[D3D12] How to correctly update constant buffers in different scenarios.

Volgogradetzzz · 2016-06-17T04:57:28

Hello, This topic is super confusing for me and I hope you'll help me to understand it better. Here several usage scenarios: 1. I have n objects and for every object I need to change a constant data before draw. Let's say the constant data ConstData have one integer field and is different for every object. I have a constant buffer consBuffer and I need to update it. In pseudo code: ConstData cd; cd.data = 1; ptr = consBuffer.Map(); memcpy(ptr, cd); constBuffer.Unmap(); draw(obj1); cd.data = 2; ptr = consBuffer.Map(); memcpy(ptr, cd); constBuffer.Unmap(); draw(obj2); ... It well known that draw doesn't happen immediately but goes to the gpu queue for later execution. And from the code I wrote I see that when the gpu is ready to actually draw object1 the constant data will be overridden by later calls to memcpy. Does that mean that I need to have a different constant buffer for every object? So if I have 100 objects and 2 back buffers I need 100 * 2 = 200 constant buffers. Is it right? 2. If I want to store a constant data directly in root signature, can I update the root signature before every draw call? Taking my previous example, I will not have calls to Map()/memcpy()/Unmap(), but will write directly to signature. Do I need to wait until previous object or frame finished rendering? 3. If I have rarely changed constant buffer, a projection matrix, for example, do I need to have different constant buffers for every render target? In other words, if projection matrix was changed can I safely update buffer only once and hope that changes will propagate to all frames in the queue?

Graphics and GPU Programming Programming

Started by Volgogradetzzz June 10, 2016 08:34 AM

40 comments, last by SoldierOfLight 8 years, 5 months ago

Dingleberry

924

June 16, 2016 11:40 PM

So if you write to a persistently mapped resource in a command list and wait for a signal, you're guaranteed to have the writes finished without any barriers? That seems odd because shouldn't the signal only be informing the cpu of the current execution state?

SoldierOfLight

2,378

June 17, 2016 04:57 AM

If you write to a persistently mapped buffer in a command list, and wait for a signal, yes the writes will complete without any barriers on the buffer. For textures in CPU-accessible heap types, the resource needs to be in the COMMON state for the CPU to read it.

[D3D12] How to correctly update constant buffers in different scenarios.

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[D3D12] How to correctly update constant buffers in different scenarios.

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