The RootLayout class.
D3D11, OpenGL and Metal use a 'table' model for binding resources.
While there are a couple differences between the APIs, if we look at HLSL's binding model:
- There is a special const buffer for uniform parameters
- All const buffers are in slots [b0; b8)
- Textures and TexBuffers share the same slots, range [t0; t128)
- Samplers have their own slots [s0; s16)
- UAV textures and buffers share the same slots, range [u0; u8)
Vulkan & D3D12 however don't follow this table model. Instead they distribute
bindings into sets and bindings. Vulkan guarantees at least 4 sets will be available.
A binding can contain ANYTHING: sampler, const buffer, textures, etc
@code
layout( set = 0, location = 0 ) texture2D myTex;
layout( set = 0, location = 1 ) sampler mySampler;
layout( set = 0, location = 2 ) buffer myBuffer { uint myUavArray[4096]; };
layout( set = 0, location = ... ) ;
layout( set = 1, location = 0 ) samplerBuffer myTexBuffer;
layout( set = 1, location = 1 ) sampler anotherSampler;
layout( set = 1, location = ... ) ;
@endcode
Since Ogre has historically followed the 'table' model and we want to reuse existing
shaders as much as possible; this RootLayout tells Ogre which resources your shader
is planning to use, so we can create the optimal DescriptorSets.
For example an HLSL shader that ONLY declares and uses the following resources:
@code
cbuffer myConstBuffer0 : register(b4) {};
cbuffer myConstBuffer1 : register(b5) {};
cbuffer myConstBuffer2 : register(b6) {};
Buffer<float4> myTexBuffer : register(t1);
Texture2D myTex0 : register(t3);
Texture2D myTex1 : register(t4);
SamplerState mySampler : register(s1);
RWTexture3D<float> myUavTex : register(u0);
RWBuffer<float> myUavBuffer : register(u1);
@endcode
Would work using the following RootLayout:
@code
{
"0" :
{
"const_buffers" : [4,7],
"tex_buffers" : [1,2],
"textures" : [3,4],
"samplers" : [1,2],
"uav_buffers" : [1,2],
"uav_textures" : [0,1]
}
}
@endcode
That is, explicitly declare that you're using const buffer range [4; 7),
tex buffer range [1; 2) etc.
In Vulkan we will automatically generate macros for use in bindings
(buffers are uppercase letter, textures lowercase):
@code
// Const buffers
#define ogre_B4 set = 0, binding = 0
#define ogre_B5 set = 0, binding = 1
#define ogre_B6 set = 0, binding = 2
// Texture buffers
#define ogre_T1 set = 0, binding = 3
// Textures
#define ogre_t3 set = 0, binding = 4
// Samplers
#define ogre_s1 set = 0, binding = 5
// UAV buffers
#define ogre_U1 set = 0, binding = 6
// UAV textures
#define ogre_u0 set = 0, binding = 7
@endcode
Thus a GLSL shader can use it like this:
@code
layout( ogre_B4 ) uniform bufferName
{
uniform float myParam;
uniform float2 myParam2;
};
layout( ogre_t3 ) uniform texture2D myTexture;
// etc...
@endcode
Could you have used e.g. "const_buffers" : [0,7] instead of [4,7]?
i.e. declare slots in range [0;4) even though they won't be used?
Yes. But you would be consuming more memory.
Note that if your vertex shader uses slots [0; 3) and pixel shader uses range
[4; 7) then BOTH shaders must use a RootLayout that at least declares range [0; 7)
so they can be paired together
RootLayouts have a memory vs performance trade off:
- Declaring unused slots helps reusing RootLayout. Reuse leads to fewer descriptor swapping
- Unused slots waste RAM and VRAM
That's why low level materials provide prefab RootLayouts, in order to maximize
RootLayout reuse while also keeping reasonable memory consumption.
See GpuProgram::setPrefabRootLayout
Note that if two shaders were compiled with different RootLayouts, they may still
be able to be used together if their layouts are compatible; and the 'bigger' RootLayout
which can be used for both shaders will be selected.
For example if vertex shader declared:
@code
{
"0" :
{
"const_buffers" : [0,3]
}
}
@endcode
and the pixel shader declared:
@code
{
"0" :
{
"const_buffers" : [0,3],
"textures" : [2,4]
}
}
@endcode
Then they're compatible; and pixel shader's will be selected to be used for both shaders.
That is because both layouts declared 3 const buffers (perfect match); and textures
ALWAYS go after const buffers (THE ORDER is important and determined by
DescBindingTypes::DescBindingTypes).
Thus the pixel shader's RootLayout can be used for the vertex shader.
However if the vertex shader had declared instead:
@code
{
"0" :
{
"const_buffers" : [0,3],
"tex_buffers" : [0,1]
}
}
@endcode
then both RootLayouts are incompatible.
This is because vertex shader uses 3 const_buf slots, then 1 tex buffer slot:
1. const
2. const
3. const
4. tex buffer
But where pixel shader uses 3 const_buf slots, then 2 texture slots:
1. const
2. const
3. const
4. texture --> ERROR. CONFLICT
5. texture
The 4th slot is in conflict because the vertex shader uses it for a tex buffer,
and the pixel shader uses it for a texture. A slot can only be used for one type of resource.
In order to fix this incompatibility, the pixel shader needs to declare a tex_buffer,
even if it won't ever use the tex_buffer slot (the vertex shader will):
@code
{
"0" :
{
"const_buffers" : [0,3],
"tex_buffers" : [0,1],
"textures" : [2,4]
}
}
@endcode
<b>Baked sets:</b>
If you intend to use UAVs or you're writing a shader with the Hlms which uses
DescriptorSetSampler, DescriptorSetTexture or DescriptorSetUav, then you need to use
a baked set.
For example if a low level material uses UAVs:
@code
cbuffer myConstBuffer0 : register(b4) {};
cbuffer myConstBuffer1 : register(b5) {};
cbuffer myConstBuffer2 : register(b6) {};
Buffer<float4> myTexBuffer : register(t1);
Texture2D myTex0 : register(t3);
Texture2D myTex1 : register(t4);
SamplerState mySampler : register(s1);
RWTexture3D<float> myUavTex : register(u0);
RWBuffer<float> myUavBuffer : register(u1);
@endcode
You'll need a set for baking the UAVs:
@code
{
"0" :
{
"const_buffers" : [4,7],
"tex_buffers" : [1,2],
"textures" : [3,4],
"samplers" : [1,2]
},
"1" :
{
"baked" : true,
"uav_buffers" : [1,2],
"uav_textures" : [0,1]
}
}
@endcode
If this is a Compute Shader or another Hlms shader, then the textures and samplers also need
to be in the baked set:
@code
{
"0" :
{
"const_buffers" : [4,7],
},
"1" :
{
"baked" : true,
"tex_buffers" : [1,2],
"textures" : [3,4],
"samplers" : [1,2],
"uav_buffers" : [1,2],
"uav_textures" : [0,1]
}
}
@endcode
Note that you can use more than one set if e.g. you intend to swap a
DescriptorSetTexture much more often than the rest:
@code
{
"0" :
{
"const_buffers" : [4,7],
},
"1" :
{
"baked" : true,
"samplers" : [1,2],
"uav_buffers" : [1,2],
"uav_textures" : [0,1]
},
"2" :
{
"baked" : true,
"tex_buffers" : [1,2],
"textures" : [3,4]
}
}
@endcode
For best performance, <b>the last set index should be the one that changes more frequently<b/>
Inheritance diagram for Ogre::RootLayout:
1.9.1