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[sfmTransform] Rewrite check on ambiguity in sfmTransform #1525

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Sep 4, 2023
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[sfmTransform] Rewrite check on ambiguity in sfmTransform #1525

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13 changes: 7 additions & 6 deletions src/aliceVision/sfm/utils/alignment.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -544,9 +544,10 @@ void computeNewCoordinateSystemFromCamerasXAxis(const sfmData::SfMData& sfmData,
//If we use the raw X, it will be in the image without the orientation
//We need to use this orientation to make sure the X spans the horizontal plane.
const Eigen::Vector3d rX = p.rotation().transpose() * oriented_X;
const Eigen::Vector3d rY = p.rotation().transpose() * oriented_Y;

meanRx += rX;
meanRy += oriented_Y;
meanRy += rY;

meanCameraCenter += p.center();
++validPoses;
Expand Down Expand Up @@ -608,18 +609,18 @@ void computeNewCoordinateSystemFromCamerasXAxis(const sfmData::SfMData& sfmData,
Eigen::Vector3d nullestSpace = solver.eigenvectors().col(minCol).real();
const Eigen::Vector3d referenceAxis = Eigen::Vector3d::UnitY();

const double d = nullestSpace.dot(meanRy);
// Compute the rotation which rotates nullestSpace onto unitY
out_R = Matrix3d(Quaterniond().setFromTwoVectors(nullestSpace, referenceAxis));
const double d = (out_R * meanRy).normalized().dot(Eigen::Vector3d::UnitY());
const bool inverseDirection = (d < 0.0);
// We have an ambiguity on the Y direction, so if our Y axis is not aligned with the Y axis of the scene
// we inverse the axis.
if(inverseDirection)
{
nullestSpace = -nullestSpace;
out_R = Matrix3d(Quaterniond().setFromTwoVectors(nullestSpace, referenceAxis));
}


// Compute the rotation which rotates nullestSpace onto unitY
out_R = Matrix3d(Quaterniond().setFromTwoVectors(nullestSpace, referenceAxis));

out_S = 1.0;
out_t = -out_R * meanCameraCenter;
}
Expand Down