diff --git a/src/webp/encode.h b/src/webp/encode.h
index 54938dfe..565c029b 100644
--- a/src/webp/encode.h
+++ b/src/webp/encode.h
@@ -292,6 +292,11 @@ typedef enum WebPEncodingError {
 #define WEBP_MAX_DIMENSION 16383
 
 // Main exchange structure (input samples, output bytes, statistics)
+//
+// Once WebPPictureInit() has been called, it's ok to make all the INPUT fields
+// (use_argb, y/u/v, argb, ...) point to user-owned data, even if
+// WebPPictureAlloc() has been called. Depending on the value use_argb,
+// it's guaranteed that either *argb or *y/*u/*v content will be kept untouched.
 struct WebPPicture {
   //   INPUT
   //////////////