將3D點投影到2D屏幕座標OpenTK

Question

使用Monotouch和OpenTK我試圖獲得一個3D點的屏幕座標。我建立了我的世界視圖投影矩陣，而OpenGL能夠理解它並完美地投影我的3D模型，但是如何使用相同的矩陣來投影從2D到3D的一個點？

我以爲我可以簡單地使用：

Vector3.Transform(ref input3Dpos, ref matWorldViewProjection, out projected2Dpos); 

然後在projected2DPos投影屏幕座標。但由此產生的Vector4似乎並不能代表正確的投影屏幕座標。我不知道如何從那裏計算。

---

我發現我需要除以Vector4.w，但我仍然得到錯誤的值。現在使用這種方法：

private static bool GluProject(OpenTK.Vector3 objPos, OpenTK.Matrix4 matWorldViewProjection, int[] viewport, out OpenTK.Vector3 screenPos) 
{ 
    OpenTK.Vector4 _in; 

    _in.X = objPos.X; 
    _in.Y = objPos.Y; 
    _in.Z = objPos.Z; 
    _in.W = 1f; 

    Vector4 _out = OpenTK.Vector4.Transform(_in, matWorldViewProjection); 

    if (_out.W <= 0.0) 
    { 
     screenPos = OpenTK.Vector3.Zero; 
     return false; 
    } 

    _out.X /= _out.W; 
    _out.Y /= _out.W; 
    _out.Z /= _out.W; 
    /* Map x, y and z to range 0-1 */ 
    _out.X = _out.X * 0.5f + 0.5f; 
    _out.Y = -_out.Y * 0.5f + 0.5f; 
    _out.Z = _out.Z * 0.5f + 0.5f; 

    /* Map x,y to viewport */ 
    _out.X = _out.X * viewport[2] + viewport[0]; 
    _out.Y = _out.Y * viewport[3] + viewport[1]; 

    screenPos.X = _out.X; 
    screenPos.Y = _out.Y; 
    screenPos.Z = _out.Z; 

    return true; 
} 

我看不到任何雖然錯誤...：S

Answer 1

在第一個問題，你錯過了最後一步：從NDC映射（規格化設備座標）視口座標。這就是你GluProject線

/* Map x,y to viewport */ 
_out.X = _out.X * viewport[2] + viewport[0]; 
_out.Y = _out.Y * viewport[3] + viewport[1]; 

做，

Answer 2

你有兩個選擇。你可以自己計算一下，或者使用glProject函數。我更喜歡第一個。

數1：

private Vector2 Convert(
    Vector3 pos, 
    Matrix4 viewMatrix, 
    Matrix4 projectionMatrix, 
    int screenWidth, 
    int screenHeight) 
{ 
    pos = Vector3.Transform(pos, viewMatrix); 
    pos = Vector3.Transform(pos, projectionMatrix); 
    pos.X /= pos.Z; 
    pos.Y /= pos.Z; 
    pos.X = (pos.X + 1) * screenWidth/2; 
    pos.Y = (pos.Y + 1) * screenHeight/2; 

    return new Vector2(pos.X, pos.Y); 
} 

數2：

public Vector2 form3Dto2D(Vector3 our3DPoint) 
{ 
    Vector3 our2DPoint; 

    float[] modelviewMatrix = new float[16]; 
    float[] projectionMatrix = new float[16]; 
    int[] viewport = new int[4]; 

    GL.GetFloat(GetPName.ModelviewMatrix, modelviewMatrix); 
    GL.GetFloat(GetPName.ProjectionMatrix, projectionMatrix); 
    GL.GetInteger(GetPName.Viewport, viewport); 

    OpenTK.Graphics.Glu.Project(our3DPoint, convertFloatsToDoubles(modelviewMatrix), 
     convertFloatsToDoubles(projectionMatrix), viewport, out our2DPoint); 

    return new Vector2(our2DPoint.X, our2DPoint.Y) 
} 

public static double[] convertFloatsToDoubles(float[] input) 
{ 
    if (input == null) 
    { 
     return null; // Or throw an exception - your choice 
    } 

    double[] output = new double[input.Length]; 
    for (int i = 0; i < input.Length; i++) 
    { 
     output[i] = input[i]; 
    } 
    return output; 
}