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POCamera extrinsics calculation wrong
 

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  1. POCamera extrinsics calculation wrong
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    <p>I am trying to find camera extrinsics from 2 images. I have the intrinsics from CameraCalibration and the scene has known dimensions (created with 3DSMAX).</p> <p>The chessboard is 1000*1000, each square thus 125*125. The camera is at (0,0,3000) looking straight down at the chessboard which is centred at the origin. In the second image, the camera is translated (-1500, 0, -402) and rotated 30° on the Y axis to point again to the centre of the chessboard: <img src="https://i.stack.imgur.com/bRBBI.jpg" alt="camera setup"></p> <p>GoodFeaturesToTrack correctly identifies the 81 corners: <img src="https://i.stack.imgur.com/kLarg.jpg" alt="chessboards"></p> <p>I create the 3d points of the chessboard corners, cvFindExtrinsicCameraParams2 to compute the intrinsics and cvRodrigues2 to get the rotation matrix. Here's the code</p> <pre><code>Imports Emgu.CV Imports Emgu.CV.Structure Imports Emgu.CV.CvInvoke Imports Emgu.CV.CvEnum Imports Emgu.CV.UI Imports System.Drawing Imports System.IO Imports System.Diagnostics Imports System.Math Module main_ Sub Main() Const MAXFEATURES As Integer = 100 Dim featuresA(0)() As PointF Dim featuresB(0)() As PointF Dim features As Integer = 0 Dim imgA As Emgu.CV.Image(Of Emgu.CV.Structure.Bgr, Byte) Dim imgB As Emgu.CV.Image(Of Emgu.CV.Structure.Bgr, Byte) Dim grayA As Emgu.CV.Image(Of Emgu.CV.Structure.Gray, Byte) Dim grayB As Emgu.CV.Image(Of Emgu.CV.Structure.Gray, Byte) Dim pyrBufferA As Emgu.CV.Image(Of Emgu.CV.Structure.Gray, Byte) Dim pyrBufferB As Emgu.CV.Image(Of Emgu.CV.Structure.Gray, Byte) Dim pointsA As Matrix(Of Single) Dim pointsB As Matrix(Of Single) Dim flags As Emgu.CV.CvEnum.LKFLOW_TYPE = Emgu.CV.CvEnum.LKFLOW_TYPE.DEFAULT Dim imagesize As Size Dim termcrit As New Emgu.CV.Structure.MCvTermCriteria(20, 0.03D) Dim status As Byte() = Nothing Dim errors As Single() = Nothing Dim red As Bgr = New Bgr(Color.Red) ' Load chessboards imgA = New Image(Of [Structure].Bgr, Byte)("chessboard centre.jpg") imgB = New Image(Of [Structure].Bgr, Byte)("chessboard left.jpg") grayA = imgA.Convert(Of Gray, Byte)() grayB = imgB.Convert(Of Gray, Byte)() ' setup for feature detection imagesize = cvGetSize(grayA) pyrBufferA = New Emgu.CV.Image(Of Emgu.CV.Structure.Gray, Byte)(imagesize.Width + 8, imagesize.Height / 3) pyrBufferB = New Emgu.CV.Image(Of Emgu.CV.Structure.Gray, Byte)(imagesize.Width + 8, imagesize.Height / 3) features = MAXFEATURES ' Find features featuresA = grayA.GoodFeaturesToTrack(features, 0.01, 25, 3) grayA.FindCornerSubPix(featuresA, New System.Drawing.Size(10, 10), New System.Drawing.Size(-1, -1), termcrit) features = featuresA(0).Length ' Compute optical flow. Not necessary here but good to remember Emgu.CV.OpticalFlow.PyrLK(grayA, grayB, pyrBufferA, pyrBufferB, featuresA(0), New Size(25, 25), 3, termcrit, flags, featuresB(0), status, errors) Debug.Assert(featuresA(0).GetUpperBound(0) = featuresB(0).GetUpperBound(0)) ' Copy features to an easier-to-use matrix and get min/max to create 3d points Dim minx As Double = Double.MaxValue Dim miny As Double = Double.MaxValue Dim maxx As Double = Double.MinValue Dim maxy As Double = Double.MinValue pointsA = New Matrix(Of Single)(features, 2) pointsB = New Matrix(Of Single)(features, 2) For i As Integer = 0 To features - 1 pointsA(i, 0) = featuresA(0)(i).X pointsA(i, 1) = featuresA(0)(i).Y pointsB(i, 0) = featuresB(0)(i).X pointsB(i, 1) = featuresB(0)(i).Y If pointsA(i, 0) &lt; minx Then minx = pointsA(i, 0) End If If pointsA(i, 1) &lt; miny Then miny = pointsA(i, 1) End If If pointsA(i, 0) &gt; maxx Then maxx = pointsA(i, 0) End If If pointsA(i, 1) &gt; maxy Then maxy = pointsA(i, 1) End If Next ' Create 3D object points that correspond to chessboard corners ' (The chessboard is 1000*1000, squares are 125*125) Dim corners As Integer = Sqrt(features) Dim obj As New Matrix(Of Double)(features, 3) Dim squaresize2dx As Double = (maxx - minx) / 8 ' pixel width of a chessboard square Dim squaresize2dy As Double = (maxy - miny) / 8 ' pixel height of a chessboard square For i As Integer = 0 To features - 1 obj(i, 0) = Math.Round((pointsA(i, 0) - minx) / squaresize2dx) * 125 ' X=0, 125, 250, 375 ... 1000 obj(i, 1) = Math.Round((pointsA(i, 1) - miny) / squaresize2dy) * 125 ' idem in Y obj(i, 2) = 0 ' Debug.WriteLine(pointsA(i, 0) &amp; " " &amp; pointsA(i, 1) &amp; " " &amp; obj(i, 0) &amp; " " &amp; obj(i, 1) &amp; " " &amp; obj(i, 2)) ' Just to verify Next ' These were calculated with CalibrateCamera using the same images Dim intrinsics As New Matrix(Of Double)({{889.1647, 0.0, 318.3721}, {0.0, 888.5134, 238.4254}, {0.0, 0.0, 1.0}}) ' Find extrinsics Dim distortion As New Matrix(Of Double)({-0.036302, 2.008797, -29.674306, -29.674306}) Dim translation As New Matrix(Of Double)(3, 1) Dim rotation As New Matrix(Of Double)(3, 1) cvFindExtrinsicCameraParams2(obj, pointsA, intrinsics, distortion, rotation, translation, False) ' Convert rotation vector to rotation matrix Dim rotmat As New Matrix(Of Double)(3, 3) Dim jacobian As New Matrix(Of Double)(9, 3) cvRodrigues2(rotation, rotmat, jacobian) ' From http://en.wikipedia.org/wiki/Rotation_representation paragraph "Conversion formulae between representations" Dim yr As Double = Asin(-rotmat(2, 0)) Dim xr As Double = Asin(rotmat(2, 1) / Cos(yr)) Dim zr As Double = Asin(rotmat(1, 0) / Cos(yr)) End Sub End Module </code></pre> <p>The results don't seem correct, I was expecting the translation/rotation but i get this:</p> <pre><code>translation 208.394425348956 -169.447506344527 -654.273807995629 rotation -0.0224937226554797 -2.13660350939653 -1.10542281290682 rotmat -0.741100224945266 0.322885083546921 -0.588655824237707 -0.293966101915684 0.632206237134128 0.716867633983572 0.603617749499279 0.704315622822328 -0.373610915174894 xr=1.08307908108382 yr=-0.648031006135158 zr=-0.377625254910525 xr=62.0558602250101° yr=-37.1294416451609° zr=-21.636333343925° </code></pre> <p>Would someone have an idea what I'm doing wrong? Thanks!</p>
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    1. CO+1 for introducing me to cvRodrigues2(). Cheers!
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