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OpenGL Camera and Viewing Transformations

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First of all, I am aware that there is no such entity called camera in OpenGL. The eye is located at the origin looking down the negative z-axis with the up vector being the positive y-axis (please correct me if I am wrong here or anywhere).

So here's the deal, I am trying to make a class that will handle the view of the scene using keyboard input. Just like in an aircraft, there is thrust which causes the camera to move forward(if we are sitting in a cockpit). Let us say, I am using W and S keys to control the thrust of this hypothetical camera and A and D keys to control my heading(the direction at which I am looking) and Up and Down keys to control the Pitch and Left and Right to control Roll.

If you have ever played Descent, there are actually ten ways to control the ship. Thrust forward, backward.. slide left, right, slide up and down, Pitch up and down, and roll left and right.

This is what I am trying to do here. By using basic opengl transformation functions I want to build my own camera class that I can control the view the scene very easily and how I want to view it.

Here is the code so far that I was able to do:
[CODE]
#ifndef CAMERA_H_INCLUDED
#define CAMERA_H_INCLUDED
#include <SDL.h>
#include <GLEW\glew.h>
#include <math.h>
#include "Misc.h"
const GLfloat INCREMENT_HEADING = 0.1f;
const GLfloat INCREMENT_PITCH = 0.1f;
const GLfloat INCREMENT_ROLL = 0.1f;
const GLfloat INCREMENT_FORWARD = 1.0f;
// From Right to Left, in Bits, the keys are
// right left down up d a s w
class EyeCam
{
private:
GLfloat Angle_Heading;
GLfloat Angle_Pitch;
GLfloat Angle_Roll;
Uint8 KeyState;
public:
GLfloat eyeX, eyeY, eyeZ;
EyeCam(GLfloat eyeX, GLfloat eyeY, GLfloat eyeZ, GLfloat aimX, GLfloat aimY, GLfloat aimZ);
void onEvent(SDL_Event* event);
void update();
};
#endif // CAMERA_H_INCLUDED
[/CODE]

[CODE]

#include "EyeCam.h"
#include <iostream>
EyeCam::EyeCam(GLfloat eyeX, GLfloat eyeY, GLfloat eyeZ, GLfloat aimX, GLfloat aimY, GLfloat aimZ)
{
this->eyeX = eyeX;
this->eyeY = eyeY;
this->eyeZ = eyeZ;
Angle_Heading = Misc::toDegrees(atan2f((eyeX - aimX), (eyeZ - aimZ)));
GLfloat base = sqrt(pow(eyeZ-aimZ, 2) + pow(eyeX - aimX, 2));
Angle_Pitch = Misc::toDegrees(atan2f( -(eyeY - aimY), base));
//Angle_Roll = 0.0;
KeyState = 0x00;
}


void EyeCam::onEvent(SDL_Event* event)
{
if (event->type == SDL_KEYDOWN)
{
if (event->key.keysym.sym == SDLK_RETURN)
{
system("cls");
std::cout << "X: " << eyeX << '\n';
std::cout << "Y: " << eyeY << '\n';
std::cout << "Z: " << eyeZ << "\n\n";
std::cout << "Heading: " << Angle_Heading << '\n';
std::cout << "Pitch: " << Angle_Pitch << '\n';
//std::cout << "Roll: " << Angle_Roll << '\n';
}
if (event->key.keysym.sym == SDLK_w)
KeyState = KeyState | 0x01;
if (event->key.keysym.sym == SDLK_s)
KeyState = KeyState | 0x02;
if (event->key.keysym.sym == SDLK_a)
KeyState = KeyState | 0x04;
if (event->key.keysym.sym == SDLK_d)
KeyState = KeyState | 0x08;
if (event->key.keysym.sym == SDLK_UP)
KeyState = KeyState | 0x10;
if (event->key.keysym.sym == SDLK_DOWN)
KeyState = KeyState | 0x20;
if (event->key.keysym.sym == SDLK_LEFT)
KeyState = KeyState | 0x40;
if (event->key.keysym.sym == SDLK_RIGHT)
KeyState = KeyState | 0x80;
}
else if (event->type == SDL_KEYUP)
{
if (event->key.keysym.sym == SDLK_w)
KeyState = KeyState & 0xFE;
if (event->key.keysym.sym == SDLK_s)
KeyState = KeyState & 0xFD;
if (event->key.keysym.sym == SDLK_a)
KeyState = KeyState & 0xFB;
if (event->key.keysym.sym == SDLK_d)
KeyState = KeyState & 0xF7;
if (event->key.keysym.sym == SDLK_UP)
KeyState = KeyState & 0xEF;
if (event->key.keysym.sym == SDLK_DOWN)
KeyState = KeyState & 0xDF;
if (event->key.keysym.sym == SDLK_LEFT)
KeyState = KeyState & 0xBF;
if (event->key.keysym.sym == SDLK_RIGHT)
KeyState = KeyState & 0x7F;
}
}
void EyeCam::update()
{
if (KeyState & 0x01) // w
{
eyeX = eyeX - INCREMENT_FORWARD * sin(Misc::toRadians(Angle_Heading));
eyeY = eyeY + INCREMENT_FORWARD * sin(Misc::toRadians(Angle_Pitch));
eyeZ = eyeZ - INCREMENT_FORWARD * cos(Misc::toRadians(Angle_Heading));
}
if (KeyState & 0x02) // s
{
eyeX = eyeX + INCREMENT_FORWARD * sin(Misc::toRadians(Angle_Heading));
eyeY = eyeY - INCREMENT_FORWARD * sin(Misc::toRadians(Angle_Pitch));
eyeZ = eyeZ + INCREMENT_FORWARD * cos(Misc::toRadians(Angle_Heading));
}
if (KeyState & 0x04) // a
{
Angle_Heading = Angle_Heading + INCREMENT_HEADING;
}
if (KeyState & 0x08) // d
{

Angle_Heading = Angle_Heading - INCREMENT_HEADING;
}
if (KeyState & 0x10) // Up
{
Angle_Pitch = Angle_Pitch + INCREMENT_PITCH;
}
if (KeyState & 0x20)
{
Angle_Pitch = Angle_Pitch - INCREMENT_PITCH;
}
if (KeyState & 0x40) // Left
{
Angle_Roll = Angle_Roll + INCREMENT_ROLL;
}
if (KeyState & 0x80) // Right
{
Angle_Roll = Angle_Roll - INCREMENT_ROLL;
}
glRotatef(-Angle_Pitch, 1.0, 0, 0);
glRotatef(-Angle_Heading, 0, 1.0, 0);
glTranslatef(-eyeX, -eyeY, -eyeZ);
glRotatef(-Angle_Roll, 0.0, 0.0, 1.0);
}

[/CODE]

The constructor of the class takes the same arguments as gluLookAt() function, except for the Up vector (I've not implemented it yet in my class). The position of the eye in x, y, z coordinates and a point of aim in x, y, z coordinates.

In the constructor, I am calculating two angles, the Heading and the Pitch by using spherical trigonometry and then using these angles to rotate the view accordingly. Please note that, I am not using gluLookAt() function at all in my class.

Then, by using keyboard commands, I am just making changes to the angles to adjust my aim and calculating eye coordinates to adjust my position.

Now, the problem I am having here is that, If I am originally looking down the -ve z- axis and my up is +ve y axis. then I know to adjust my heading I have to Rotate the scene around the y - axis, if I want to adjust pitch I have to Rotate around the x - axis and if I want to adjust roll I have to Rotate around the z-axis. Hence, I am using equation of circle with heading angle to calculate my eyeX and eyeZ and pitch angle to calculate y - axis. But unfortunately, this will only work till I stay in the XZ plane. Any change in the plane, I will not get the desired results.

For example, at the start if I make a 90 degree Roll such that my up now is -ve x-axis, then if I want to Pitch up relative to the camera, I have rotate around the y-axis. Originally I was rotating about the x-axis when roll was 0. Similarly, If I am looking down the -ve z-axis and then roll will perfectly fine because I am rotating around the +ve z axis, however, If I move the camera to aim at the +ve x-axis and then roll it will not work, in that case I have to rotate on the -ve x-axis to make the roll work. I was able to solve this problem by changing the glRotate function for Roll

from:
glRotatef(Angle_Roll, 0, 0, 1);
to:
glRotatef(Angle_Roll, sin(toRadians(Angle_Heading), 0, cos(toRadians(Angle_Heading));

but what if I am looking down the -ve or +ve y-axis, this again will not work as desired and also I try to change the heading then it will automatically affect the roll transformation.

I hope that I am not confusing you guys too much with the my description, please correct me if I am wrong in my assumptions.

So, the bottom line question is, what I need to do make this camera work in every orientation so that it pitchs, rolls, and turns correctly relative to its orientation.

I am not very skilled with mathematics, but I have studied trigonometry and equations of curves and shapes. Please let me know If I need to cover more mathematics in order to implement what I want.

Any input will be appreciated, Thanks.

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If you are up for it, I recommend storing and manipulating your camera's orientation as a [url="http://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation"]quaternion[/url]. Especially if you are also implementing a roll for rotation, this will save you quite a bit of trouble. There are a few benefits of using quaternions:
- You gain the ability to Interpolate between two orientations, which is very valuable if you want to move your camera along a spline for instance.
- By not having separate angles, there are no "duplicate" poses (look up, and either rotate by yaw or roll).
- Concatenating rotations (say you want a camera fixed to an object, and you rotate the object) becomes a lot easier.

Regardless of what method of orientation you use, camera's will have some position and orientation in space. Combining them will give you the camera's transformation. To apply this to the scene, you will need to use the inverse of this transformation on the whole scene.
First imagine doing several rotations and translations on your camera. Then, as you "undo" each of those in the inverse order, but also apply them to your scene, you will end up with your camera back where it was, but your scene has moved back along with it. This is exactly what you want to achieve.

Let's take Euler angles as an example. In matrix form, you can view it like this:
Camera Transform = Txyz * Ry * Rp * Rr
where Txyz is the translation, and R denotes Rotation, yaw, pitch and roll. The inverse of this, is the following:
Inverse Transform = Rr[sup]-1[/sup] * Rp[sup]-1[/sup] * Ry[sup]-1[/sup] * Txyz[sup]-1[/sup].
Apply that to your scene, and you are done.

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IMHO one has to understand the concept of spaces. Statements like "looking along the negative z-axis", " rotating around the y axis", and "you'll end up with your camera back where it was" are somewhat meaningless without knowing the reference in use. Sometimes it becomes clear from the context, but often enough it doesn't do so.

E.g. when one applies the said inverse camera transformation (a.k.a. view transformation) to the entirety of objects including your camera, the scene doesn't really change. Instead, one is switching over to another reference system (usually called the view space) in which the camera is located at the origin and orientated normally. (Whether looking is done along the negative z-axis in view space or something else belongs to the projection matrix and clipping.)

When considering the concept of space, the questions of when to apply which transformation at what position in the pipeline becomes clear.

[quote name='Ignifex' timestamp='1345366628' post='4971044']
If you are up for it, I recommend storing and manipulating your camera's orientation as a [url="http://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation"]quaternion[/url]. ...
[/quote]
Only unit quaternions represent ure rotations, to be precise. This is important insofar that one need to normalize quaternions from time to time (not so often as rotation matrices need to be normalized, I agree).

[quote name='Ignifex' timestamp='1345366628' post='4971044']
- By not having separate angles, there are no "duplicate" poses (look up, and either rotate by yaw or roll).
[/quote]
A unit quaternion has 2 representations for each orientation, namely q and -q, so it isn't unique as well. Especially when doing interpolation one has the choice of following the short or the long arc (ignoring pathological cases here).

[quote name='Ignifex' timestamp='1345366628' post='4971044']
- Concatenating rotations (say you want a camera fixed to an object, and you rotate the object) becomes a lot easier.
[/quote]
Why this? Concatenation means to multiply quaternions on the one or matrices on the other hand. Spaces and order play their role regardless whether one uses quaternions or matrices.

IMHO especially the example "you want a camera fixed to an object" is less intuitive when using quaternions, because "fixed to an object" implies a fixed position, too. This can be expressed as a single homogenous matrix and a simple matrix product. Edited by haegarr

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I agree the concept of spaces is essential for a good understanding of how transformations work, especially in the case of view transformations where there are quite a few distinct spaces you have to understand (object space, world space, view/eye space, clip space).
IMO, there are many ways in which to view transformations. As you progress in your understanding of how these work, your view will likely change.
You might well be right that one should start by understanding spaces. How about reading something like [url="http://http.developer.nvidia.com/CgTutorial/cg_tutorial_chapter04.html"]this[/url].

My earlier post was mainly an attempt to intuitively explain some of the core mechanics of transformations, without diving too deep into the maths behind it.
[quote name='haegarr' timestamp='1345371483' post='4971060']
Why this? Concatenation means to multiply quaternions on the one or matrices on the other hand. Spaces and order play their role regardless whether one uses quaternions or matrices.
[/quote]
This is mainly a comparison between quaternions and euler angles. Finding the euler angles of a concatination from two rotations is not very optimal. I couldn't think of a proper use case where concatenating euler angles makes sense though, instead of the matrices derived from them.

Euler angles are good for allowing a user to specify an angle, but are far less useful when applying these angles as transformations.
Matrices are the default approach for applying transformations.
Quaternions are useful for manipulating rotation, such as interpolation.

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@Ignifex

As you suggested, I will take a look at quaternions, I never heard this term before.

As far as camera and inverse transformations are concerned, this is what I am trying to do here already, the knowledge that I have with Opengl and viewing transformations come from the red book.

From what I understand, there is no such thing called a 'Camera' in Opengl library. If you want view an object that is drawn and centered at the origin, then you can either use the gluLookAt(0, 0, +z, 0, 0, 0, 0, 1, 0) or insted you can use glTranslatef(0, 0, -z), both of these will have the same effect (for this case at least).

From what I have studied and understand, the function name " LookAt " is like a false advertisement. You are not actually looking somewhere in the scene, it just applies inverse transformations to the whole scene and you think that you are moving a camera in the scene when you are acutally just transforming the scene. But then again, I might be incorrect here and its a completely different debate.

In my class, I am doing a similar thing, taking vector input in euclidean space and then finding the appropriate angle of rotations of heading and pitch, and then apply the inverse of this when I am rotating about an axis and it works fine as I please.

The problem I am having, is that if you take a look at my code, as long you give the initial arguments such that your hypothetical camera is located at the XZ plane, the heading will work fine, however, If I pitch or Roll to 180 degrees, and then I apply my heading calculation, then they will seem to work in reverse order. If I give the input to turn left, it will turn right and vice versa, because I am still rotating around this +ve y-axis.

Please note that, when I was trying to implement this class, I was not thinking in terms of matrices, I was thinking in terms of these transformation commands (glRotatef and glTranslatef). Edited by uzipaz

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      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.
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