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OpenGL Shader engine generel questions

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while were developing our graphics engine on OpenGL were at the point where it is important to use shaders to our
scenes. I looked at the unreal engine shaders and were confused about there system of different parts of shaders onnected
to the scene.

My Question now is, if i have one lightsource i use one shader for my scene but what is if i have several different lightsources and
something else like water-shader, sky-shader and a particle-shader for fire in one scene?

Are they connected to one shader and whats going on with eah of these shaders main-function? or are they differently loaded and rendered but
how is this done in rendering process and then dropped to scene? Edited by Shaarigan

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You should have many different shaders. Each material type you use should use the fixed function pipeline or have a custom shader. The number of shaders and what they do is entirely up to you.

You should look into GLSL. I would also recommend the OpenGL Orange Book. This is a very broad topic and is going to require some time before making some of the advanced effects seen in games.

But, in a netshell this is what needs to happen.

-Load and compile the shaders you are going to use.
-For every object drawn in the scene. use a shader using glUseProgram and draw the object. The object will be drawn with that shader. you may also pass a 0 to glUseProgram and openGL will use the fixed rendering pipeline.

Hope this helps

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I already know and implemented shaders in my engine.

I dont understand eg i have a scene with a character and that character has a torch on it. How could that done when i use
a shader to iluminate my scene by the torch, a second one that lets the character glow magicaly blue and a third one thats
illuminating the scene (the moon or a bigger fire) also effects my character. This ment i had to render my character three times
with different shaders or should i compile my shaders to one shader during runtime or how?
I already read that there is no #include in GlSl implemented at the moment so i had to merge my shaders by myselfe?

And as is sayed,i dont know how big engines like Unreal do that when using hundrets of different shaders to a scene.

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Hi,
Lighting is difficult subject and there are lots of information around the internet about different implementations.

First to answer your question:

- you may go down the path that for each object you calculate which are the affecting lights and then render the object to light accumulation buffer as many times as there are lights. This is rather slow of course.

- To optimize the case, you may make a long shader which handles multiple light sources of different types in one pass. you may make a shader with lots of #ifdef's to create an optimized shader for each required lighting/material case. This will result tons of different kind of shaders to manage. Also, you may choose to limit the amount of lights affecting each object by choosing the strongest/nearest light sources. This is better of course. However, it still takes lots of efforts to see which light affects which object. Also, drawing lots of objects in same time (instancing) is difficult.

To over come the problems presented in the cases above, many people choose to use deferred shading/lighting approach which basically allows lots of lights to affect single object / pixel. Of course deferred shading comes with it's down sides too.

Cheers!

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For your torch example you pass the light position and properties to each shader. Your world shader will get those properties and then your animation shader for the character will get a 2nd copy of those properties. They will have partially the same code with the lighting, but the character will have animated bones and/or other computations. So you will be doing a lot of copy/paste type stuff inside of several shaders.

Each shader is separate, there is no 1 file. You use whatever shader you want at any time.

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There are different techniques to handle shader management.

- You may create few so-called über-shaders with lots of #defines ands #ifdefs to handle different lighting / material permutations.

- You may also use something called dynamic shader linkage (I'm not totally familiar with this) which is AFAIK practically about having small pieces of shader functionaly and linking them together to form a complete shader for each situation.

- Or you may take a different route and implement deferred shading. Of course you'll need to look what you really need to accomplish and what are the limitations of each technique.

Cheers!

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So i make something that manages #include tags, copys shader files together that were included and merge the main functions to get
one File that contains functions on different situations for my models.

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So i make something that manages #include tags, copys shader files together that were included and merge the main functions to get
one File that contains functions on different situations for my models.


This doesn't really make sense. There shouldn't be any reason to "manage #include tags or copy shader files together". It is more like compiling a shader with certain set of flags (the übershader case) or compiling shader parts and joining them together with the opengl api (the dynamic linkage case).

Well, now I notice that you are using opengl and I'm not the person to respond to opengl specific questions.

Perhaps you should present a real world problem you are facing with your shaders so that a more precise piece of advice may be given.

Cheers!

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This doesn't really make sense. There shouldn't be any reason to "manage #include tags or copy shader files together". It is more like compiling a shader with certain set of flags (the übershader case) or compiling shader parts and joining them together with the opengl api (the dynamic linkage case).


There isnt any way to combining compiled shaders in opengl because you can link only one shader of each type to a program. Also there isnt any #include-statement in GLSL.
In generating a shader i push my shader file as a set of strings to the api and ink al shaders (fragment, vertex) to a program. So i dont know any dynamic linking mechanic.

The example i've given above is what it is pointing on !!

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There isnt any way to combining compiled shaders in opengl because you can link only one shader of each type to a program. Also there isnt any #include-statement in GLSL.
In generating a shader i push my shader file as a set of strings to the api and ink al shaders (fragment, vertex) to a program. So i dont know any dynamic linking mechanic.
The example i've given above is what it is pointing on !!


Yeah I realized that we are talking about opengl here and that it doesn't have existing solutions for dynamic shader linkage (as far as I know from a short research on the net). Yes, you can make your own parser for the shader files to handle #including files for redundant shader code.

Have you considered deferred shading or übershaders?


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I think im orienting on bigger solutions like Unreal, that uses a system of basic and included shaders so i use the deffered version
with a set of basic shaders and some specielized ones with includes

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[quote name='kauna' timestamp='1320565832' post='4881004']
This doesn't really make sense. There shouldn't be any reason to "manage #include tags or copy shader files together". It is more like compiling a shader with certain set of flags (the übershader case) or compiling shader parts and joining them together with the opengl api (the dynamic linkage case).


There isnt any way to combining compiled shaders in opengl because you can link only one shader of each type to a program. Also there isnt any #include-statement in GLSL.
In generating a shader i push my shader file as a set of strings to the api and ink al shaders (fragment, vertex) to a program. So i dont know any dynamic linking mechanic.

The example i've given above is what it is pointing on !!
[/quote]

Actually, you can link several shaders together as long as there is only 1 main() function.

Also, there is the include feature now. It is an extension for now
http://www.opengl.or...age_include.txt

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Also #include is pretty straight-forward to implement on your own anyway. Since you are the ones writing the shaders, there is no reason a most-basic parser will not work.

I suggest going with uber shaders though. It is straight-forward and simple to implement.
You basically just associate one #ifdef with one bit flag in your code.
You ask for shaders from your shader manager (at the time of rendering) by sending bit flags that tell the manager what kind of shader you want (one with normal mapping, one without, etc.) and when it sees a given flag it sends the matching macro to the shader. Now you have permutations, and it was that simple.
The shader creation flags are remembered by the manager, so next time you ask for the same shader you will get the one that was already compiled.

The important thing in any case is to request shaders at the actual time of rendering. This is the only time you can check for lighting being enabled and other system states.


L. Spiro

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Actually, you can link several shaders together as long as there is only 1 main() function.

Also, there is the include feature now. It is an extension for now
http://www.opengl.or...age_include.txt


Ah ok, good to know. But i think it isnt availible on older cards (like mine radean hd 2600) isnt it?


Also #include is pretty straight-forward to implement on your own anyway. Since you are the ones writing the shaders, there is no reason a most-basic parser will not work.

I suggest going with uber shaders though. It is straight-forward and simple to implement.
You basically just associate one #ifdef with one bit flag in your code.
You ask for shaders from your shader manager (at the time of rendering) by sending bit flags that tell the manager what kind of shader you want (one with normal mapping, one without, etc.) and when it sees a given flag it sends the matching macro to the shader. Now you have permutations, and it was that simple.
The shader creation flags are remembered by the manager, so next time you ask for the same shader you will get the one that was already compiled.


Creating a shader preprocessor isnt as strength to me as you might think. I already build a 'compiler' for a scripting alguage with dynamic loaded syntax and
commands based on an ebnf pattern struct. I only have to change this a little, managing several includes ionto one file an well it is.

To had the idea with the flags too but in a different way. My engine only contains the main propertys per object (like only position and orientation of a mesh) and the other
propertys are modified by either there is an existing property (like color) so there is a filter object added, that modifies the property or it is a completly new ability (like a clipping function)
then there is a propperty handler added. Now im able to add a flag handler to a shader object that modifies some known flags or uniforms every time the shader is activated.

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      Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
      // Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
      // 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.
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