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OpenGL encounter weird problem when turn on the color blending.

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hi guys! I am a rookie in OpenGL, so, please do not laugh at me about my silly questions, aha.

 

here is the thing.

 

I need to turn on the color blending to deliver transparent effect. But weird thing happens, the front side, which is facing the light, looks not bad, like this:[attachment=22381:QQ??20140629143801.png]

 

but the back side looks awful, seems like some parts of surface are missing, latticed holes, like this:

[attachment=22382:QQ??20140629143822.png]

 

I tried to add another light on the back side, but both sides turn out to be awful with holes then.

 

Is it a issue related to lights? or texture? or depth test? because if I disable the GL_DEPTH_TEST, holes are gone, but the whole body looks awful, not the way I want it to be looked like.

 

Or it is something else? I even do not know where to get started to solve this problem, someone please help me!

 

 

 

 

For more details, now down to only one object in the scene.

 

This is the front side of the lung:

[attachment=22411:1.png]

 

This is the back side of the lung:

[attachment=22412:2.png]

 

 

BUT, if I change the light in the scene to pointing at the back side:

 

The front side:

[attachment=22413:3.png]

 

The back side:

[attachment=22414:4.png]

 

If I turn on two lights, one pointing at the front, the other pointing at the back, both sides of the object covered with meshes. If remove all lights in the scene, the object looks dark, which is obvious, and both sides of meshes remain. This makes me believe that the light is not the cause of the issue, right?

 

More angles of view, might help you guys to see the meshes more clearly:

[attachment=22415:333333.png]

 

[attachment=22416:QQ??20140630224309.png]

 

[attachment=22417:QQ??20140630224328.png]

Edited by eric_lie

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if you show us your code , we will see what is causing this trouble

we need to see something like this

 

all opengl calls [initialization and drawing)

then drawing modelitself

all glcalls after drawing

just show us your code, 

 

 

=====

try to set up only one light in the back and see if problem occurs for back of the model) (or set ambient to the same color like diffusion color)

 

its really hard to say what is causing this problem (it may be depth test, wrong order of face drawing, wrong normals for polygons etc)

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The most common problem with transparency is that rendering order of faces does not match the requirements of the rendering algorithm. The simplest algorithm needs faces to be rendered in back to front order. This implies meshes to be ordered in dependence of the view, that meshes must not be concave (or else they need to be divided up if a free view is allowed), and meshes must not touch or overlap (or else z-figthing will occur).

 

As WiredCat mentioned we need more details, but also above the code level. What algorithm is used? How are the meshes organized?

 

When you have a problem with a complex scene, reducing complexity first helps to narrow down the cause. E.g. Does the problem occur even if only a single organ is rendered, ...

Edited by haegarr

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if you show us your code , we will see what is causing this trouble
we need to see something like this
 
all opengl calls [initialization and drawing)
then drawing modelitself
all glcalls after drawing
just show us your code, 
 
 
=====
try to set up only one light in the back and see if problem occurs for back of the model) (or set ambient to the same color like diffusion color)
 
its really hard to say what is causing this problem (it may be depth test, wrong order of face drawing, wrong normals for polygons etc)

 


I set ambient to the same color like diffusion color, problem remains.

If I set a light from the back side, the back side of the model looks fine but the front face becomes the weird side instead, and if turn out all lights in the scene, then the whole model is covered with meshes. I thought the light can solve this problem someway somehow at first, but when I turn on all the lights, both in front and back, both sides of meshes remain.

So now I think may be the light is not the key point here.

My whole solution is a little bit complex, it takes time for me to the extract all OpenGL parts out, I will paste my codes here later.

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Sounds like you may have a culling problem.

 

Try turning on the depth buffer and setting cull mode to none and see what it looks like.

 

you mean glDisable(GL_CULL_FACE) ?

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The most common problem with transparency is that rendering order of faces does not match the requirements of the rendering algorithm. The simplest algorithm needs faces to be rendered in back to front order. This implies meshes to be ordered in dependence of the view, that meshes must not be concave (or else they need to be divided up if a free view is allowed), and meshes must not touch or overlap (or else z-figthing will occur).

 

As WiredCat mentioned we need more details, but also above the code level. What algorithm is used? How are the meshes organized?

 

When you have a problem with a complex scene, reducing complexity first helps to narrow down the cause. E.g. Does the problem occur even if only a single organ is rendered, ...

 

All organ models are 3DS files which generated by 3DS MAX, and be rendered into the scene from inside to outside, but the thing is, I am rendering a human body here, they can not be simply organized by inside and outside, or front and back, right? Just like you said, I need a free view, I need the whole body can be rotated so that you can see the body from any angle of view. So, at this point, organ A covers organ B (but you still can see organ B through organ A because of transparency), but if rotated around, organ B covers organ A instead. 

 

But if I disable the depth test, if organ A is rendered last, organ A is always covering organ B no matter which side you look at the model, then the whole body looks creepy and weird. It is totally not what I want. The funny part is, now meshes are gone.

 

What's more, if set all color alpha value to 1, which means no transparency at all, and enable the depth test back on, everything looks fine.

 

Like I said to WiredCat, I will paste codes up here after I review all over my solution.

 

Problem still remains even if only one organ in the scene.

 

thanks a lot!

Edited by eric_lie

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Since you appear to be in the medical industry (a colleague of jenny_wui’s?) where performance is not the main ingredient, you should research depth peeling.
 
http://developer.download.nvidia.com/SDK/10/opengl/src/dual_depth_peeling/doc/DualDepthPeeling.pdf
http://gamedevs.org/uploads/interactive-order-independent-transparency.pdf
 
 
L. Spiro Edited by L. Spiro

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The following is not meant to turn you away, but since you described yourself as a "rookie in OpenGL", I think it should be pointed out to you to prevent any misconceptions:

OpenGL is a thin (most will argue still too thick) api towards the GPU, providing you with the most basic interface to render and shade triangles. You may have noticed, that it doesn't provide any means to load models or textures. The newer versions of OpenGL don't even support lighting out of the box. The idea is that you implement those things on top of OpenGL. This holds for transparency as well. Transparency is not as simple as enabling blending, you have to implement some form of algorithm for it on top of OpenGL. "Depth peeling", as suggested by L.Spiro, is on of those techniques. Splitting the model into parts, whose rendering order gets determined by the camera position (what haegarr suggested), is another one. There are quite a few more.

Using OpenGL (or, for that matter, OpenGL ES, Direct3D, Metal, Mantle, ...) means that you will have to write a lot of code around it,
as these are not intended to be full fledged rendering engines.

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so less details, but anyway try this:

divide whole model to different organs, then try to render every organ where you begin from those that are deep in the body and you end with drawing lungs

 

also glBlendFunc needs to be set properly (and sometimes glColor4f <- alpha value)

 

we also dont know if your textures have alpha channel but i guess they don't)

 

At first you need to draw parts that are not transparent and then you draw transparent organs in proper order.

 

 

if you manage to sort your faces you will get almost the same effect like disabling GL_DEPTH_TEST but you'll see properly displayed model

 

 

this also try to lower z_near value (in glFrustum) 

 

or scale in ex lungs so lungs faces wont touch other organs

 

 

 

=====================

anyway youll have to sort faces or that would be far way faster and easier -organs

 

 

this is what ahppens when you sort faces.

yu.jpg

 

 

 

 

one that i can say is i see there z fighting problem or wrong face order or wrong blending factors

Edited by WiredCat

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Looking at those images, I don't think sorting the meshes will help.

 

It looks like some of them actual wrap around the other sub meshes.

 

If that is the case then you have a much more complex problem.

 

I would suggest using volume rendering, but as you stated you a newb at OpenGL, that's going to be a big issue for you. You might be able to just grab an existing OpenGL volume renderer and use it without understanding it though.

 

Have a look at http://idav.ucdavis.edu/~okreylos/ResDev/VolVis/MainPage.html

 

If that seems too much for you, then see if you can split the meshes that cause problems into two, then sort them before rendering.

 

Hope this gives you some ideas.

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you mean glDisable(GL_CULL_FACE) ?

never diable cullling, instead reverse what "front face" is.

 

You have a well defined geometry (veru well :)) so:

 

gl.enable(this.m_pGL.CULL_FACE); // yes, I wish to cull
      gl.cullFace(this.m_pGL.BACK); // I want to cull back face
     gl .frontFace(this.m_pGL.CCW); // the front face is counter clock wise face

 

 

... so if after this you change fronts face difintion to

 

  gl .frontFace(this.m_pGL.CW);

 

you will establish front faces as the back faces, thus getting culled front faces "the back faces"

 

and back faces as front faces, being processed, with their vertex attributes in take

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mind yet, that if you shade back faces with general set up of lighting or other parameters, you may get stranger results, unless you realy understand the geometry with its attributes. (you cannot observe back faces through culled front faces, if you get what I mean- very metaphysical stuff)

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so did you make any tests?

 

I am working on it. I am sorry to say that what you mentioned did not solve my problem.

 

Now it seems like not that easy to solve this issue. Some of you guys recommend me to resort all objects in a proper way, but even if only a single organ displayed in the scene, the problem still remains. And the thing is, in my case, each object is mapping to other stuff, resorting means changing everything. It does take time to solve this. Seems like a major problem to a rookie like me. 

 

I think Ohforf sake is right, Transparency is not as simple as enabling color blending, maybe this is the key point here!

 

Thanks anyway!smile.png smile.png

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Now it seems like not that easy to solve this issue. Some of you guys recommend me to resort all objects in a proper way, ...

Yep. Or else you try depth peeling as is suggested above by LS.

 


... but even if only a single organ displayed in the scene, the problem still remains. ...

Maybe this is because of the already mentioned concavity of the meshes? Question is, if you simplify the scene down to a single organ, do you have a chance to notice whether the problem occurs only if you look through a concavity. If so, then we are on the right track when suspecting the drawing order. A solution then will be to use sub-meshes.

 
But if concavity is not the cause, then we need to investigate further.
 


… And the thing is, in my case, each object is mapping to other stuff, resorting means changing everything. It does take time to solve this.  ...

This isn't a problem solely related to yours. It is common in game engines and elsewhere. And hence there is a solution :)

 

It is possible to have more than a single order on objects. Notice that it is recommended to have several organizing structures, one for each task to do. It is absolutely fine to have a logical organization of the objects, a spatial organization (if collision or proximity is an issue), a render order, and perhaps more. Don't stuck with the über scene graph approach, or you will be lost sooner or later!

 

For example, you iterate the scene description and detect all objects that need to be rendered. You insert each object into a list (which is emptied before the scene is iterated). After finishing, you sort the list by some criterion, in your case using the distance from the current camera. Object rendering then is done in the order given by the list. So rendering has no influence on other aspects of object organization, and nevertheless is done in the required way.

 


I think Ohforf sake is right, Transparency is not as simple as enabling color blending, maybe this is the key point here!

Absolutely. 

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Now it seems like not that easy to solve this issue. Some of you guys recommend me to resort all objects in a proper way, but even if only a single organ displayed in the scene, the problem still remains. 

 

can you upload somewhere this model i want to see it (one organ that is not properly displayed), maybe some face normals are reversed

Edited by WiredCat

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Now it seems like not that easy to solve this issue. Some of you guys recommend me to resort all objects in a proper way, but even if only a single organ displayed in the scene, the problem still remains. 

 

can you upload somewhere this model i want to see it (one organ that is not properly displayed), maybe some face normals are reversed

 

 

you mean the 3DS model? I am sure that all my 3DS model are accurate. 

 

Is there any possibility that when I render a 3DS model into an OpenGL scene, the normals are reversed by some inconspicuous mistakes somehow?

 

I upload more detail images, you can check it out.rolleyes.gif rolleyes.gif

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Yep. Or else you try depth peeling as is suggested above by LS.

 

I am reading those articles she gives to me, I think they would be helpful.

 


if you simplify the scene down to a single organ, do you have a chance to notice whether the problem occurs only if you look through a concavity.

 

I upload some detail images.

 


This isn't a problem solely related to yours. It is common in game engines and elsewhere. And hence there is a solution
 
It is possible to have more than a single order on objects. Notice that it is recommended to have several organizing structures, one for each task to do. It is absolutely fine to have a logical organization of the objects, a spatial organization (if collision or proximity is an issue), a render order, and perhaps more. Don't stuck with the über scene graph approach, or you will be lost sooner or later!
 
For example, you iterate the scene description and detect all objects that need to be rendered. You insert each object into a list (which is emptied before the scene is iterated). After finishing, you sort the list by some criterion, in your case using the distance from the current camera. Object rendering then is done in the order given by the list. So rendering has no influence on other aspects of object organization, and nevertheless is done in the required way.

 

This is quite inspiring, I will try it out! thanks!biggrin.png

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haegarr, on 30 Jun 2014 - 10:45 AM, said:

snapback.png


This isn't a probl.......he required way.

 

This is quite inspiring, I will try it out! thanks!biggrin.png

 

 

few post earilier you said that you already checked this solution.

You lie and post another images that say nothing.

I cut out of this topic,

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I am reading those articles she gives to me, I think they would be helpful.

She is actually a he. Just pointing out.

The problem you are seeing stems from the nature of your blend operation. Every surface slightly darkens what lies behind them, which allows the user to percieve an order of the surfaces, but also means that the order of mixing together the colors is important. There are other blend operations, like additive blending (glBlendFunc(GL_ONE, GL_ONE);) where the order doesn't matter. Can you get away with additive blending?

If not, mixing the colors of different surfaces seen on top of each other must be performed in the correct order: Mixing from back to front. As you already know, there are multiple, very different approaches, to achieving this. Some require you to write shaders, some only require you to do s.th. with your meshes, some work in all cases, some only if you can make certain geometric assumptions for your modell or allow for certain artifacts.

For example, consider a single convex object. If you render the back facing triangles first, their color will get blended with the background color. If you now render the front facing triangles, their color will get blended with the result of the previous blending operation and you get the correct result. So for a single convex object, you can get away with the simple trick of rendering the back faces first, and then the front faces which will, for each pixel, always be closer to the camera. Automatic and perfect back to front sorting. Sadly, you have more than one object, and those intestines are far from convex.

So exploiting convexity is out of the question, at least directly. Which leaves you with 2 questions: Do you want "perfect" results? Do you want to use shaders, or do you prefer a geometric solution?

Haegarr is suggesting a geometric solution: Cut your scene into pieces (or your objects into subobjects) and sort them back to front. But what pieces? Is it enough to cut them into convex pieces? The answer is no, just cutting them into convex pieces will not guarentee artifact free pictures. For example, assume that you make one subobject out of each triangle (a single triangle is convex). Then there are certain situations where you can not find an ordering for the triangles, that lead to a back to front rendering. For example, consider these 4 quads (also works with 3 triangles), where each quad has one end in front of the previous but behind the next.
  DD        BB
AAAAAAAAAAAABBAA
AAAAAAAAAAAABBAA
  DD        BB
  DD        BB
  DD        BB
  DD        BB
CCDDCCCCCCCCCCCC
CCDDCCCCCCCCCCCC
  DD        BB
For these, no correct rendering order exists. At least one of the quads has to be split. This is the point, where you have to decide, if you can live with the occasional artifact, or if you want to descend into the ugliness of splitting faces, and inserting them into a BSP-tree or s.th. similar.

If you go the geometric route, I would follow haegarr's advise: Automatically cut the objects into smaller, approximately convex and not elongated pieces, sort those pieces back to front then render each object twice, first by culling away the front faces (only rendering back faces) then by culling away the back faces (only rendering front faces). Then take a look at it and decide, if the remaining artifacts are acceptable, preferably while experimenting with different parameters for your cutting algorithm. However, keep in mind, that the geometric sorting approach can get very ugly, so if you have the compute power to spare, and the time to read further into OpenGL, then Depth-peeling and other "order independent transparency" techniques can be a reasonable choice.

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few post earilier you said that you already checked this solution.
You lie and post another images that say nothing.
I cut out of this topic,

 

I am really really sorry if I said or did something to upset you. If so, I have to say I definitely didn't mean it! there must be some misunderstanding between us.sad.png

I deeply appreciate your help here, why would I lie about anything?

 

 

 


try to set up only one light in the back and see if problem occurs for back of the model) (or set ambient to the same color like diffusion color)

 

I tried, problem does still remain. I already upload the images.

 


divide whole model to different organs, then try to render every organ where you begin from those that are deep in the body and you end with drawing lungs

 

I have not tried this yet, as I said before, my objects are all mapping other stuff, I need to figure it out how to do it in a proper way before I do any modification on models.

 


also glBlendFunc needs to be set properly (and sometimes glColor4f <- alpha value)

 

I tried a couple in glBlendFunc, and also tried change the alpha value. If I use a lower value, like 0.3, the meshes looks thinner but still exist.

 


we also dont know if your textures have alpha channel but i guess they don't)

 

Yes, my textures have no alpha channel, I got bitmap files, I just load the texture by gluBuild2DMipmaps. If there has any alpha channel in texture, I don't know where to modify it.

 


At first you need to draw parts that are not transparent and then you draw transparent organs in proper order.

 

This is still about resorting, I have to say again, I will try it, since modifying my model is a little bit complex, I prefer to try other way first. And I have no non-transparent objects in the scene.

 


this also try to lower z_near value (in glFrustum) 

 

I use gluPerspective, the near value is already 0.01f.

 


anyway youll have to sort faces or that would be far way faster and easier -organs
 
 
this is what ahppens when you sort faces.

 

I will try it later.

 

 

 

 

 

Again, I am really sorry.

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I use gluPerspective, the near value is already 0.01f.

A low near value is the main reason behind z-fighting (which manifests as flickering between objects near each other).
Always use the highest value possible that does not cause visual artifacts with near-clipping.
If the far plane is at 1,000.0 for example then a good near might be 10.0 or 5.0.


L. Spiro

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      Introduction
      Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
      There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
      Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. 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 C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
      Overview
      Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
      Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
      Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
      An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
      The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
      In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
      Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
      Render device, device contexts and swap chain are created during the engine initialization.
      Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
      Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
      Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
      Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
      API Basics
      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. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
      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 ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
      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 ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
      Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
      Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
      Initializing the Pipeline State
      As it was mentioned earlier, Diligent Engine follows next-gen APIs 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.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
      Creating Shaders
      While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 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. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
      The following is an example of shader initialization:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      Creating the Pipeline State Object
      After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
      PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
      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 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 michaeldodis
      I've started building a small library, that can render pie menu GUI in legacy opengl, planning to add some traditional elements of course.
      It's interface is similar to something you'd see in IMGUI. It's written in C.
      Early version of the library
      I'd really love to hear anyone's thoughts on this, any suggestions on what features you'd want to see in a library like this? 
      Thanks in advance!
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