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DX11 [DX11] Blending Lights

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I'm implementing deferred rendering, everything was going smooth until I got to the part where in order to support multiple directional lights. Just for testing purposes, I've created a red light coming from the positive x-axis, and a blue light coming from the negative x-axis. The problem I'm having is I render one after the other in the same pass before I call present(), but only the first color is drawn. In PIX, I've debugged a pixel that should be colored with the blue light, the debugger shows that the pixel would be blue as expected, but it's failed the depth test.

Here is my blend state description:
D3D11_BLEND_DESC omDesc;
ZeroMemory( &omDesc, sizeof( D3D11_BLEND_DESC ) );
omDesc.RenderTarget[0].BlendEnable = TRUE;
omDesc.RenderTarget[0].BlendOp = D3D11_BLEND_OP_ADD;
omDesc.RenderTarget[0].SrcBlend = D3D11_BLEND_ONE;
omDesc.RenderTarget[0].DestBlend = D3D11_BLEND_ONE;
omDesc.RenderTarget[0].BlendOpAlpha = D3D11_BLEND_OP_ADD;
omDesc.RenderTarget[0].SrcBlendAlpha = D3D11_BLEND_ONE;
omDesc.RenderTarget[0].DestBlendAlpha = D3D11_BLEND_ONE;
omDesc.RenderTarget[0].RenderTargetWriteMask = 0x0F;

And here is my depth stencil description:
dsDesc.DepthEnable = true;
dsDesc.DepthWriteMask = D3D11_DEPTH_WRITE_MASK_ALL;
dsDesc.DepthFunc = D3D11_COMPARISON_LESS;

// Stencil test parameters
dsDesc.StencilEnable = true;
dsDesc.StencilReadMask = D3D11_DEFAULT_STENCIL_READ_MASK;
dsDesc.StencilWriteMask = D3D11_DEFAULT_STENCIL_WRITE_MASK;

// Stencil operations if pixel is front-facing
dsDesc.FrontFace.StencilFunc = D3D11_COMPARISON_ALWAYS;
dsDesc.FrontFace.StencilPassOp = D3D11_STENCIL_OP_KEEP;
dsDesc.FrontFace.StencilFailOp = D3D11_STENCIL_OP_DECR;
dsDesc.FrontFace.StencilDepthFailOp = D3D11_STENCIL_OP_KEEP;

// Stencil operations if pixel is back-facing
dsDesc.BackFace.StencilFunc = D3D11_COMPARISON_ALWAYS;
dsDesc.BackFace.StencilPassOp = D3D11_STENCIL_OP_KEEP;
dsDesc.BackFace.StencilFailOp = D3D11_STENCIL_OP_DECR;
dsDesc.BackFace.StencilDepthFailOp = D3D11_STENCIL_OP_KEEP;

I am drawing this as a fullscreen quad, built from components of my g-buffer, so I'm not sure I fully understand how the depth buffer / stencil thing works. I'd appreciate any help anyone can provide, this has had me pretty stumped all afternoon.

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You don't want depth writes on on when rendering your lighting. Having depth writes on with LESS as your comparison is like saying "don't render anything else to these pixels, unless it has a closer depth value than me". So if you render two fullscreen quads that have the same depth value, only the first will render since the second will fail the depth test. So you should disable depth writes (set DepthWriteMask to 0) when rendering lighting, and you should also set the comparison to LESS_EQUAL.You probably also want to disable stencil for now...it can be useful for optimizing your lighting pass but you should get your lighting working correctly first before optimizing.

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Up until now I'd gotten by with just pasting in someone else's generic initialization code, but I think now I'm going to have to spend some time and understand the whole depth aspect. Thanks for the reply.

Follow up question going forward, how expensive is OMSetDepthStencilState()? Is it ok to be called a few times per frame, or is it pretty costly?

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Like any D3D call you'll want to minimize calling it, but you don't need to go crazy. You would probably need to call it several hundreds of times a frame for it to be a problem.

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