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Aqua Costa

Reducing state changes

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Do you use any subsystem to help reduce state changes (like ignore a call to bind a texture/shader/render target if it is already bound)? How did you designed it?

Currently I keep track of what is bound to every resource/shader/render target slots using arrays and before binding a new resource I check if it's already in the array, however the code is kind of messy because I'm trying to batch all resource bindings to a single API call so for example if 3 textures need to be bound to the PS I only have to call PSSetShaderResources() once.

Does calling [CODE]PSSetShaderResources(0, D3D11_COMMONSHADER_INPUT_RESOURCE_SLOT_COUNT, resources);[/CODE] increases driver overhead even if 'resources' is an array where all pointers but one are NULL because I actually only need to bind a single texture. Edited by TiagoCosta
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most expensive GPU operation is binding a texture to a slot. I order the rendered objects by an algorithm which produses leat possible texture slots resets.
The render state changes are cheap, and to my surprise, the redner target binding too.
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I follow a similar method to what you described in Hieroglyph 3, except I do it with all of the states (for shader stages and fixed stages too). And you are right, it does get kind of messy at times. The general concept that I followed was to have a single class that represents the state of a pipeline stage. Then I keep two copies of that object - one for the desired state, and one for the current state.

Then I track at state setting time if there is a difference between the two. If so, then the next time I bind the states for a draw call, then I push the API changes accordingly. That is the best way I have found to do this management. However, this doesn't try to find the best order of object rendering or anything like that - something like that is left up to either the application itself, or the objects being rendered.
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I concur that textures are the most costly thing not to redundancy-check in DirectX 11. Although I only have one card for testing DirectX 11, it is likely to be universal because textures are the most costly on bandwidth in most cases.
Setting shaders is more expensive in DirectX 9 but DirectX 11 has eliminated the internal state changes that caused that to be the case, and has made bandwidth the most prominent issue instead of internal state changes, flushes, etc.

My method for redundancy-checking is to allow the user to set states at any time, but all that happens is that my own variables get changed and nothing is actually issued to DirectX 11.
Then I have one function for rendering, and within it I check my own state copy against the state copy last time things were drawn and send only commands to DirectX 11 that are not redundant.

The reason I wait until the actual draw call is 2-fold:
#1: You can add a “set default render states” function with no impact on performance as we do at work (but I do not personally do within my engine). Like what Hodgman does.
#2: It is the only way to set multiple textures at once, avoiding unnecessary calls to PSSetShaderResources().


And then I have a render queue to ensure that redundancies are maximized, resulting in the fewest possible state changes between draw calls.


L. Spiro Edited by L. Spiro
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