stencil buffer通常在实现一些特殊的效果时使用的,这里我们以实现一个object的shadow为例,来看看具体怎么使用它。
首先需要创建一个和要实现shadow的object一样的object,作为shadow object。然后对其应用shadow matrix变换:
XMMATRIX world = XMLoadFloat4x4(&object->mWorldMatrix);
XMVECTOR shadowPlane = XMVectorSet(0.0f, 1.0f, 0.0f, 0.0f); // xz plane
XMVECTOR toMainLight = XMVectorSet(-0.57735f, 0.57735f, -0.57735f, 0.0f);
XMMATRIX shadow = XMMatrixShadow(shadowPlane, toMainLight);
XMMATRIX shadowOffsetY = XMMatrixTranslation(0.0f, 0.001f, 0.0f);
XMStoreFloat4x4(&object->mWorldMatrix, world * shadow * shadowOffsetY);
这里的shadowOffsetY
是用来避免阴影本身和接收阴影的平面重合导致z-fighting而设置的。接下来,讲道理我们只需要直接绘制阴影,让其与接收阴影的平面blend即可,但是这样可能会出现double blending的现象,即一个像素可能是由多个阴影像素同时blend而成,这样会导致绘制出的阴影颜色不均,有的地方深有的地方浅。
因此,为了阻止double blending,需要引入模板缓存,思路其实很简单,就是绘制过阴影像素的地方做下标记,后面如果还有阴影像素要在此处绘制,直接返回失败。启用模板缓存,模板测试相关的设置都在D3D12_GRAPHICS_PIPELINE_STATE_DESC
数据结构中,我们只需要设置好,创建相应的pipelineStateObject即可:
D3D12_DEPTH_STENCIL_DESC stencilDesc;
stencilDesc.BackFace.StencilDepthFailOp = D3D12_STENCIL_OP_KEEP;
stencilDesc.BackFace.StencilFailOp = D3D12_STENCIL_OP_KEEP;
stencilDesc.BackFace.StencilFunc = D3D12_COMPARISON_FUNC_EQUAL;
stencilDesc.BackFace.StencilPassOp = D3D12_STENCIL_OP_INCR;
stencilDesc.DepthEnable = true;
stencilDesc.DepthFunc = D3D12_COMPARISON_FUNC_LESS;
stencilDesc.DepthWriteMask = D3D12_DEPTH_WRITE_MASK_ALL;
stencilDesc.FrontFace.StencilDepthFailOp = D3D12_STENCIL_OP_KEEP;
stencilDesc.FrontFace.StencilFailOp = D3D12_STENCIL_OP_KEEP;
stencilDesc.FrontFace.StencilFunc = D3D12_COMPARISON_FUNC_EQUAL;
stencilDesc.FrontFace.StencilPassOp = D3D12_STENCIL_OP_INCR;
stencilDesc.StencilEnable = true;
stencilDesc.StencilReadMask = 0xff;
stencilDesc.StencilWriteMask = 0xff;
psoDesc.DepthStencilState = stencilDesc;
每次绘制前,我们都会清空深度缓存和模板缓存,并把模板参考值设置为0:
mCommandList->ClearDepthStencilView(dsv, D3D12_CLEAR_FLAG_DEPTH | D3D12_CLEAR_FLAG_STENCIL, 1.0f, 0, 0,
nullptr);
mCommandList->OMSetStencilRef(0);
这样就能保证阴影像素第一次写入时,模板测试是通过的;然后模板缓存里的值就会+1,这样后续再有阴影像素写入相同位置时,因为参考值(0)与此时模板缓存里的值(1)不相等,导致模板测试失败,进而无法写入像素,也就达到了我们的预期。运行效果如下:
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