# OpenGL Calculate depth of front sphere from back side

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Currently sitting on an issue which i can't solve due to my lack of mathematical knowledge.

Here is a picture i made which sums up what i'm trying to do:

To put it simply:

I render spheres (simple 3D meshes) into the scene on a seperate FBO by using frontface culling (so that the backside is rendered. The red part of the sphere on the screenshot.)

Now, in the fragment shader i can access the depth value of the rendered pixel by using "gl_Fragcoord.z". Now what i want to do is to calculate the depth value of the front facing side of the sphere of the exact same pixel. (so that i have a min and max depth value in order to know what the start depth and end depth value of the sphere on the given pixel is.) I need those values for post processing purposes.

My attempt to solve this was:

1.  pass the current vertex position into the fragment shader
2. subtract the vertex position from the origin point (in view space) to retrieve a normal pointing from the origin to the backface point
3. Mirror the z-component of this normal (as we are in view space)
4. add the mirrored normal to the origin point which gives us the front facing (vertex) position of the sphere
5. use this position to calculate the depth value like in an openGL depth buffer. (haven't done this properly.)

I may or may not have an error in my shader code. (Maybe the way i multiply matrices is wrong?)

Here is my current code (a bit messy but i tried to comment it.)

//-------------- Vertex Shader --------------------
#version 330

layout (location = 0) in vec3 position;
layout (location = 1) in vec3 normal;
layout (location = 2) in vec4 color;
layout (location = 3) in vec2 uv;

uniform mat4 uProjectionMatrix;
uniform mat4 uModelViewMatrix;

out vec4 oColor;
out vec2 vTexcoord;

out vec4 vFragWorldPos;
out vec4 vOriginWorldPos;
out mat4 vProjectionMatrix;

void main()
{
oColor = color;
vTexcoord = uv;

//coordinates are in view space!
vec4 tFragWorldPos = (uModelViewMatrix * vec4(position,1.0));
vec4 tOriginWorldPos = (uModelViewMatrix * vec4(0.0,0.0,0.0,1.0));

vFragWorldPos = (uModelViewMatrix * vec4(position,1.0));
vOriginWorldPos = (uModelViewMatrix * vec4(0.0,0.0,0.0,1.0));

gl_Position = uProjectionMatrix * uModelViewMatrix * vec4(position,1.0);

//send projection matrix to the fragment shader
vProjectionMatrix = uProjectionMatrix;
}
//---------------- Fragment Shader --------------
#version 330

in vec4 oColor;
in vec2 vTexcoord;
out vec4 outputF;
in vec4 gl_FragCoord;

uniform sampler2D sGeometryDepth;
in vec4 gl_FragCoord;

in vec4 vFragWorldPos;
in vec4 vOriginWorldPos;
in mat4 vProjectionMatrix;

void main()
{

//get texture coordinates of the screenspace depthbuffer
vec2 relativeTexCoord = vec2(gl_FragCoord.x,gl_FragCoord.y);
relativeTexCoord = relativeTexCoord-0.5+1.0;
relativeTexCoord.x = relativeTexCoord.x/1280.0;
relativeTexCoord.y = relativeTexCoord.y/720.0;

//depth
float backDepth = gl_FragCoord.z;//back depth
float geometryDepth = texture2D(sGeometryDepth,relativeTexCoord).r;//geometry depth

//--------------- Calculation of front depth---------------

//get distance from origin to the fragment (normal in viewspace)
vec3 offsetNormal = (vFragWorldPos/vFragWorldPos.w).xyz-(vOriginWorldPos/vOriginWorldPos.w).xyz;

//mirror depth normal (z)
offsetNormal.z*=-1.0;

//add normal to origin point in order to get the mirrored coordinate point of the sphere
vec4 sphereMirrorPos = vOriginWorldPos;
sphereMirrorPos.xyz += offsetNormal.xyz;

//apply perspective calculation
vec4 projectedMirrorPos = vProjectionMatrix * sphereMirrorPos;
projectedMirrorPos/=projectedMirrorPos.w;

//TODO: CALCULATE PROPERLY
float frontDepth = projectedMirrorPos.z * 0.5 + 0.5; // no idea what do do here further

//want to color only the pixels where the scene depth (provided by a screen space texture which is a depthbuffer of a different FBO)
//is exactly in between the min/max depth values of the sphere
if(backDepth>geometryDepth && frontDepth<geometryDepth){
outputF = vec4(1.0,1.0,1.0,1.0);
}else{
}

}

I suspect that maybe transforming the coordinates into viewspace in the vertex shader (and working with those coordinates) may be an issue. (No idea where/when to divide by "W" for example.) Also i'm currently stuck at the part where i have to calculate the depth values in the same range as the OpenGL depth buffer in order to compare them in the if statement shown at the end of the fragment shader.

Hints/help would be greatly appreciated.

Edited by Lewa

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I nearly got it working.

But there still seems to be a minor error in the calculation of the mirrored depth value.

Here is the current fragment shader (vertex shader is the same as above:)

#version 330

in vec4 oColor;
in vec2 vTexcoord;
out vec4 outputF;
in vec4 gl_FragCoord;

uniform sampler2D sGeometryDepth;
in vec4 gl_FragCoord;

in vec4 vFragWorldPos;
in vec4 vOriginWorldPos;
in mat4 vProjectionMatrix;

float linearizeDepth(float depthVal,float zNear,float zFar)
{
float n = zNear; // camera z near
float f = zFar; // camera z far
float z = depthVal;
return (2.0 * n) / (f + n - z * (f - n));
}

void main()
{
float uZnear = 0.01;
float uZfar = 500.0;

//get texture coordinates of the screenspace depthbuffer
vec2 relativeTexCoord = vec2(gl_FragCoord.x,gl_FragCoord.y);
relativeTexCoord = relativeTexCoord-0.5+1.0;
relativeTexCoord.x = relativeTexCoord.x/1280.0;
relativeTexCoord.y = relativeTexCoord.y/720.0;

//depth of the backfacing sphere pixels and of the level geometry (depth texture of different FBO)
float backDepth = linearizeDepth(gl_FragCoord.z,uZnear,uZfar);//back depth
float geometryDepth = linearizeDepth(texture2D(sGeometryDepth,relativeTexCoord).r,uZnear,uZfar);//geometry depth

//Now we have to calculate the front depth

//--------------- Calculation of front depth---------------

//get distance from origin to the fragment (in viewspace)

float depthDiff = (vFragWorldPos.z-vOriginWorldPos.z);

//substract depth difference from origin point in order to get the mirrored coordinate point of the sphere
vec4 sphereMirrorPos = vec4(vFragWorldPos.xy,vOriginWorldPos.z - depthDiff,vOriginWorldPos.w);

//apply perspective calculation
vec4 projectedMirrorPos = vProjectionMatrix * sphereMirrorPos;
projectedMirrorPos/=projectedMirrorPos.w;

//depth calculation
float frontDepth = (projectedMirrorPos.z + 1.0) / 2.0;
frontDepth = linearizeDepth(frontDepth,uZnear,uZfar);

//want to color only the pixels where the scene depth (provided by a screen space texture which is a depthbuffer of a different FBO)
//is exactly in between the min/max depth values of the sphere

if(backDepth>geometryDepth && frontDepth<geometryDepth){
outputF = vec4(1.0,1.0,1.0,1.0);
}else{
}

}

I believe the issue is somewhere here:

float depthDiff = (vFragWorldPos.z-vOriginWorldPos.z);

//add normal to origin point in order to get the mirrored coordinate point of the sphere
vec4 sphereMirrorPos = vec4(vFragWorldPos.xy,vOriginWorldPos.z - depthDiff,vOriginWorldPos.w);

"vFragWorldPos" is the position of the current vertex in ModelviewSpace. "vOriginWorldPos" is the origin of the sphere in modelviewSpace.

I simply calculate the z difference of both points by substracting the z components of both vectors.

Then i reconstruct the mirrored vertex coordinate by using the xy coordiantes of "vFragWorldPos" while the z-coordinate is calculated by substracting the depthDifference from the origin z-coordinate.

The issue is that it doesn't seem to give correct results by doing so.

I tested if this reconstruction method by changing this line:

vec4 sphereMirrorPos = vec4(vFragWorldPos.xy,vOriginWorldPos.z - depthDiff,vOriginWorldPos.w);

to this:

vec4 sphereMirrorPos = vec4(vFragWorldPos.xy,vOriginWorldPos.z + depthDiff,vOriginWorldPos.w);

which effectively calculates the depth of the back side of the sphere which i then compared with the values of the depth buffer. They are exactly the same. (which is correct.) But substracting the depthDiff value doesn't yield correct results.

Is there something that i'm missing? Maybe the z coordinates of the vertices which were transformed to modelview space aren't linear?

Edited by Lewa

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This sounds like an XY problem to me. Try backing up a step or two and describe what you're trying to accomplish. I suspect someone on these forums will be able to suggest a different approach that might work better.

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I only looked over the code quickly, so apologies if I'm misunderstanding something.  But, assuming your description of how you're trying to go about solving this, and that you're doing all the math in view space as you said... then it wont work because negating the z value wont give you what you think it gives you.  It wont give you a point backwards along the line of sight.

You can solve this by getting the point of intersection between the line of sight to the pixel and the line from the sphere origin that intersects that line at a right angle.  Once you get this point (lets call it midPoint) you're basically home free as you can just use the pixel position and midPoint to get the point your looking for.

Here's some pseudo code:

vec3 pixelPos;	// position in view space of the pixel on the sphere back face.  Known.
vec3 spherePos;	// position in view space of sphere origin.  Known.

vec3 tempDir = CrossProduct(spherePos, pixelPos);	// vector pointing up/down from plane
tempDir = CrossProduct(pixelPos, tempDir);		// vector pointing from line-of-sight towards spherePos
tempDir = Normalize(tempDir);

float d = DotProduct(tempDir, spherePos);	// distance along the "right" vector towards the sphere origin

vec3 midPoint = spherePos - tempDir * d;	// midPoint = point midway from front to backface along ling-of-sight

vec3 frontFacePos = 2 * midPoint - pixelPos;	// the frontface view-space point you want

Of course this code doesnt check to see if pixelPos and spherePos are parallel.  You'll need to check for that and handle the situation accordingly.  But, I think this will give you what you want... a point backwards along the line of sight (in view-space) from the backface of the sphere towards the frontface.

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On 7.08.2017 at 11:51 AM, 0r0d said:

I only looked over the code quickly, so apologies if I'm misunderstanding something.  But, assuming your description of how you're trying to go about solving this, and that you're doing all the math in view space as you said... then it wont work because negating the z value wont give you what you think it gives you.  It wont give you a point backwards along the line of sight.

You can solve this by getting the point of intersection between the line of sight to the pixel and the line from the sphere origin that intersects that line at a right angle.  Once you get this point (lets call it midPoint) you're basically home free as you can just use the pixel position and midPoint to get the point your looking for.

Here's some pseudo code:


vec3 pixelPos;	// position in view space of the pixel on the sphere back face.  Known.
vec3 spherePos;	// position in view space of sphere origin.  Known.

vec3 tempDir = CrossProduct(spherePos, pixelPos);	// vector pointing up/down from plane
tempDir = CrossProduct(pixelPos, tempDir);		// vector pointing from line-of-sight towards spherePos
tempDir = Normalize(tempDir);

float d = DotProduct(tempDir, spherePos);	// distance along the "right" vector towards the sphere origin

vec3 midPoint = spherePos - tempDir * d;	// midPoint = point midway from front to backface along ling-of-sight

vec3 frontFacePos = 2 * midPoint - pixelPos;	// the frontface view-space point you want

Of course this code doesnt check to see if pixelPos and spherePos are parallel.  You'll need to check for that and handle the situation accordingly.  But, I think this will give you what you want... a point backwards along the line of sight (in view-space) from the backface of the sphere towards the frontface.

That is EXACTLY what i needed/what i was looking for. Thank you!

Although i'm having a hard time understanding why this formula works. (As i seem to misunderstand how the viewspace works.)

Quote

Of course this code doesnt check to see if pixelPos and spherePos are parallel.

What exactly do you mean with "parallel"? If they are axis aligned in view space?

Edited by Lewa

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6 hours ago, Lewa said:

That is EXACTLY what i needed/what i was looking for. Thank you!

Although i'm having a hard time understanding why this formula works. (As i seem to misunderstand how the viewspace works.)

What exactly do you mean with "parallel"? If they are axis aligned in view space?

Here's a diagram that might help:

I think the problem is that you're thinking that view space means the Z values point along the line of sight to the camera.  But, view space is just camera space.  So when you take the vector (=> pixelPos - spherePos) and then negate the Z component, you get the "incorrect midPoint" seen above.

So what you need is to find the correct midPoint by first finding the "tempDir" in the image, which is found by first finding the vector normal to the plane and then doing a cross product to find this new vector which is orthogonal to both the plane normal and the line of sight vector.  Once you have that vector you easily get the midPoint and then easily the frontFacePos.

Does that make sense?

As far as why it matters to check if spherePos and pixelPos are parallel... if they are parallel (ie they both lie on the line from camera to spherePos) then the first 2 cross products will give you a 0 length vector, and then the normalize operation will cause a divide by 0.  So, you need to check if the pixelPos is parallel to spherePos, in which case the frontFacePos would just be

frontFacePos = 2 * spherePos - pixelPos;

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• This article uses material originally posted on Diligent Graphics web site.
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:
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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.
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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:
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.
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// 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.
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:
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 reenigne
For those that don't know me. I am the individual who's two videos are listed here under setup for https://wiki.libsdl.org/Tutorials
I also run grhmedia.com where I host the projects and code for the tutorials I have online.
Recently, I received a notice from youtube they will be implementing their new policy in protecting video content as of which I won't be monetized till I meat there required number of viewers and views each month.

Frankly, I'm pretty sick of youtube. I put up a video and someone else learns from it and puts up another video and because of the way youtube does their placement they end up with more views.
Even guys that clearly post false information such as one individual who said GLEW 2.0 was broken because he didn't know how to compile it. He in short didn't know how to modify the script he used because he didn't understand make files and how the requirements of the compiler and library changes needed some different flags.

At the end of the month when they implement this I will take down the content and host on my own server purely and it will be a paid system and or patreon.

I get my videos may be a bit dry, I generally figure people are there to learn how to do something and I rather not waste their time.
I used to also help people for free even those coming from the other videos. That won't be the case any more. I used to just take anyone emails and work with them my email is posted on the site.

I don't expect to get the required number of subscribers in that time or increased views. Even if I did well it wouldn't take care of each reoccurring month.
I figure this is simpler and I don't plan on putting some sort of exorbitant fee for a monthly subscription or the like.
I was thinking on the lines of a few dollars 1,2, and 3 and the larger subscription gets you assistance with the content in the tutorials if needed that month.
Maybe another fee if it is related but not directly in the content.
The fees would serve to cut down on the number of people who ask for help and maybe encourage some of the people to actually pay attention to what is said rather than do their own thing. That actually turns out to be 90% of the issues. I spent 6 hours helping one individual last week I must have asked him 20 times did you do exactly like I said in the video even pointed directly to the section. When he finally sent me a copy of the what he entered I knew then and there he had not. I circled it and I pointed out that wasn't what I said to do in the video. I didn't tell him what was wrong and how I knew that way he would go back and actually follow what it said to do. He then reported it worked. Yea, no kidding following directions works. But hey isn't alone and well its part of the learning process.

So the point of this isn't to be a gripe session. I'm just looking for a bit of feed back. Do you think the fees are unreasonable?
Should I keep the youtube channel and do just the fees with patreon or do you think locking the content to my site and require a subscription is an idea.

I'm just looking at the fact it is unrealistic to think youtube/google will actually get stuff right or that youtube viewers will actually bother to start looking for more accurate videos.

• i got error 1282 in my code.