<Unity-UGUI>使用Mask, 正确的裁减非默认材质UI对象

使用UGUI中的Mask的时候，如何正确裁减非默认材质的UI对象

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。

/// Stencil calculation time!
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 we are at the first level...
// we want to destroy what is there
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;
}

//otherwise we need to be a bit smarter and set some read / write masks
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;
}

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

/// <summary>
/// Add a new material using the specified base and stencil ID.
/// </summary>
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;
}

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]
}