Emgu CV: CvInvoke.ProjectPoints throws "OpenCV: d == 2 && (sizes[0] == 1 || sizes[1] == 1 || sizes[0]sizes[1] == 0)" despite valid inputs

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1

I’m using Emgu CV (OpenCV C# wrapper) and trying to project 3D points onto a 2D image plane using CvInvoke.ProjectPoints. However, I’m getting this OpenCV error:

OpenCV: d == 2 && (sizes[0] == 1 || sizes[1] == 1 || sizes[0]*sizes[1] == 0)

I’ve carefully verified the shape and type of all inputs. Here’s my full setup:

:wrench: Setup:

Emgu.CV version: 4.11.0

Language: C#

Platform: .NET (WinForms)

:pushpin: My code: (csharp)

// Assume i is the current camera index
Mat rVec = new Mat();
Mat tVec = new Mat();

// Intrinsic parameters (from calibration)
Matrix<double> camMatrix = K[i];       // 3x3, Cv64F
Matrix<double> distCoeffs = D[i];      // 1x4, Cv64F

// 3D points in meters
var coords = new List<float[]>
{
    new float[] { 0.1f, 0.2f, 0.5f },
    new float[] { 0.2f, 0.1f, 0.5f },
    new float[] { 0.3f, 0.1f, 0.5f },
    new float[] { 0.4f, 0.1f, 0.5f },
    new float[] { 0.5f, 0.1f, 0.5f },
    new float[] { 0.6f, 0.1f, 0.5f }
};

var coordsArray = coords.Select(p => new MCvPoint3D32f(p[0], p[1], p[2])).ToArray();

// Corresponding 2D image points
List<PointF> pointsF = new List<PointF>
{
    new PointF(1234f, 567f),
    new PointF(1250f, 570f),
    new PointF(1270f, 575f),
    new PointF(1290f, 580f),
    new PointF(1305f, 583f),
    new PointF(1320f, 586f)
};

// SolvePnP
CvInvoke.SolvePnP(
    coordsArray,
    pointsF.ToArray(),
    camMatrix,
    distCoeffs,
    rVec,
    tVec
);

// Convert translation to meters if needed
// tVec.ConvertTo(tVec, tVec.Depth, 1.0 / 1000.0); // optional depending on units

// Project 3D points
var proj_obj = new Emgu.CV.Util.VectorOfPointF();
CvInvoke.ProjectPoints(
    coordsArray,
    rVec,
    tVec,
    camMatrix,
    distCoeffs,
    proj_obj // this throws the error
);

:white_check_mark: Inputs (confirmed): coordsArray.Length == 6

rVec: 3x1, type Cv64F, 1 channel

tVec: 3x1, type Cv64F, 1 channel

camMatrix: 3x3, type Cv64F, 1 channel e.g.:

[2416,   0, 960]
[   0,2416, 540]
[   0,   0,   1]

distCoeffs: 1x4, type Cv64F, 1 channel

:red_exclamation_mark: Problem: Even with all input dimensions and types seemingly correct, I still get this error from ProjectPoints.

:red_question_mark: Question: What could cause CvInvoke.ProjectPoints in Emgu CV to throw this OpenCV error, even when all dimensions seem correct? Are there any edge cases or known issues I might be missing?

:magnifying_glass_tilted_left: Tried: Verified dimensions of all inputs.

Replaced 0 Z values with small epsilon like 0.0001f.

Tried flattening tVec, or converting to different shapes — no luck.

Ensured all coordinate units are consistent (meters or millimeters).

Any help is greatly appreciated!

2

also posted on https://stackoverflow.com/questions/79694299/emgu-cv-cvinvoke-projectpoints-throws-opencv-d-2-sizes0-1-size