Add a note about Trigg's correction
Add a pointer about how the theory and practice of Trigg's correction
for loss function differs when the second derivative of the loss
function becomes positive.
https://github.com/ceres-solver/ceres-solver/issues/573
Change-Id: Ic22ce91cc230f7ed7fa7b70a1cc2050919039828
diff --git a/docs/source/nnls_modeling.rst b/docs/source/nnls_modeling.rst
index 9f62aa6..cac3d1c 100644
--- a/docs/source/nnls_modeling.rst
+++ b/docs/source/nnls_modeling.rst
@@ -1154,7 +1154,6 @@
matrix such that the robustified Gauss-Newton step corresponds to an
ordinary linear least squares problem.
-
Let :math:`\alpha` be a root of
.. math:: \frac{1}{2}\alpha^2 - \alpha - \frac{\rho''}{\rho'}\|f(x)\|^2 = 0.
@@ -1178,6 +1177,11 @@
problems.
+While the theory described above is elegant, in practice we observe
+that using the Triggs correction when :math:`\rho'' > 0` leads to poor
+performance, so we upper bound it by zero. For more details see
+`corrector.cc <https://github.com/ceres-solver/ceres-solver/blob/master/internal/ceres/corrector.cc#L51>`_
+
:class:`LocalParameterization`
==============================
