http://blog.csdn.net/ykmzy/article/details/51366396
在Unity中的UGUI中实现区域裁剪可以使用Mask组件，但是当改变被裁剪对象的材质后，裁剪效果可能会失效，也就是说Mask对该UI失效，这是为什么呢？
这我们得了解UGUI中Mask的工作原理。Mask得裁剪效果是使用Stencil计算来实现裁剪，关于Stencil（模板）可以仔细了解一下渲染管线得相关知识就可以知道是怎么回事了，可参见维基百科,也可自行搜索其它资料。
通过参考Unity中UGUI中的Mask.cs源码，可以管窥一二。
public virtual Material GetModifiedMaterial(Material baseMaterial){ if (graphic == null) return baseMaterial; var rootSortCanvas = MaskUtilities.FindRootSortOverrideCanvas(transform); var stencilDepth = MaskUtilities.GetStencilDepth(transform, rootSortCanvas); if (stencilDepth >= 8) { Debug.LogError("Attempting to use a stencil mask with depth > 8", gameObject); return baseMaterial; } int desiredStencilBit = 1 << stencilDepth; if (desiredStencilBit == 1) { var maskMaterial = StencilMaterial.Add(baseMaterial, 1, StencilOp.Replace, CompareFunction.Always, m_ShowMaskGraphic ? ColorWriteMask.All : 0); StencilMaterial.Remove(m_MaskMaterial); m_MaskMaterial = maskMaterial; var unmaskMaterial = StencilMaterial.Add(baseMaterial, 1, StencilOp.Zero, CompareFunction.Always, 0); StencilMaterial.Remove(m_UnmaskMaterial); m_UnmaskMaterial = unmaskMaterial; graphic.canvasRenderer.popMaterialCount = 1; graphic.canvasRenderer.SetPopMaterial(m_UnmaskMaterial, 0); return m_MaskMaterial; } var maskMaterial2 = StencilMaterial.Add(baseMaterial, desiredStencilBit | (desiredStencilBit - 1), StencilOp.Replace, CompareFunction.Equal, m_ShowMaskGraphic ? ColorWriteMask.All : 0, desiredStencilBit - 1, desiredStencilBit | (desiredStencilBit - 1)); StencilMaterial.Remove(m_MaskMaterial); m_MaskMaterial = maskMaterial2; graphic.canvasRenderer.hasPopInstruction = true; var unmaskMaterial2 = StencilMaterial.Add(baseMaterial, desiredStencilBit - 1, StencilOp.Replace, CompareFunction.Equal, 0, desiredStencilBit - 1, desiredStencilBit | (desiredStencilBit - 1)); StencilMaterial.Remove(m_UnmaskMaterial); m_UnmaskMaterial = unmaskMaterial2; graphic.canvasRenderer.popMaterialCount = 1; graphic.canvasRenderer.SetPopMaterial(m_UnmaskMaterial, 0); return m_MaskMaterial;}
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通过上面得代码我们得知Mask会对目标UI得材质使用StencilMaterial.Add方法来传递模板相关参数
StencilMaterial.Add(baseMaterial, desiredStencilBit - 1, StencilOp.Replace, CompareFunction.Equal, 0, desiredStencilBit - 1, desiredStencilBit | (desiredStencilBit - 1));
继续查看StencilMaterial.Add方法，有如下代码
public static Material Add(Material baseMat, int stencilID, StencilOp operation, CompareFunction compareFunction, ColorWriteMask colorWriteMask, int readMask, int writeMask) { if ((stencilID <= 0 && colorWriteMask == ColorWriteMask.All) || baseMat == null) return baseMat; if (!baseMat.HasProperty("_Stencil")) { Debug.LogWarning("Material " + baseMat.name + " doesn't have _Stencil property", baseMat); return baseMat; } if (!baseMat.HasProperty("_StencilOp")) { Debug.LogWarning("Material " + baseMat.name + " doesn't have _StencilOp property", baseMat); return baseMat; } if (!baseMat.HasProperty("_StencilComp")) { Debug.LogWarning("Material " + baseMat.name + " doesn't have _StencilComp property", baseMat); return baseMat; } if (!baseMat.HasProperty("_StencilReadMask")) { Debug.LogWarning("Material " + baseMat.name + " doesn't have _StencilReadMask property", baseMat); return baseMat; } if (!baseMat.HasProperty("_StencilReadMask")) { Debug.LogWarning("Material " + baseMat.name + " doesn't have _StencilWriteMask property", baseMat); return baseMat; } if (!baseMat.HasProperty("_ColorMask")) { Debug.LogWarning("Material " + baseMat.name + " doesn't have _ColorMask property", baseMat); return baseMat; } for (int i = 0; i < m_List.Count; ++i) { MatEntry ent = m_List[i]; if (ent.baseMat == baseMat && ent.stencilId == stencilID && ent.operation == operation && ent.compareFunction == compareFunction && ent.readMask == readMask && ent.writeMask == writeMask && ent.colorMask == colorWriteMask) { ++ent.count; return ent.customMat; } } var newEnt = new MatEntry(); newEnt.count = 1; newEnt.baseMat = baseMat; newEnt.customMat = new Material(baseMat); newEnt.customMat.hideFlags = HideFlags.HideAndDontSave; newEnt.stencilId = stencilID; newEnt.operation = operation; newEnt.compareFunction = compareFunction; newEnt.readMask = readMask; newEnt.writeMask = writeMask; newEnt.colorMask = colorWriteMask; newEnt.useAlphaClip = operation != StencilOp.Keep && writeMask > 0; newEnt.customMat.name = string.Format("Stencil Id:{0}, Op:{1}, Comp:{2}, WriteMask:{3}, ReadMask:{4}, ColorMask:{5} AlphaClip:{6} ({7})", stencilID, operation, compareFunction, writeMask, readMask, colorWriteMask, newEnt.useAlphaClip, baseMat.name); newEnt.customMat.SetInt("_Stencil", stencilID); newEnt.customMat.SetInt("_StencilOp", (int)operation); newEnt.customMat.SetInt("_StencilComp", (int)compareFunction); newEnt.customMat.SetInt("_StencilReadMask", readMask); newEnt.customMat.SetInt("_StencilWriteMask", writeMask); newEnt.customMat.SetInt("_ColorMask", (int)colorWriteMask); newEnt.customMat.SetInt("_UseAlphaClip", newEnt.useAlphaClip ? 1 : 0); m_List.Add(newEnt); return newEnt.customMat; }
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通过对StencilMaterial.Add方法得查看我们知道该方法在对目标材质得_StencilOp，_StencilComp等等属性进行操作，这样操作后才可以实现它得模板裁剪。我们自定义的Shader一般没有这几个属性，所以我们在自己的shader中添加如下代码便可以实现自定义材质得正确Mask了
_StencilComp ("Stencil Comparison", Float) = 8_Stencil ("Stencil ID", Float) = 0_StencilOp ("Stencil Operation", Float) = 0_StencilWriteMask ("Stencil Write Mask", Float) = 255_StencilReadMask ("Stencil Read Mask", Float) = 255............................................................................Stencil{ Ref [_Stencil] Comp [_StencilComp] Pass [_StencilOp] ReadMask [_StencilReadMask] WriteMask [_StencilWriteMask]}
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