https://msdn.microsoft.com/en-us/library/windows/desktop/dn508285(v=vs.85).aspx
Map with D3D11_MAP_WRITE_DISCARD, the runtime returns a pointer to a new region of memory instead of the old buffer data.
This allows the GPU to continue using the old data while the app places data in the new buffer.
No additional memory management is required in the app; the old buffer is reused or destroyed automatically when the GPU is finished with it.
好神奇
Note When you map a buffer with D3D11_MAP_WRITE_DISCARD, the runtime always discards the entire buffer.
You can't preserve info in unmapped areas of the buffer by specifying a nonzero offset or limited size field.
When you call Map on a static vertex buffer while the GPU is using the buffer, you get a significant performance penalty. In this situation,
Map must wait until the GPU is finished reading vertex or index data from the buffer before Map can return to the calling app, which causes a significant delay.
Calling Map and then rendering from a static buffer several times per frame also prevents the GPU from buffering rendering commands
because it must finish commands before returning the map pointer.
Without buffered commands, the GPU remains idle until after the app is finished filling the vertex buffer or index buffer and issues a rendering command.
D3D11_USAGE_IMMUTABLE is well-suited to data such as textures because such data is typically read into memory from some file format.
Therefore, when you create a texture with D3D11_USAGE_IMMUTABLE, the GPU directly reads that texture into memory.
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shit原先那個頁面被更新了。。。
https://docs.microsoft.com/zh-cn/windows/desktop/api/d3d11/ne-d3d11-d3d11_usage
https://docs.microsoft.com/en-us/windows/desktop/api/d3d11/ne-d3d11-d3d11_map
====================
兜兜轉轉 又回來繼續弄這個
先捋一下dx11
map是為了可以拿到這塊memory的地址 來操作裡面的內容
如果memory型別用static 就需要gpu等cpu用完再用 hang在那裡 很慢 所以memory型別要選dynamic
這樣顯示卡會自己處理這些事情不會hang在那裡等gpu用完再用 處理的方式取決於map type
D3D11_MAP_WRITE_DISCARD --如果是write discard，這時 如果gpu在用cpu就直接開一塊新的往裡寫，所以裡面的內容是未定義的。舊內容discard了
D3D11_USAGE_DEFAULT GPU讀寫--預設的GPU資源
D3D11_USAGE_IMMUTABLE GPU讀 CPU不能操作---不變
D3D11_USAGE_DYNAMIC CPU寫 GPU讀 map出來
D3D11_USAGE_STAGING GPUcopy to CPU --save GPU和CPU讀寫
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https://docs.microsoft.com/en-us/windows/desktop/direct3d12/memory-management-strategies
下面寫dx12了
有三種
committed
placed
reserved
如果cpu要寫資源 不會像dx11 的write discard那樣driver管理資源，就等於只有NO_OVERWRITE flag需要自己管理資源
保證GPU要用的資源沒有被cpu寫壞 比如加fence 用uploading resorces
就是每份gpu要用的資料cpu copy一份出來操作 等於手動 write discard
我現在用的fence就讓他hang在那裡了，dynamic的map unmap肯定不能這樣，這樣太慢了 但應該是可以用的
https://docs.microsoft.com/en-us/windows/desktop/direct3d12/uploading-resources