Monday, April 4, 2011

[Direct3D] Handling Lost Devices

Hello and welcome to another topic about 3D programming.

Let's suppose we're programming in Direct3D9.

Today I'll be talking about one of the most annoying problems found in 3D applications: lost devices.

Lost Devices
First things first: what's a lost device anyway? When a device goes in a lost state it means it can't place it's results anywhere. It then isn't able to put it's buffers to any place on screen. This might happen when a user Alt Tabs out of a fullscreen game for example: the GPU then can't place it's frames anywhere anymore, so it becomes lost.

Lost devices can't be fully accessed by the Direct3D API anymore. For example, draw calls will return D3DERR_INVALIDCALL instead of D3D_OK (success). This means you can't tell the GPU to do anything useful anymore. To get it working again, you need to reset it.

Resetting your GPU Card
Just to assure you, you don't need to reset your PC or anything, you only need to empty it's memory manually by software (API). Remember, most of Direct3D's API isn't working anymore. There's just about five calls we can make:
  • devicepointer->TestCooperativeLevel(). Think of this one as a doorbell. No one coming at the door (not returning D3D_OK)? Your GPU is lost.
  • devicepointer->Reset(). This one will wipe GPU RAM and reset all States you've set (like SetRenderState and SetSamplerState).
  • resourceinheritingfromIUnknown->Release(). You need to tell the GPU you don't need resources in GPU RAM anymore with this function. You can't even forget a single resource pointer: it will make your app crash when reseting.
  • resourcebackedupincpuram->OnLostDevice(). Call this on any object that has a backup in CPU RAM.
  • resourcebackedupincpuram->OnResetDevice(). Call this on any object that has a backup in CPU RAM.
As I've said, you need to empty the GPU's memory before you can reset it. One minor note first though: as any experienced PC user does *cough* you make backups *cough*:
  • Resources put in D3DPOOL_DEFAULT are no-backup resources and can be found in the best RAM possible (GPU RAM). If we run out of GPU RAM, we put it in CPU RAM. If we can't store it there, leave it on the drive, crash, or put it in Page File. Clear enough.
  • But we've also got D3DPOOL_MANAGED. These resources are copied to CPU RAM, and only when needed they get copied (not moved) to GPU RAM. This means there's always a backup available of these resources.
Let's put together a TODO-list of our reset then. What needs to be done is (in this order):
  1. Release() any resources that are stored in GPU RAM (D3DPOOL_DEFAULT).
  2. Put any resources that are stored both in GPU RAM and in CPU RAM (D3DPOOL_MANAGED) on hold and remove them from GPU RAM. Do this by calling OnLostDevice() on them.
  3. Reset the device with devicepointer->Reset(D3DPRESENT_PARAMETERS).
  4. Then tell all resources backed up in CPU RAM to copy back to GPU RAM by calling OnResetDevice() on them.
  5. Recreate your resources that were put in D3DPOOL_DEFAULT.
An example
Let's say we've got ourselves a basic engine with the following resources:
  • A GPU font, called ID3DXFont, used to draw our tooltip text on the GPU.
  • The FX Framework, called ID3DXEffect, used to modify shader parameters.
  • A shadowmap (color + depth), created by CreateTexture and CreateDepthStencilSurface. These are IDirect3D9Surface's and a IDirect3D9Texture.
In this case, the reset sequence will look like this:

void D3D::resetD3D() {
// Saveable resources in D3DPOOL_MANAGED

// Unsavable resources that don't have a backup

// Let's throw everything away

// And copy the CPU RAM resources to GPU RAM

// Recreate D3DPOOL_DEFAULT stuff

I hope this will be clear enough, and if not, tell me!

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