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Tordin

DX11 Directx11 - Shader Constantbuffers vs the world.

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Hey!

i working on some dx11 stuff and are at the point where i need to be able to set shader variables easy and fast, since it gonna happen in runtime i am looking for the fastest way to do this.

At the moment i am struggeling with Constant buffers and i havent realy found another way to set shader input variabels, so my big question is :

How do i in the fastest CPU SPEED way set shadervariables? i need them to change on the fly for each frame.

Is CBuffers the way to do it?
Is there any other way to do it?
Is that otherway faster than remapping the cbuffer?

Would be glad for articels, information, tutorials and more!

cheers !

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If you want performance, than constant buffers are absolutely the way to go. No question. You obviously want to limit the amount of data that you map and update, and the way to do that is to group your constants based on how often they change. So for example if your view * projection matrix never changes the entire frame, put in a PerFrames cb and only set them once a frame. If you set a world matrix for each draw call, put that in a PerDraws cb.

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is PreFrame cb a different constant buffer than a regular Cbuffer?
is PreDraw cb a different constant buffer than a regular Cbuffer?

How do they work? (if they exsist?)

or do you just mean to have different cb in the shader with less data than a full global one?

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That does actually touch on something I've been wanting to ask about MJP. Say you want to set a VP matrix on a per frame basis. How would you go about that in detail.

In my framework I'm working without the DX effects as I wanted to learn the nitty gritty details. And when I reflect my shaders to find the bind information a given CB containing the VP matrix (for the sake of this example let it contain only that) it might give me different bind points for different shaders.

How exactly would I avoid having to bind the information for each shader. Even when specifying the CB register that doesn't change the bind points, and I just haven't been able to find any information regarding them.

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If you manually bind a constant buffer to a register using "register : (cb0)", then you can use that index to bind the D3D11 constant buffer at runtime. If you want you can keep your commonly-used constant buffers in a header file and #include that in all of your shaders, which makes it easier to manage.

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Another way to improve performance for CPU related configuration of the pipeline is to use the multithreading capabilities of D3D11. If you are using 2 or 4 cores to submit the requested changes into command lists, and then submit the command lists to the immediate context then you will almost surely be faster than doing it in a single thread on a single core.

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Ah, i see.
Than this brings me to another question.

Say i have 3 sets of CBs,
When i set thees and do the call "SetVSConstantBuffer" for the second one for example, should i set the "numBuff" to 1? or 2?

or is this where the register thing come into play?

Can some one give me a example & explenation of the cb & registery system?

i have understood it like this :

ConstantBuffer WorldViewProjection {
matrix World : World <--- in my c++ buffer i have to have a variable named that name?
};

Or is this wrong?

Cheers!

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Hey Tordin - Sorry for hijacking your thread like that.

As far as I can tell what you're doing is wrong. A constant buffer is a buffer like any other. It's a structure containing some data you would like your shader instance to use. In DX9-era shaders you would have a shader just declare the variables it would need - But in DX10/11 they switched to batching variables to the GPU to avoid the driver overhead caused by repeated updating of variables. Enter the constant buffer. It's a way to group up variables (be it matricies, floats, arrays whatever) based on usage.

An excample of a constant buffer declaration in a shader could be:

cbuffer CBObject : register(cb0) <-- The register here would be the slot you need to set.
{
float4x4 m_kWorldView;
float4x4 m_kWorldViewIT;
float4x4 m_kWorldViewProjection;
}

Hope this helped.

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So if i have three buffers

i would define them like this : ?

cbuffer CBObject : register(cb0)
{
float4x4 m_kWorldView;
float4x4 m_kWorldViewIT;
float4x4 m_kWorldViewProjection;
}

cbuffer CBObject : register(cb1)
{
float4x4 m_kWorldView;
float4x4 m_kWorldViewIT;
float4x4 m_kWorldViewProjection;
}

cbuffer CBObject : register(cb2)
{
float4x4 m_kWorldView;
float4x4 m_kWorldViewIT;
float4x4 m_kWorldViewProjection;
}


if i call the SetVsConstantbuffer(1,....);

i would assign stuff to the second buffer? not the first or the third one?
just because i sat the registery(cb1) there? is this correct?

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The VSSetConstantBuffers works like this: you specify which slot you want to start at, and how many constant buffers you want to set contiguously starting from the slot you specified. So if you pass 1 as StartSlot and then pass an array of 3 constant buffers, you will set those three constant buffers to slots 1, 2, and 3 respectively. If you want to just set one constant buffer, then you pass the slot you want to set it to and pass 1 for NumBuffers.

That said, that example you just outlined with three identical constant buffers declared will not compile. This is because you can't have multiple buffers with the same name declared in the same file, and you can't have the same parameter name declared in multiple constant buffers.

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Yes of corse that wont compile, that wasent the point eiter, the point was that i had understood it and hadent the time to write a correct shader!

Anyhow, to bring the subject up again, I have this shader :

cbuffer perFrame : register(cb0)
{
float3 floatP1;
float3 floatP2;
float3 floatP3;
float3 floatP4;
};

struct VSOUT
{
float4 pos : SV_POSITION;
float2 uv : TEXCOORD0;
};


Texture2D diffuse;
SamplerState samLin;


float4 mainPS(VSOUT inPS) : SV_Target
{
float4 color = diffuse.Sample(samLin,inPS.uv);
return color*floatP2.x;
}



And i try to set a row of 4 3d vectors, the first vector floatP1 gets it information, but the rest dosent!

and this is how i set the buffer! if you see ways to improve this, please tell me!


bool D3D11Device::SetPixelShaderVariableConstantBuffer(MORN_VARIABLE_BUFFER_PERFRAME data)
{
HRESULT hr;
D3D11_BUFFER_DESC cbDesc;
D3D11_SUBRESOURCE_DATA cbSubData;
D3D11_MAPPED_SUBRESOURCE cbMapped;
MORN_VARIABLE_BUFFER_PERFRAME* Buffer;

if(m_pPixelShaderCBuffer == NULL)
{
Buffer = &data;

ZeroMemory(&cbDesc,sizeof(cbDesc));
ZeroMemory(&cbSubData,sizeof(cbSubData));

cbDesc.ByteWidth = sizeof(MORN_VARIABLE_BUFFER_PERFRAME);
cbDesc.Usage = D3D11_USAGE_DYNAMIC;
cbDesc.BindFlags = D3D11_BIND_CONSTANT_BUFFER;
cbDesc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE;
cbDesc.MiscFlags = 0;
cbDesc.StructureByteStride = 0;

cbSubData.pSysMem = Buffer;
cbSubData.SysMemPitch = 0;
cbSubData.SysMemSlicePitch = 0;

int size = sizeof(MORN_VARIABLE_BUFFER_PERFRAME);

hr = m_pD3DDevice->CreateBuffer( &cbDesc, &cbSubData,
&m_pPixelShaderCBuffer );
if(ChekReturnError(hr))
return false;

m_pD3DContex->PSSetConstantBuffers( 0, 1, &m_pPixelShaderCBuffer );
return true;
}
else if(m_pPixelShaderCBuffer != NULL)
{
hr = m_pD3DContex->Map(m_pPixelShaderCBuffer,0,D3D11_MAP_WRITE_DISCARD,0,&cbMapped);
if(ChekReturnError(hr))
return false;

Buffer = (MORN_VARIABLE_BUFFER_PERFRAME*)cbMapped.pData;
for(int i = 0; i<4; ++i)
Buffer->fff[i] = data.fff[i];

m_pD3DContex->Unmap(m_pPixelShaderCBuffer,0);
m_pD3DContex->PSSetConstantBuffers( 0, 1, &m_pPixelShaderCBuffer );

return true;
}
}



So when i try to get information from the second 3d vector in the array, it´s pichblack!

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In constant buffers, float3's will be aligned to 16-byte boundaries due to the fact that registers are 16 bytes in size. So on the CPU side you need to either mirror that alignment using pragmas, or put in appropriate padding. See this for a full set of packing rules.

[Edited by - MJP on November 25, 2010 1:34:25 PM]

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