CSharpGL(21)用鼠标拾取、拖拽VBO图元内的点、线或本身
效果图
以最常见的三角形网格（用GL_TRIANGLES方式进行渲染）为例。
在拾取模式为GeometryType.Point时，你可以拾取单个的顶点。
在拾取模式为GeometryType.Line时，你可以拾取任意一个三角形里的任意一条线。即同时拾取此线段的两个顶点。
在拾取模式为GeometryType.Triangle时，你可以拾取任意一个三角形。即同时拾取此三角形的三个顶点。
实际上，CSharpGL实现了在所有渲染模式下拾取Point、Line、Triangle、Quad和Polygon的功能。（当然，你可以想象，如果想在一个GL_TRIANGLES渲染方式下拾取一个Quad，那是什么都拾取不到的）下面是描述这一功能的图示。由于我的白板小，就没有列出GL_TRIANGLES_ADJACENCY、GL_TRIANGLE_STRIP_ADJACENCY、GL_LINES_ADJACENCY、GL_LINE_STRIP_ADJCANCEY这几个情况。
下载
CSharpGL已在GitHub开源，欢迎对OpenGL有兴趣的同学加入（https://github.com/bitzhuwei/CSharpGL）
规定
为了简便描述，我用GL_LINE*代表GL_LINES、GL_LINE_STRIP、GL_LINES_ADJACENCY、GL_LINE_STRIP_ADJACENCY，用GL_TRIANGLE*代表GL_TRIANGLES、GL_TRIANGLE_STRIP、GL_TRIANGLE_FAN、GL_TRIANGLES_ADJACENCY、GL_TRIANGLE_STRIP_ADJACENCY，用GL_QUAD*代表GL_QUADS、GL_QUAD_STRIP。
如何使用
使用方式十分简单，只需给RenderEventArgs传入如下的参数：
GeometryType PickingGeometryType = Geometry.Point;
var arg = new RenderEventArgs(
// 为了拾取而进行的渲染
RenderModes.ColorCodedPicking,
this.glCanvas1.ClientRectangle,
this.camera,
// 我想拾取的类型(Geometry)
this.PickingGeometryType);
// 要拾取的位置（鼠标位置）
Point mousePostion = GetMousePosition();
// 支持Picking的Renderer列表
PickableRenderer[] pickableElements = GetRenderersInScene();
// 执行拾取操作
PickedGeometry pickedGeometry = ColorCodedPicking.Pick(arg, mousePostion, pickableElements);
具体用法详见（CSharpGL(20)用unProject和Project实现鼠标拖拽图元）
如何实现
在GL_POINTS时拾取Point，在GL_LINE*时拾取Line，在GL_TRIANGL*时拾取Triangle，在GL_QUAD*时拾取Quad，在GL_POLYGON时拾取Polygon，这都是已经实现了的（CSharpGL(18)分别处理glDrawArrays()和glDrawElements()两种方式下的拾取(ColorCodedPicking)）。这些不再详述。
拾取Point
ZeroIndexRenderer
在除了GL_POINTS时，想拾取一个Point，只能用 glDrawArrays(GL_POINTS, ..); 来代替原有的 glDrawArrays(OriginalMode, ..); 。但这会渲染所有的顶点。而在OriginalMode下，未必渲染所有的顶点。所以在拾取到一个Point后要判断一下是否真的应该拾取到它。
/// <summary>
/// 现在，已经判定了鼠标在某个点上。
/// 我需要判定此点是否出现在图元上。
/// now that I know the mouse is picking on some point,
/// I need to make sure that point should appear.
/// </summary>
/// <param name="lastVertexId"></param>
/// <param name="mode"></param>
/// <returns></returns>
private bool OnPrimitiveTest(uint lastVertexId, DrawMode mode)
{
bool result = false;
int first = this.zeroIndexBufferPtr.FirstVertex;
if (first < ) { return false; }
int vertexCount = this.zeroIndexBufferPtr.VertexCount;
if (vertexCount <= ) { return false; }
int last = first + vertexCount - ;
switch (mode)
{
case DrawMode.Points:
result = true;
break;
case DrawMode.LineStrip:
result = vertexCount > ;
break;
case DrawMode.LineLoop:
result = vertexCount > ;
break;
case DrawMode.Lines:
if (vertexCount > )
{
if (vertexCount % == )
{
result = (first <= lastVertexId && lastVertexId <= last);
}
else
{
result = (first <= lastVertexId && lastVertexId <= last - );
}
}
break;
case DrawMode.LineStripAdjacency:
if (vertexCount > )
{
result = (first < lastVertexId && lastVertexId < last);
}
break;
case DrawMode.LinesAdjacency:
if (vertexCount > )
{
var lastPart = last - (last + - first) % ;
if (first <= lastVertexId && lastVertexId <= lastPart)
{
var m = (lastVertexId - first) % ;
result = (m == || m == );
}
}
break;
case DrawMode.TriangleStrip:
if (vertexCount > )
{
result = vertexCount > ;
}
break;
case DrawMode.TriangleFan:
if (vertexCount > )
{
result = vertexCount > ;
}
break;
case DrawMode.Triangles:
if (vertexCount > )
{
if (first <= lastVertexId)
{
result = ((vertexCount % == ) && (lastVertexId <= last))
|| ((vertexCount % == ) && (lastVertexId < last))
|| ((vertexCount % == ) && (lastVertexId + < last));
}
}
break;
case DrawMode.TriangleStripAdjacency:
if (vertexCount > )
{
var lastPart = last - (last + - first) % ;
if (first <= lastVertexId && lastVertexId <= lastPart)
{
result = (lastVertexId - first) % == ;
}
}
break;
case DrawMode.TrianglesAdjacency:
if (vertexCount > )
{
var lastPart = last - (last + - first) % ;
if (first <= lastVertexId && lastVertexId <= lastPart)
{
result = (lastVertexId - first) % == ;
}
}
break;
case DrawMode.Patches:
// not know what to do for now
break;
case DrawMode.QuadStrip:
if (vertexCount > )
{
if (first <= lastVertexId && lastVertexId <= last)
{
result = (vertexCount % == )
|| (lastVertexId < last);
}
}
break;
case DrawMode.Quads:
if (vertexCount > )
{
if (first <= lastVertexId && lastVertexId <= last)
{
var m = vertexCount % ;
if (m == ) { result = true; }
else if (m == ) { result = lastVertexId + < last; }
else if (m == ) { result = lastVertexId + < last; }
else if (m == ) { result = lastVertexId + < last; }
else { throw new Exception("This should never happen!"); }
}
}
break;
case DrawMode.Polygon:
if (vertexCount > )
{
result = (first <= lastVertexId && lastVertexId <= last);
}
break;
default:
throw new NotImplementedException();
} return result;
}
bool OnPrimitiveTest(uint lastVertexId, DrawMode mode)
OneIndexBuffer
如果是用glDrawElements(OriginalMode, ..);渲染，此时想拾取一个Point，那么我就不做类似的OnPrimitiveTest了。因为情况太复杂，且必须用MapBufferRange来检测大量的顶点情况。而这仅仅是因为导入的IBufferable模型本身没有使用某些顶点。没用你就删了它啊！这我就不管了。
/// <summary>
/// I don't know how to implement this method in a high effitiency way.
/// So keep it like this.
/// Also, why would someone use glDrawElements() when rendering GL_POINTS?
/// </summary>
/// <param name="lastVertexId"></param>
/// <param name="mode"></param>
/// <returns></returns>
private bool OnPrimitiveTest(uint lastVertexId, DrawMode mode)
{
return true;
}
拾取Line
ZeroIndexRenderer
如果是在GL_LINE*下拾取线，那么这是上一篇文章已经实现了的情况。如果是想在GL_TRIANGLE*、GL_QUAD*、GL_POLYGON模式下拾取其某个图元的某条Line，那么就分两部走：第一，像上一篇一样拾取图元；第二，设计一个新的小小的索引，即用GL_LINES模式渲染此图元（三角形、四边形、多边形）的所有边的索引。用此索引重新执行渲染、拾取，那么就可以找到鼠标所在位置的Line了。
例如，下面是在一个三角形图元中找到那个你想要的Line的过程。
class ZeroIndexLineInTriangleSearcher : ZeroIndexLineSearcher
{
/// <summary>
/// 在三角形图元中拾取指定位置的Line
/// </summary>
/// <param name="arg">渲染参数</param>
/// <param name="x">指定位置</param>
/// <param name="y">指定位置</param>
/// <param name="lastVertexId">三角形图元的最后一个顶点</param>
/// <param name="modernRenderer">目标Renderer</param>
/// <returns></returns>
internal override uint[] Search(RenderEventArgs arg,
int x, int y,
uint lastVertexId, ZeroIndexRenderer modernRenderer)
{
// 创建临时索引
OneIndexBufferPtr indexBufferPtr = null;
using (var buffer = new OneIndexBuffer<uint>(DrawMode.Lines, BufferUsage.StaticDraw))
{
buffer.Alloc();
unsafe
{
var array = (uint*)buffer.FirstElement();
array[] = lastVertexId - ; array[] = lastVertexId - ;
array[] = lastVertexId - ; array[] = lastVertexId - ;
array[] = lastVertexId - ; array[] = lastVertexId - ;
} indexBufferPtr = buffer.GetBufferPtr() as OneIndexBufferPtr;
} // 用临时索引渲染此三角形图元（仅渲染此三角形图元）
modernRenderer.Render4InnerPicking(arg, indexBufferPtr);
// id是拾取到的Line的Last Vertex Id
uint id = ColorCodedPicking.ReadPixel(x, y, arg.CanvasRect.Height); indexBufferPtr.Dispose(); // 对比临时索引，找到那个Line
if (id + == lastVertexId)
{ return new uint[] { id + , id, }; }
else
{ return new uint[] { id - , id, }; }
}
}
OneIndexBuffer
用glDrawElements()时，实现思路与上面一样，只不过Index参数变化一下而已。
在（CSharpGL(18)分别处理glDrawArrays()和glDrawElements()两种方式下的拾取(ColorCodedPicking)），已经能够找到目标图元的所有顶点，所以就简单了。
继续用"在一个三角形图元中找到那个你想要的Line的过程"来举例。
class OneIndexLineInTrianglesSearcher : OneIndexLineSearcher
{
internal override uint[] Search(RenderEventArgs arg,
int x, int y,
RecognizedPrimitiveIndex lastIndexId,
OneIndexRenderer modernRenderer)
{
if (lastIndexId.IndexIdList.Count != ) { throw new ArgumentException(); }
List<uint> indexList = lastIndexId.IndexIdList;
if (indexList[] == indexList[]) { return new uint[] { indexList[], indexList[], }; }
else if (indexList[] == indexList[]) { return new uint[] { indexList[], indexList[], }; }
else if (indexList[] == indexList[]) { return new uint[] { indexList[], indexList[], }; } OneIndexBufferPtr indexBufferPtr = null;
using (var buffer = new OneIndexBuffer<uint>(DrawMode.Lines, BufferUsage.StaticDraw))
{
buffer.Alloc();
unsafe
{
var array = (uint*)buffer.FirstElement();
array[] = indexList[]; array[] = indexList[];
array[] = indexList[]; array[] = indexList[];
array[] = indexList[]; array[] = indexList[];
} indexBufferPtr = buffer.GetBufferPtr() as OneIndexBufferPtr;
} modernRenderer.Render4InnerPicking(arg, indexBufferPtr);
uint id = ColorCodedPicking.ReadPixel(x, y, arg.CanvasRect.Height); indexBufferPtr.Dispose(); if (id == indexList[])
{ return new uint[] { indexList[], indexList[], }; }
else if (id == indexList[])
{ return new uint[] { indexList[], indexList[], }; }
else if (id == indexList[])
{ return new uint[] { indexList[], indexList[], }; }
else
{ throw new Exception("This should not happen!"); }
}
}
Polygon
这里顺便提一下GL_POLYGON，这是个特别的图元，因为它的顶点数是不确定的。它产生的临时小索引就可能不再小。但神奇的是，它不再需要OneIndexBufferPtr类型的临时索引，而只需一个几乎不占空间的ZeroIndexBufferPtr。
class ZeroIndexLineInPolygonSearcher : ZeroIndexLineSearcher
{
internal override uint[] Search(RenderEventArgs arg,
int x, int y,
uint lastVertexId, ZeroIndexRenderer modernRenderer)
{
var zeroIndexBufferPtr = modernRenderer.GetIndexBufferPtr() as ZeroIndexBufferPtr;
ZeroIndexBufferPtr indexBufferPtr = null;
// when the temp index buffer could be long, it's no longer needed.
// what a great OpenGL API design!
using (var buffer = new ZeroIndexBuffer(DrawMode.LineLoop,
zeroIndexBufferPtr.FirstVertex, zeroIndexBufferPtr.VertexCount))
{
indexBufferPtr = buffer.GetBufferPtr() as ZeroIndexBufferPtr;
}
modernRenderer.Render4InnerPicking(arg, indexBufferPtr);
uint id = ColorCodedPicking.ReadPixel(x, y, arg.CanvasRect.Height); indexBufferPtr.Dispose(); if (id == zeroIndexBufferPtr.FirstVertex)
{ return new uint[] { (uint)(zeroIndexBufferPtr.FirstVertex + zeroIndexBufferPtr.VertexCount - ), id, }; }
else
{ return new uint[] { id - , id, }; }
}
}
拾取本身
所谓拾取本身，就是：如果用GL_TRIANGLE*进行渲染，就拾取一个Triangle；如果用GL_QUAD*进行渲染，就拾取一个Quad；如果用GL_POLYGON进行渲染，就拾取一个Polygon。这都是在（CSharpGL(18)分别处理glDrawArrays()和glDrawElements()两种方式下的拾取(ColorCodedPicking)）中已经实现了的功能。
整合
三种情况都解决了，下面整合进来就行了。
ZeroIndexRenderer
这是对ZeroIndexRenderer的Pick。
public override PickedGeometry Pick(RenderEventArgs arg, uint stageVertexId,
int x, int y)
{
uint lastVertexId;
if (!this.GetLastVertexIdOfPickedGeometry(stageVertexId, out lastVertexId))
{ return null; } GeometryType geometryType = arg.PickingGeometryType; if (geometryType == GeometryType.Point)
{
DrawMode mode = this.GetIndexBufferPtr().Mode;
if (this.OnPrimitiveTest(lastVertexId, mode))
{ return PickPoint(stageVertexId, lastVertexId); }
else
{ return null; }
}
else if (geometryType == GeometryType.Line)
{
DrawMode mode = this.GetIndexBufferPtr().Mode;
GeometryType typeOfMode = mode.ToGeometryType();
if (geometryType == typeOfMode)
{ return PickWhateverItIs(stageVertexId, lastVertexId, mode, typeOfMode); }
else
{
ZeroIndexLineSearcher searcher = GetLineSearcher(mode);
if (searcher != null)// line is from triangle, quad or polygon
{ return SearchLine(arg, stageVertexId, x, y, lastVertexId, searcher); }
else if (mode == DrawMode.Points)// want a line when rendering GL_POINTS
{ return null; }
else
{ throw new Exception(string.Format("Lack of searcher for [{0}]", mode)); }
}
}
else
{
DrawMode mode = this.GetIndexBufferPtr().Mode;
GeometryType typeOfMode = mode.ToGeometryType();
if (typeOfMode == geometryType)// I want what it is
{ return PickWhateverItIs(stageVertexId, lastVertexId, mode, typeOfMode); }
else
{ return null; }
//{ throw new Exception(string.Format("Lack of searcher for [{0}]", mode)); }
}
}
public override PickedGeometry Pick(RenderEventArgs arg, uint stageVertexId, int x, int y)
OneIndexRenderer
这是对OneIndexRenderer的Pick。
public override PickedGeometry Pick(RenderEventArgs arg, uint stageVertexId,
int x, int y)
{
uint lastVertexId;
if (!this.GetLastVertexIdOfPickedGeometry(stageVertexId, out lastVertexId))
{ return null; } GeometryType geometryType = arg.PickingGeometryType; if (geometryType == GeometryType.Point)
{
DrawMode mode = this.GetIndexBufferPtr().Mode;
if (this.OnPrimitiveTest(lastVertexId, mode))
{ return PickPoint(stageVertexId, lastVertexId); }
else
{ return null; }
}
else if (geometryType == GeometryType.Line)
{
// 找到 lastIndexId
RecognizedPrimitiveIndex lastIndexId = this.GetLastIndexIdOfPickedGeometry(
arg, lastVertexId, x, y);
if (lastIndexId == null)
{
Debug.WriteLine(
"Got lastVertexId[{0}] but no lastIndexId! Params are [{1}] [{2}] [{3}] [{4}]",
lastVertexId, arg, stageVertexId, x, y);
{ return null; }
}
else
{
// 获取pickedGeometry
DrawMode mode = this.GetIndexBufferPtr().Mode;
GeometryType typeOfMode = mode.ToGeometryType();
if (geometryType == typeOfMode)
{ return PickWhateverItIs(stageVertexId, lastIndexId, typeOfMode); }
else
{
OneIndexLineSearcher searcher = GetLineSearcher(mode);
if (searcher != null)// line is from triangle, quad or polygon
{ return SearchLine(arg, stageVertexId, x, y, lastVertexId, lastIndexId, searcher); }
else if (mode == DrawMode.Points)// want a line when rendering GL_POINTS
{ return null; }
else
{ throw new Exception(string.Format("Lack of searcher for [{0}]", mode)); }
}
}
}
else
{
// 找到 lastIndexId
RecognizedPrimitiveIndex lastIndexId = this.GetLastIndexIdOfPickedGeometry(
arg, lastVertexId, x, y);
if (lastIndexId == null)
{
Debug.WriteLine(
"Got lastVertexId[{0}] but no lastIndexId! Params are [{1}] [{2}] [{3}] [{4}]",
lastVertexId, arg, stageVertexId, x, y);
{ return null; }
}
else
{
DrawMode mode = this.GetIndexBufferPtr().Mode;
GeometryType typeOfMode = mode.ToGeometryType();
if (typeOfMode == geometryType)// I want what it is
{ return PickWhateverItIs(stageVertexId, lastIndexId, typeOfMode); }
else
{ return null; }
//{ throw new Exception(string.Format("Lack of searcher for [{0}]", mode)); }
}
}
}
public override PickedGeometry Pick(RenderEventArgs arg, uint stageVertexId, int x, int y)
总结
在完成后，我以为彻底解决了拾取问题。等完成本文后，我不再这么想了。还是谦虚点好。
原CSharpGL的其他功能（3ds解析器、TTF2Bmp、CSSL等），我将逐步加入新CSharpGL。
欢迎对OpenGL有兴趣的同学关注（https://github.com/bitzhuwei/CSharpGL）