在direct3d11中映射资源并分配数据

Question

Hi i'm working with shaders and i've just got a quick question about whether or not i can do something here. When mapping data to a buffer i normally see it done like this. Define class or struct to represent whats in the buffer.

class MatrixBuffer
{
public:
    D3DXMATRIX worldMatrix;
    D3DXMATRIX viewMatrix;
    D3DXMATRIX projectionMatrix;
} 

then set up a input element description when you make when you create the input layout, i got all that. Now, when i see people update the buffers before rendering, i see it mostly done like this:

MatrixBuffer * dataPtr;
if(FAILED(deviceContext->Map(m_matrixBuffer, 0, D3D11_MAP_WRITE_DISCARD, 0, &mappedResource)))
        return false;

dataPtr = (MatrixBuffer *) mappedResource.pData;
dataPtr->world = worldMatrix;
dataPtr->view = viewMatrix;
dataPtr->projection = projectionMatrix;

deviceContext->Unmap(m_matrixBuffer, 0);

where the values assigned are actually valid D3DXMATRIX's. seeing as that's okay, would anything go wrong if i just did it like this?

MatrixBuffer* dataPtr;
if(FAILED(deviceContext->Map(m_matrixBuffer, 0, D3D11_MAP_WRITE_DISCARD, 0, &mappedResource)))
            return false;

dataPtr = (MatrixBuffer *) mappedResource.pData;
*dataPtr = matrices;

deviceContext->Unmap(m_matrixBuffer, 0);

where matrices is a valid filled out MatrixBuffer object? Would there be dire consequences later because of some special direct X handles this or is it fine?

Answer 1

I don't see anything wrong with your version, from a technical perspective.

It might be slightly harder to grasp what actually gets copied there, and from a theoretical viewpoint the resulting code should be pretty much the same -- I would expect that any decent optimizer would notice that the "long" version just copies several successive items, and lump them all into one larger copy.

Answer 2

Try avoiding using mapping as it stalls the pipeline. For stuff that gets updated once a frame it's better to have D3D11_USAGE_DEFAULT buffer and then use UpdateSubresource method to push new data in. More on that here: http://blogs.msdn.com/b/shawnhar/archive/2008/04/14/stalling-the-pipeline.aspx