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OpenGL Alpha blending and the depth test

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I'm sure you've heard this a hundred times, and I apologize, but I still don't quite understand it. I am having the classic problem of not being able to see certain objects through certain other transparent / translucent objects. I know why this is happening: because the transparent objects are being drawn to soon, and things behind them are losing the depth test. I've heard the proposed solution of drawing all the opaque objects first, disabling writing to the depth buffer, and drawing the transparent objects in a certain optimal order. Still this is not practical for a number of reasons. First, the viewer is constantly moving, which means that the order objects must be optimally drawn in could be vastly different from frame to frame. Second, many of my objects (and textures) have both alpha channeled and opaque parts. And third, I have a lot of complex alpha channeled objects in my scene, and it's quite possible that part of an object would need to be drawn before part of another object but after a different part of the first object. Basically, the "optimal drawing order" would need to be more granular than a object-by-object approach, or possibly even a face-by-face approach, to get the desired effect. I know very little about OpenGL's built-in alpha handling features, but I was wondering if it would be possible to use the depth buffer to pre-calculate the distance from the viewer on a per-pixel basis, record the results, and then use that in place of the depth buffer for drawing in a second pass. I obviously don't know exactly what I'm talking about in terms of what I need to do here, but my ultimate goal is a system of rendering my alpha enabled models so that everything is visible through everything else, no matter where the viewer is located. Sorry again for not understanding this better, and thank you. EDIT: Slight modification to my original plan: What if you you made the videocard render every pixel (with depth data still intact), without discarding any at all. Than, once the entire frame was rendered, it could go back and do a depth test on the rendered buffer before copying it to the screen. It would take more memory, but it would make render order irrelevant. [Edited by - punmaster on June 10, 2009 1:29:08 PM]

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glEnable( GL_ALPHA_TEST );
glALphaTest( GL_GREATER 0.0 );

alternatively u can look into MSAA see sample to coverage

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What you're looking for is called "depth peeling".

BTW, I have no idea what zeds is going on about here.

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zedz: I am assuming you were suggesting I put the following code in my initialization routine:

glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.0f);

I tried this and nothing changed. Could you please be more specific.

Sneftel: I did a quick google search of "depth peeling" and found relevant results, but I'm afraid I don't understand how to actually implement it in OpenGL. The closest I came was this "http://developer.nvidia.com/object/Interactive_Order_Transparency.html" document from nVidia, but I haven't had a chance to read the entire thing. Do you have any better suggestions on where I should begin? I have never used GLSL before, will that be a requirement? Thank you for your helpful advice.

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That paper describes what I'm talking about. GLSL is actually not a requirement -- the technique can be implemented totally with fixed functionality.

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After reading the nVidia article, I fully understand depth peeling in concept. Still, the implementation they described went pretty far over my head. I searched around a little, but didn't find anything too helpful. Can anyone here give me an overview and possibly some resources to go about implementing this. Also, is there any way to avoid needing a ton of rendering passes (I saw something about "double depth peeling" which also makes sense in theory, but the implementation details for this were even worse). Thank you.

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A few solutions, all with their respective drawbacks:

A) The common way: render all opaque objects, disable depth writes, and draw all transparent objects in back to front order. The transparent objects need to be sorted on some granularity, usually per-object. This can still create artifacts, because self intersecting or concave objects need smaller granularity sorting. But it's relatively cheap. This is the usual system as it is used by 99% of all games for translucent geometry.

B) Alpha tested geometry. Essentially binary transparency. Perfect choice for things like leaves or objects shaped by using binary opacity masks. Very cheap, and entirely unproblematic since sorting is not required for pixel perfect rendering. Modern hardware with coverage sample AA can very well anti-alias the hard edges of the alpha mask to produce nice fades, without requiring any kind of sorting. This will not work for semi-transparent or translucent objects though.

C) Depth peeling. Basically per-pixel sorting. Given enough layers, it will generate a pixel perfect solution for even complex self-intersecting transparent or translucent geometry. It's very performance heavy though, and (as you noticed) not trivial to implement and optimize. Possibilities of optimizing it are:

* Double depth peeling
* ZT-buffer (ShaderX5, ch 2.8). Currently relies on undefined behaviour, since it requires simultaneous read/write access to a render target. May be impossible on most mainstream hardware, but could become practical in the future.
* Stencil routing (ShaderX6, ch 3.5). Can render many layers simultaneously without separate passes. Very advanced. Requires D3D10 and advanced shader support.

D) Screendoor transparency. Easy technique, doesn't require sorting, but can generate weird artifacts. Usually doesn't look very good, unless you don't have a lot of overlapping transparent objects and you're on a very high screen resolution.

E) Super-sampling with randomly rotated stipple patterns. Essentially sub-texel screendoor transparency with an additional resolve pass. With high enough supersampling resolution, this technique can look very good and doesn't require sorting on overlapping geometry. It completely murders fillrate though, is very memory intensive, and requires special shaders.

F) FSAA coverage sample stipple masks. Similar to E, but using the onboard multisampling circuitry. This technique would be optimal, but unfortunately most (read: 99.9%) of all consumer level GPUs use screenspace aligned sub-texel masks, thus making the technique impossible. Next generation GPUs could solve this by providing user-offset subtexel masks.

There are some more obscure techniques for very specific application scenarios.

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zedz: After a little more reading, I have found that the code you provided earlier would have worked perfectly to solve my problem I were only dealing with purely transparent objects. Unfortunately, it will not help with objects that have fully linear alpha channels. Still, I appreciate the input.

Yann L: Thank you for the very informative post. This is some of the best information I have ever seen presented in one place on this subject. After reading through the options, it seems that depth peeling is still the best way to get what I am looking for. The question now really comes down to getting it working in my application. Fortunately, the models I am working with at this point are not very high-cost in any other category, and, ironically, make heavy use of alpha channels partially to make up for the fact that they are relatively plain in terms of other rendering "frills". For that reason, I think I can live with the unoptimized multi-pass system, at least for the moment.

I am more than willing to write what ever code is necessary to make this happen, and learn what I need to along the way. Unfortunately, this seems to be a fairly sparsely documented concept (that appears to be relatively old, but has never really taken off), and I really do not know where to begin. All of the information I have seen regarding its implementation in OpenGL is either too vague to follow, or does things I completely don't understand. If anyone can help me on my way to actually seeing this work, I would be very happy. Thank you, everyone, for your help so far.

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      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains 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.
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