From 9509b59beae1876326101282982879eda2aa4b00 Mon Sep 17 00:00:00 2001
From: Peter Chapman <peter@conglomo.co.nz>
Date: Mon, 14 Feb 2022 10:43:08 +1300
Subject: [PATCH] Fixed incorrect value for fully transparent pixels

---
 include/SFML/Graphics/Image.hpp | 10 +++++++---
 src/SFML/Graphics/Image.cpp     | 19 +++++++++++++------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/include/SFML/Graphics/Image.hpp b/include/SFML/Graphics/Image.hpp
index d58989135c..309058a1cb 100644
--- a/include/SFML/Graphics/Image.hpp
+++ b/include/SFML/Graphics/Image.hpp
@@ -228,9 +228,13 @@ class SFML_GRAPHICS_API Image
     /// kind of feature in real-time you'd better use sf::RenderTexture.
     ///
     /// If \a sourceRect is empty, the whole image is copied.
-    /// If \a applyAlpha is set to true, the transparency of
-    /// source pixels is applied. If it is false, the pixels are
-    /// copied unchanged with their alpha value.
+    /// If \a applyAlpha is set to true, alpha blending is
+    /// applied from the source pixels to the destination pixels
+    /// using the \b over operator. If it is false, the source
+    /// pixels are copied unchanged with their alpha value.
+    ///
+    /// See https://en.wikipedia.org/wiki/Alpha_compositing for
+    /// details on the \b over operator.
     ///
     /// \param source     Source image to copy
     /// \param destX      X coordinate of the destination position
diff --git a/src/SFML/Graphics/Image.cpp b/src/SFML/Graphics/Image.cpp
index 010023c5f3..d6dfe6b506 100644
--- a/src/SFML/Graphics/Image.cpp
+++ b/src/SFML/Graphics/Image.cpp
@@ -257,12 +257,19 @@ void Image::copy(const Image& source, unsigned int destX, unsigned int destY, co
                 const Uint8* src = srcPixels + j * 4;
                 Uint8*       dst = dstPixels + j * 4;
 
-                // Interpolate RGBA components using the alpha value of the source pixel
-                Uint8 alpha = src[3];
-                dst[0] = static_cast<Uint8>((src[0] * alpha + dst[0] * (255 - alpha)) / 255);
-                dst[1] = static_cast<Uint8>((src[1] * alpha + dst[1] * (255 - alpha)) / 255);
-                dst[2] = static_cast<Uint8>((src[2] * alpha + dst[2] * (255 - alpha)) / 255);
-                dst[3] = static_cast<Uint8>(alpha + dst[3] * (255 - alpha) / 255);
+                // Interpolate RGBA components using the alpha values of the destination and source pixels
+                Uint8 src_alpha = src[3];
+                Uint8 dst_alpha = dst[3];
+                Uint8 out_alpha = static_cast<Uint8>(src_alpha + dst_alpha - src_alpha * dst_alpha / 255);
+
+                dst[3] = out_alpha;
+
+                if (out_alpha)
+                    for (int k = 0; k < 3; k++)
+                        dst[k] = static_cast<Uint8>((src[k] * src_alpha + dst[k] * (out_alpha - src_alpha)) / out_alpha);
+                else
+                    for (int k = 0; k < 3; k++)
+                        dst[k] = src[k];
             }
 
             srcPixels += srcStride;