0. 版本
源码版本：2017.3.0
着色器版本：2017.3.0
1. PerformClipping调用顺序
CanvasUpdateRegistry.PerformUpdate()
public class CanvasUpdateRegistry
protected CanvasUpdateRegistry()
// 每一帧都会调用
Canvas.willRenderCanvases += PerformUpdate;
private void PerformUpdate()
// 更新Layout
......
// now layout is complete do culling...
ClipperRegistry.instance.Cull();
// 更新Graphic
......
ClipperRegistry.Cull()
public class ClipperRegistry
// RectMask2D实现了IClipper接口
readonly IndexedSet<IClipper> m_Clippers = new IndexedSet<IClipper>();
public void Cull()
for (var i = 0; i < m_Clippers.Count; ++i)
m_Clippers[i].PerformClipping();
RectMask2D.PerformClipping()
public class RectMask2D : UIBehaviour, IClipper, ICanvasRaycastFilter
private List<RectMask2D> m_Clippers = new List<RectMask2D>();
// MaskableGraphic实现了IClippable接口
private List<IClippable> m_ClipTargets = new List<IClippable>();
public virtual void PerformClipping()
//TODO See if an IsActive() test would work well here or whether it might cause unexpected side effects (re case 776771)
// if the parents are changed
// or something similar we
// do a recalculate here
if (m_ShouldRecalculateClipRects)
// m_Clippers = this的Parent路径上的，所有的RectMask2D组件。
MaskUtilities.GetRectMasksForClip(this, m_Clippers);
m_ShouldRecalculateClipRects = false;
// get the compound rects from
// the clippers that are valid
bool validRect = true;
// clipRect = m_Clippers中所有的RectMask2D的区域的交集。
// vaildRect = 区域是否有交集。
Rect clipRect = Clipping.FindCullAndClipWorldRect(m_Clippers, out validRect);
bool clipRectChanged = clipRect != m_LastClipRectCanvasSpace;
if (clipRectChanged || m_ForceClip)
// 重点1：向MaskGraphic设置剪裁区域。
// m_ClipTargets有哪些后面会分析。
foreach (IClippable clipTarget in m_ClipTargets)
clipTarget.SetClipRect(clipRect, validRect);
m_LastClipRectCanvasSpace = clipRect;
m_LastValidClipRect = validRect;
foreach (IClippable clipTarget in m_ClipTargets)
// hasMoved : True if any change has occured that would invalidate the positions of generated geometry.
var maskable = clipTarget as MaskableGraphic;
if (maskable != null && !maskable.canvasRenderer.hasMoved && !clipRectChanged)
continue;
// 重点2：MaskGraphic进行剪裁。
clipTarget.Cull(m_LastClipRectCanvasSpace, m_LastValidClipRect);
MaskGraphic.SetClipRect
MaskGraphic.Cull
public abstract class MaskableGraphic : Graphic, IClippable, IMaskable, IMaterialModifier
public virtual void SetClipRect(Rect clipRect, bool validRect)
if (validRect) // Mask有交接，设置区域。修改材质的事被CanvasRenderer做了，黑盒看不到。
canvasRenderer.EnableRectClipping(clipRect);
canvasRenderer.DisableRectClipping(); // Mask没有交集。
public virtual void Cull(Rect clipRect, bool validRect)
var cull = !validRect || !clipRect.Overlaps(rootCanvasRect, true);
UpdateCull(cull);
private void UpdateCull(bool cull)
var cullingChanged = canvasRenderer.cull != cull;
canvasRenderer.cull = cull;
if (cullingChanged)
UISystemProfilerApi.AddMarker("MaskableGraphic.cullingChanged", this);
m_OnCullStateChanged.Invoke(cull);
SetVerticesDirty(); // 疑问：为什么cull变了，要重建网格？猜测是CanvasRender根据cull的情况，做了网格的优化，减少了一些Overdraw。可以实验看看。
2. 剪裁区域
GetCanvasRect
internal class RectangularVertexClipper
readonly Vector3[] m_WorldCorners = new Vector3[4];
readonly Vector3[] m_CanvasCorners = new Vector3[4];
public Rect GetCanvasRect(RectTransform t, Canvas c)
if (c == null)
return new Rect();
t.GetWorldCorners(m_WorldCorners);
var canvasTransform = c.GetComponent<Transform>();
for (int i = 0; i < 4; ++i)
m_CanvasCorners[i] = canvasTransform.InverseTransformPoint(m_WorldCorners[i]);
return new Rect(m_CanvasCorners[0].x, m_CanvasCorners[0].y, m_CanvasCorners[2].x - m_CanvasCorners[0].x, m_CanvasCorners[2].y - m_CanvasCorners[0].y);
UI/Default
疑问：OUT.worldPosition = v.vertex; 读取的是Local坐标，怎么是World坐标呢？
猜测：CanvasBuildBatch生成的Mesh，这个Mesh直接放到世界坐标系下了，Local坐标是World坐标是一样的。
Shader "UI/Default"
Properties
[PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
_Color ("Tint", Color) = (1,1,1,1)
_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
_ColorMask ("Color Mask", Float) = 15
[Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
SubShader
"Queue"="Transparent"
"IgnoreProjector"="True"
"RenderType"="Transparent"
"PreviewType"="Plane"
"CanUseSpriteAtlas"="True"
Stencil
Ref [_Stencil]
Comp [_StencilComp]
Pass [_StencilOp]
ReadMask [_StencilReadMask]
WriteMask [_StencilWriteMask]
Cull Off
Lighting Off
ZWrite Off
ZTest [unity_GUIZTestMode]
Blend SrcAlpha OneMinusSrcAlpha
ColorMask [_ColorMask]
Name "Default"
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma target 2.0
#include "UnityCG.cginc"
#include "UnityUI.cginc" // 2D Mask 剪裁。
#pragma multi_compile __ UNITY_UI_CLIP_RECT
#pragma multi_compile __ UNITY_UI_ALPHACLIP
struct appdata_t
float4 vertex : POSITION;
float4 color : COLOR;
float2 texcoord : TEXCOORD0;
UNITY_VERTEX_INPUT_INSTANCE_ID
struct v2f
float4 vertex : SV_POSITION;
fixed4 color : COLOR;
float2 texcoord : TEXCOORD0;
float4 worldPosition : TEXCOORD1; // 2D Mask 剪裁。
UNITY_VERTEX_OUTPUT_STEREO
fixed4 _Color;
fixed4 _TextureSampleAdd;
float4 _ClipRect; // 2D Mask 剪裁。
v2f vert(appdata_t v)
v2f OUT;
UNITY_SETUP_INSTANCE_ID(v);
UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
OUT.worldPosition = v.vertex; // 2D Mask 剪裁。
OUT.vertex = UnityObjectToClipPos(OUT.worldPosition);
OUT.texcoord = v.texcoord;
OUT.color = v.color * _Color;
return OUT;
sampler2D _MainTex;
fixed4 frag(v2f IN) : SV_Target
half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;
#ifdef UNITY_UI_CLIP_RECT
color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect); // 2D Mask 剪裁。
#endif
#ifdef UNITY_UI_ALPHACLIP
clip (color.a - 0.001);
#endif
return color;
ENDCG
3. m_ClipTargets
// MaskGraphic.cs
protected override void OnEnable()
......
UpdateClipParent();
......
protected override void OnDisable()
......
UpdateClipParent();
......
protected override void OnTransformParentChanged()
......
UpdateClipParent();
......
protected override void OnCanvasHierarchyChanged()
......
UpdateClipParent();
......
public virtual void RecalculateClipping()
UpdateClipParent();
// MaskGraphic.cs
private void UpdateClipParent()
// 返回最近的ParentMask
var newParent = (maskable && IsActive()) ? MaskUtilities.GetRectMaskForClippable(this) : null;
// if the new parent is different OR is now inactive
if (m_ParentMask != null && (newParent != m_ParentMask || !newParent.IsActive()))
m_ParentMask.RemoveClippable(this);
UpdateCull(false);
// don't re-add it if the newparent is inactive
if (newParent != null && newParent.IsActive())
newParent.AddClippable(this);
m_ParentMask = newParent;
// RectMask2D.cs
public void AddClippable(IClippable clippable)
if (clippable == null)
return;
m_ShouldRecalculateClipRects = true;
if (!m_ClipTargets.Contains(clippable))
m_ClipTargets.Add(clippable);
m_ForceClip = true;
4. 总结
RectMask2D在每帧都会检查剪裁区域是否变化了。PerformUpdate中执行。
剪裁区域 = 当前RectMask2D到Parent路径上的所有有效的RectMask2D的剪裁区域的交集。
如果剪裁区域变化了，会通过CanvasRenderer来间接修改MaskGraphic的材质参数，把剪裁区域传进去。
