Direct3D c++ api how to update vertex buffer?

Question

So how can one update values in vertex buffer bound into device object using IASetVertexBuffers method? Also will changing values in this buffer before call to Draw() and Present()? Also will the image be updated according to these new values in buffer?

Answer 1

To update a vertex buffer by the CPU, you must first create a dynamic vertex buffer that allows the CPU to write to it. To do this, call ID3D11Device::CreateBufferwith Usage set to D3D11_USAGE_DYNAMIC and CPUAccessFlags set to D3D11_CPU_ACCESS_WRITE. Example:

D3D11_BUFFER_DESC desc;
ZeroMemory( &desc, sizeof( desc ) );
desc.Usage = D3D11_USAGE_DYNAMIC;
desc.ByteWidth = size;
desc.BindFlags = D3D11_BIND_VERTEX_BUFFER;
desc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE;
d3dDevice->CreateBuffer( &desc, initialVertexData, &vertexBuffer );

Now that you have a dynamic vertex buffer, you can update it using ID3D11DeviceContext::Map and ID3D11DeviceContext::Unmap. Example:

D3D11_MAPPED_SUBRESOURCE resource;
d3dDeviceContext->Map( vertexBuffer, 0, D3D11_MAP_WRITE_DISCARD, 0, &resource );
memcpy( resource.pData, sourceData, vertexDataSize );
d3dDeviceContext->Unmap( vertexBuffer, 0 );

where sourceData is the new vertex data you want to put into the buffer.

This is one method for updating a vertex buffer where you are uploading a whole new set of vertex data and discarding previous contents. There are also other ways to update a vertex buffer. For example, you could leave the current contents and only modify certain values, or you could update only certain regions of the vertex buffer instead of the whole thing.

Each method will have its own usage and performance characteristics. It all depends on what your data is and how you intend on using it. This NVIDIA presentation gives some advice on the best way to update your buffers for different usages.

Yes, you will want to call this and IASetVertexBuffers before Draw() and Present() to see the updated results for the current frame. You don't necessarily need to update the vertex buffer contents before calling IASetVertexBuffers. Those can be in either order.