From 59d2d9d00ccb476901fcaaea0f017f134ff47b58 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Thu, 29 Dec 2022 12:47:52 +1100
Subject: [PATCH] Document the meaning of "premultiplied alpha"

---
 docs/handbook/concepts.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/docs/handbook/concepts.rst b/docs/handbook/concepts.rst
index ed25e186508..26c8d1b3ae1 100644
--- a/docs/handbook/concepts.rst
+++ b/docs/handbook/concepts.rst
@@ -64,6 +64,12 @@ Pillow also provides limited support for a few additional modes, including:
     * ``BGR;24`` (24-bit reversed true colour)
     * ``BGR;32`` (32-bit reversed true colour)
 
+Premultiplied alpha is where the values for each other channel have been
+multiplied by the alpha. For example, an RGBA pixel of `(10, 20, 30, 127)`
+would convert to an RGBa pixel of `(5, 10, 15, 127)`. The values of the R,
+G and B channels are halved as a result of the half transparency in the alpha
+channel.
+
 Apart from these additional modes, Pillow doesn't yet support multichannel
 images with a depth of more than 8 bits per channel.