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OpenGL Shadow mapping help

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I have implemented shadow mapping in my engine, and I understand the basic principles, however I have some problems: 1) My game is a car game, and cars travel far. I was hoping to have shadows for long distances (2km (1 opengl unit is a meter for me) approx.). When I render the depth map, I only render looking at the origin, so its impossible for me to also render the car, unless I make the perspective really wide (like 160 degrees viewing angle from the light). If I do this, then I lose resolution, and I get huge aliasing (obviously). How does one render a pont light, or even a directional light long-range? 2) I think the solution to #1 would be PSM, and I don't mind rendering only close to the car, because I only want dynamic shadows on the car, for the environment I could use static or pre-baked lighting (if I figure it out). If PSM (perspective shadow mapping) is the solution, then does anyone have a good resource other than the paper (goes over my head) for understanding how opengl does perspective transforms, and what this "unit cube" everything I read is talking about? I can't find (don't know where to look) a good resource for learning how perspective transformations are used in opengl. Thanks! PS: I was thinking of a simple idea where I would use a directional light (so orthographic projection) to render the lightmap from an offset of the car's position so that no clipping happens, but I don't know how I would correctly set up glOrtho and use it with gluLookAt, or why I haven't read about this (maybe because it may not shadow other objects?).

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Read about PSSM (Parallel-Split Shadow Maps). The main idea is to use, for example 3 shadow maps - one works for objects near the camera, the second for those in larger distance, and the third works for thos that are far away from camera. You could combine this with VSM (Variance Shadow Maps; very easy to implement) and have quit fast and nice-looking sun shadows.
As for PSSM, many game use them today. So far I have noticed Crysis, Assassin'Creed, Call of Duty 4 use it.

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That's interesting, Ill definitely implement it once I understand it!

From what I understand: you split the scene based on some heuristic, and then I get lost.

It's not a surprise, since I have no clue as to how glFrustum works (I use gluPerspective), and would love to know, however everything I see doesn't explain it well. Could you possibly point me to somewhere that does?

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PSSM or PSM is not a solution for your first question. You need at least to focus the shadow map on the current view frustum, so You will render into the shadow map only the relevant stuff. There is no point in redering shadows for something what isn't visible :). For focusing, just see "Parallel-Split Shadow Maps" by Zhang at al. and/or "Light Space Perspective Shadow Maps" by Wimmer et al.

About VSM, IMHO it isn't very useful, as it introduces some artifacts and requires additional GPU power.

About perspective matrix (the one build by the gluPerspective), just read some math/graphic programming book. Like "3D Math Primer for Graphics and Game Development" or "Real time rendering".

[Edited by - krisiun on November 15, 2008 1:46:27 PM]

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Quote:

About VSM, IMHO it isn't very useful, as it introduces some artifacts and requires additional GPU power.

It indeed is a lot useful. Light-bleeding isn't a big problem - at first you may just cut off "the tail" from variance. Besides Lauritzen writes about (GPU Gems 3) Summed-Area VSM which gives great results and no artifacts.

Quote:

Could you possibly point me to somewhere that does?

Lets forget about any other than of light than the sun :). So, you have sun. You use gluLookAt and put it somewhere on the scene. Then you use glOrtho (not gluPerspective) to create projection for parallel light (sun is actaully parallel light for us). And right now you can, for example multiply this matrix by, for example, scale matrix (10, 10, 10). After this every object in the shadow map will get larger (you get better shadows quality). Some of them will disappear because of that. You may also want to do a translation of the shadow map to and center the look at camera position. This translation and scaling allows you to "focus" the shadow map.
I realize it is not easy at the beginning and you get confused - everyone gets :P. I would advice you to start here [http://developer.download.nvidia.com/SDK/10.5/opengl/src/cascaded_shadow_maps/doc/cascaded_shadow_maps.pdf] (Cascaded Shadow Maps == Parallel-Split Shadow Maps). Try to understand whats going on with this focusing and try to implement shadow map tracing the camera position (because that's what we want - focus the shadow map at the area close to the camera).
At first try to derive a standard shadow map matrix, and after gluLookAt and glOrtho, multiply it by:
1. glScale(5, 5, 5)
2. glTranslate(0.5, 0, 0)
See how shadow map will look after one of these additional transformations

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Ok, I'll try to combine your description with my knowledge later, for now I'm thinking about VSM, since it seems easier (and looks very nice).

I've come up with a solution for the resolution of the shadow map, that will work as long as I only have shadows for one object at a time :), but I will implement PSSM later when I finish the physics behind my game (I didn't expect shadow algorithms to be so complex, its alot easier in a raytracer). (I just render an orthogonal light and keep it looking at the car at the same angle).

I'm going to try doing VSM, and I found a great thread on VSM here.

I understand it, just need to worry about how to actually render into the shadow map the depth and depth squared values. Right now I'm using DEPTH_COMPONENT FBO which handles this all for me, but I guess I'll need to encode the info in a color texture.

Thanks for the help!


edit

I think I understand your description, scaling objects doesn't affect the orthographic projection. So the rest is simply spliting the view and find the specific scaling in order to keep everything in view, then sample that depth texture. It makes more sense now.

edit2

I think I've realized a way to do the depth thing, use glsl shader to render depth values to r, and depthsquared values to g. Only problem is that how do I find the depth values? I just looked it up, and it seems gl_FragCoord.z is the answer I want! So I just gotta render gl_FragCoord.z to r, and gl_FragCoord.z^2 to g, and the texture I render should contain the proper stuff. Time to try it!

[Edited by - solinent on November 15, 2008 8:17:52 PM]

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Certainly Parallel-Split Shadow Maps aka Cascaded Shadow Maps are a great and simple solution to resolution problems.

Quote:
Original post by solinent
I think I've realized a way to do the depth thing, use glsl shader to render depth values to r, and depthsquared values to g. Only problem is that how do I find the depth values? I just looked it up, and it seems gl_FragCoord.z is the answer I want! So I just gotta render gl_FragCoord.z to r, and gl_FragCoord.z^2 to g, and the texture I render should contain the proper stuff. Time to try it!

That should work, but you can also just pass the light space position (i.e. "view space" of the light) into the fragment shader, then evaluate whatever depth metric you want. For instance, length(PosInLightSpace) or just PosInLightSpace.z.

There's sample code on the GPU Gems 3 DVD if you have access to that. It's DirectX, but the concepts are the same in OpenGL.

Also just a side note, if you run into any problems with light bleeding, you can look into "Exponential Variance Shadow Maps"... as small variation on VSMs that almost completely eliminates any light bleeding and has fewer artifacts than pretty much any shadow filtering technique out there currently. More info in my thesis here and some discussion in the thread here.

Cheers,
Andrew

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Great, it works (some problems that I have right now will be solved when I figure out PSSM).

I want to do soft shadows now, but I can't figure out how to blur the opengl texture. I guess that's beyond the scope of this forum though?

EDIT: Yeah, sounds good andy, I'll probably implement PSSM. I'm going to uwaterloo next year, hopefully!

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Quote:
Original post by solinent
Great, it works (some problems that I have right now will be solved when I figure out PSSM).

The aforementioned sample code actually includes a simple implementation of PSSM + VSM as well. It also has blurring code to do edge softening.

Quote:
Original post by solinent
EDIT: Yeah, sounds good andy, I'll probably implement PSSM. I'm going to uwaterloo next year, hopefully!

Cool! It's a good school :)

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Quote:
Original post by AndyTX
Quote:
Original post by solinent
Great, it works (some problems that I have right now will be solved when I figure out PSSM).

The aforementioned sample code actually includes a simple implementation of PSSM + VSM as well. It also has blurring code to do edge softening.

Quote:
Original post by solinent
EDIT: Yeah, sounds good andy, I'll probably implement PSSM. I'm going to uwaterloo next year, hopefully!

Cool! It's a good school :)


I have no books, it's just me and the computer screen.

In all honesty, I've always found resources online and never had to refer to books for most things. I don't really have much of a budget, with me buying all these games :)

I would understand how to do blur on an image if I had the pixels, but blurring a vram texture is another thing. Do I really have to retrieve the texture, blur it, then upload it, or is there a way to iterate over the texture with a pixel shader?

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Ok, with variance shadow mapping:
Click

without:
Click

What's going on? Relevant code is within those images.

When I turn on filtering, then I get these ugly staircase patterns.

Ok, I am looking at your fx composer code, and you have this:

float lit_factor = (rescaled_dist_to_light <= moments.x);

What does this do? Doesn't that assign a float to a bool? Or does that assign 1.0 or 0.0 depending on the value of the bool?

[Edited by - solinent on November 16, 2008 12:51:55 PM]

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Quote:

Ok, with variance shadow mapping:
Click

Don't know whats going on but one advice - when testing shadows always check how it looks with standard dot(L, N) shading :)

Quote:

Cool! It's a good school :)

Wish I could be there but it's about one-half equator's distance from my home :P

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Yeah, there was a problem with how I was computing the shading, and it's almost fixed now (still some weird artifacts on the car, but I'll fix them later).

I'm trying to do a gaussian blur, but it seems that the algorithm isn't as easy as I thought it'd be, so I guess I'll be doing it another day. But I can apply custom glsl filters to my shadow maps now, and applying filters is doing something, so I think I'll eventually get it.

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Quote:

I'm trying to do a gaussian blur, but it seems that the algorithm isn't as easy as I thought it'd be

Yeah, I guess blurring is the most time-consuming part of this algorithm. However, even though it is much more efficient than PCF. Besides, a really good-looking sun shadows are worth that :)

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Just updating my progress: I got VSM to work nicely, and gaussian blur works as well!

I think I might just put off PSSM until I can get my hands on a nice race-track environment or make one of my own, because there's no point until then.

edit

I think I know why I have weird artifacts on my stuff, it's a precision problem.

For the brief second before my simulator blows up (due to large rendering times: I'm perplexed) when I use RGBA16, the artifacts go away.

But why is RGBA16 so slow? RGBA12 doesn't seem to make any improvement.

[Edited by - solinent on November 17, 2008 8:35:58 PM]

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My environment so far only consists of one object, and I haven't implemented PSSM (right now the light's position and direction is calculated at an offset to the car based on the drection of the light).

I'll probably get light bleeding when I add more things, but I'll cross that bridge when it comes.

Or will it help the precision issues I'm having?

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