3 replies to this topic

[XVincentX]

I've got this constant buffer.

// Buffer Definitions: 
//
// cbuffer cMultiFrame
// {
//
//   row_major float4x4 WorldMatrix;    // Offset:    0 Size:    64 [unused]
//   bool Lamp;                         // Offset:   64 Size:     4 [unused]
//   float SampleWeights[7];            // Offset:   80 Size:   100
//   float2 SampleOffsets[7];           // Offset:  192 Size:   104
//
// }
//

As you can see, while real sizeof(cMultiFrame) in Visual Studio = 160, when i call Draw function, it say me a warning:

Quote

D3D10: WARNING: ID3D10Device::Draw: The size of the Constant Buffer at slot 0 of the Pixel Shader unit is too small (160 bytes provided, 304 bytes, at least, expected).

At first i can't understand why 7 floats = 100 bytes while it should be only 28!

All this due to packing rules and floating problems.
The way is to use packoffset to force packing rules, i tried in this way following the documentation

At first i increased SAMPLE_COUNT to 8 to have float * 8 = 2 float4 and 8 * float2 = 4 float4.
And wrote this code.

cbuffer cMultiFrame
{
	row_major float4x4	WorldMatrix	:	packoffset(c0);
	float SampleWeights[SAMPLE_COUNT]	:	packoffset(c4);
	float2 SampleOffsets[SAMPLE_COUNT]	:	packoffset(c6);
	bool Lamp				:	packoffset(c10.x);
};

But it say me i'm overriding other values.
Can you suggest me right way to organize this constant buffer?

[XVincentX]

Close please, i resolved.

[Goz]

Its always nice to share how you resolved in case someone else comes across the problem :)

[XVincentX]

I resolved making another constant buffer and forcing all arrays to float4...but it's a very horrible solution and i would like to better understand how packing is done by HLSL compiler and how to avoid this problems.

Anyway, in my opinion D3D10 can also waste 1 mega for constant buffer, my question is: how can i recreate same dummy values in my struct to have the right alignement offset?? I tried using floats, but D3D10 gave me an error in CB creation becouse size was not a 16-multiple.

It looks like HLSL compiler can pack, without offsets, only float4 arrays.