From 9bb1b6f71291a816619cdeeb1598e13bbc2e040d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adri=C3=A0=20Arrufat?=
 <1671644+arrufat@users.noreply.github.com>
Date: Thu, 31 Mar 2022 07:55:27 +0900
Subject: [PATCH] Fix focal loss

The loss value should reflect the impact of the gamma parameter, as well.
---
 dlib/dnn/loss.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/dlib/dnn/loss.h b/dlib/dnn/loss.h
index cfb2402ea..bb017d1dc 100644
--- a/dlib/dnn/loss.h
+++ b/dlib/dnn/loss.h
@@ -850,14 +850,14 @@ namespace dlib
                     {
                         const float temp = log1pexp(-out_data[idx]);
                         const float focus = std::pow(1 - g[idx], gamma);
-                        loss += y * scale * temp;
+                        loss += y * scale * temp * focus;
                         g[idx] = y * scale * focus * (g[idx] * (gamma * temp + 1) - 1);
                     }
                     else
                     {
                         const float temp = -(-out_data[idx] - log1pexp(-out_data[idx]));
                         const float focus = std::pow(g[idx], gamma);
-                        loss += -y * scale * temp;
+                        loss += -y * scale * temp * focus;
                         g[idx] = -y * scale * focus * g[idx] * (gamma * temp + 1);
                     }
                 }