Replace deprecated Eigen nonZeros() call for most recent Eigen versio…

…ns. (colmap#1494)

* Remove nonZeros() call from Eigen::DenseBase

Removal of nonZeros() call which was deprecated in https://gitlab.com/libeigen/eigen/-/commit/08da52eb8537107e2853452bb13c369856d1f84a corresponding to a redundant function call to size().

Issue: COLMAP does not build on Eigen 3.4.90
Fix: Adjust nonZeros() call to be size().

https://gitlab.com/libeigen/eigen/-/issues/2382

* Compute l1-norm to check for non-zeros

Compute the L1-norm of the vectors to check for non-zeros. If vector has no non-zero elements, the norm will be zero, else the norm will be greater than zero.

Updated with clarification from COLMAP devs re: checking for nonZero elements following Eigen implementation deprecation (and clarifying that original nonZeros() always returned size()).

src/exe/model.cc

@@ -765,10 +765,10 @@ int RunModelOrientationAligner(int argc, char** argv) {
     const Eigen::Matrix3d frame = EstimateManhattanWorldFrame(
         frame_estimation_options, reconstruction, *options.image_path);
 
-    if (frame.col(0).nonZeros() == 0) {
+    if (frame.col(0).lpNorm<1>() == 0) {
       std::cout << "Only aligning vertical axis" << std::endl;
       tform = RotationFromUnitVectors(frame.col(1), Eigen::Vector3d(0, 1, 0));
-    } else if (frame.col(1).nonZeros() == 0) {
+    } else if (frame.col(1).lpNorm<1>() == 0) {
       tform = RotationFromUnitVectors(frame.col(0), Eigen::Vector3d(1, 0, 0));
       std::cout << "Only aligning horizontal axis" << std::endl;
     } else {