever (if implemented properly)

 

 

Essentially something like this.

My current design seperates the engine's data from it's own.When The engine calls the renderer, the rendering logic does not need to know about the particulars of the actual engine's data. Instead, it's api recieves copies and transforms the data to a state that is needed.

 

When it comes time to render, the culling system will work independently of the current game state, it'll display a latent frame instead, and cull data based on what it has. This also means that the Renderer uses it's own octree for it's own processes. Primarily culling, But also as a way of determining some broader spectrum of LOD.

 

The engine's logic has it's own octree for logical processes. RayCasting, Scripts that effect certain regions of land, Navmesh Collisions, etc.

 

For occlusion culling I'm using view frustum culling + Hi-Z algorithm (actually it's Lo-Z because of inverted z-buffer).

 

Main engine script does this:

  SetRenderMode("PrepForHiZ"); // This will invoke setting of render targets, rendering quads, and after rendering is done, setting shader textures of those render targets, etc...

  RenderOcclussionSpheres(); // this is main engine command - it keeps track of current objects (each has his own id and pos/radius)

  void * flags = LockGraphicBuffer("occlusion_test_render_target", (X + 10)%10); // I'm having 10 frames delay (and 10 occlusion buffers)

  SetOcclussionFlagsForObjects(flags);

 

Graphic engine doesn't even know that it did occlusion calculation. It doesn't understand what data sent to it means - only how to send it to graphic card.

That way it can be forward renderer, deffered rendered, forward+ rendered, ray tracer, <some new renderer that hasn't been invented yet>. It doesn't care what the data is - only how to render it efficiently.

 

As for token processor:

 

Rendering_technique_name = <Render Mode Name> + <Remaining Mesh Tokens> + <Remaining Material Tokens> + <Remaining Vertex Tokens>

 

SAMPLE: When rendering depth, there is no need to have tokens that have something to do with color. Then if all texture tokens are removed from material, Vertex token that represents texture coordinates will be removed (if there is no alpha mask texture). At the end you end up with only few tokens. Vertex token that represent NORMAL will probably also be removed (since it isn't even registered for RenderDepth render mode).

 

Once you know rendering technique name, you know which render parameters need to be sent to graphic card - and RenderParamTracker does that efficiently (without repetition).

 

The point is: there is no harcoding of anything graphic wise (shaders + graphic data come with the game files - not engine).

You write graphic engine once and then you don't touch it for years.

If some new way of rendering/post process effect gets published you don't change the engine - you just stuff the shader into first game package and add a few lines in the INI file. Maybe add few SetRenderModes("blablabla") into rendering script for some new post-processes.

Edited by JohnnyQ

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I prefer generic design where graphic engine only consist of a RenderQue, RenderParamTracker and TokenProcessor (meshes, materials and vertex definitions have those). INI file has definition of render modes, render targets, render params, token rules, etc...

 

It's tiny and efficient and you don't have to touch engine code ever (if implemented properly)

 

 

Essentially something like this.

My current design seperates the engine's data from it's own.When The engine calls the renderer, the rendering logic does not need to know about the particulars of the actual engine's data. Instead, it's api recieves copies and transforms the data to a state that is needed.

 

When it comes time to render, the culling system will work independently of the current game state, it'll display a latent frame instead, and cull data based on what it has. This also means that the Renderer uses it's own octree for it's own processes. Primarily culling, But also as a way of determining some broader spectrum of LOD.

 

The engine's logic has it's own octree for logical processes. RayCasting, Scripts that effect certain regions of land, Navmesh Collisions, etc.

 

For oclussion culling I'm using view fustrum culling + Hi-Z algorithm (actually it's Lo-Z because of inverted z-buffer).

 

Main engine script does this:

  SetRenderMode("PrepForHiZ"); // This will invoke setting of render targets, rendering quads, and after rendering is done, setting shader textures of those render targets, etc...

  RenderOcclussionSpheres(); // this is main engine command - it keeps track of current objects (each has his own id and pos/radius)

  void * flags = LockGraphicBuffer("oclussion_test_render_target", (X + 10)%10); // I'm having 10 frames delay (and 10 oclussion buffers)

  SetOcclussionFlagsForObjects(flags);

 

Graphic engine doesn't even know that it did oclussion claculation. It doesn't understand what data sent to it means - only how to send it to graphic card.

That way it can be forward renderer, deffered rendered, forward+ rendered, ray tracer, <some new renderer that hasn't been invented yet>. It doesn't care what the data is - only how to render it efficiently.

 

As for token processor:

 

Rendering_technihnique_name = <Render Mode Name> + <Remaining Mesh Tokens> + <Remaining Material Tokens> + <Remaining Vertex Tokens>

 

SAMPLE: When rendering depth, there is no need to have tokens that have something to do with color. Then if all texture tokens are removed from material, Vertex token that represents texture coordinates will be removed (if there is no alpha mask texture). At the end you end up with only few tokens. Vertex token that represent NORMAL will probably also be removed (since it isn't even registered for RenderDepth render mode).

 

Once you know rendering tehnique name, you know which render parameters need to be sent to graphic card - and RenderParamTracker does that efficiently (without repetition).

 

The point is: there is no harcoding of anything graphic wise (shaders + graphic data come with the game files - not engine).

You write graphic engine once and then you don't tuch it for years.

If some new way of rendering/postprocess effect gets published you don't change the engine - you just stuff the shader into first game package and add a few lines in the INI file. Maybe add few SetRenderModes("blablabla") into rendering script for some new post-processes.

 

 

Never said the data was hard coded :P Only that the renrderer recieves copies of relevant data, but does not care for how the game engine manages it.

Currently, Rendering data is defined through json like Lua scripts. and is instantiated as called upon.

Edited by Tangletail

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So yeah,

Renderer are pretty much always done through abstract interface (as demonstrated by Hodgman and Tangletail).

 

This is the abstract graphic interface in my case (used by main engine either through scripts or hardcoded):

 

  SetRenderMode("bla bla"); // This will set render targets and prepare everything (and remove prev render targets and bind them to shader texture variables for the future use)

  // Main engine will loop through visible objects and call these two commands (similar is done for light-shadow pass):

  SetRenderParam(renderParamID, value); // camData or objPos or obj bones or some custom constant or whatever (done by object script).

  RenderMesh(meshID, availableDesiredLODDistance); // This will add mesh to render que and calculate render stuff based on material/mesh tokens

 

 

That is it - It is very very abstract. I don't think it is possible to have it more abstract than that. There are also some callbacks that graphic engine will do to calc some required rendering parameter that was not set, but that's another story...

 

NOTE: Sometimes I assign some material to some geometry that are not compatible (defined by meshID). In that case graphic engine will issue a warning that it doesn't know how to render this particular combination of render_mode/mesh/material/geometry tokes. Then I have to add exception rule to the INI file or add a shader for that particular combination.

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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 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.
    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
      Regards,
      LifeArtist
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