add masks need Alpha channel

diff --git a/parts/Windows.tex b/parts/Windows.tex
index 760a69601ed98a7a7e6b384bd7f2aed42368dbf4..1d618ac5e047743efda9d93743b3938a6c880db6 100644 (file)
--- a/parts/Windows.tex
+++ b/parts/Windows.tex
@@ -1100,6 +1100,8 @@ The compositing pipeline graph has a masking stage (figure~\ref{fig:temporary-02
 
 The Mask popup menu can be overwhelming upon first encounter.  However, if you follow the next
 few steps you can create a single simple mask without having to understand every possible parameter.
+Be sure you are working in a Format that includes the Alpha channel, YUVA or RGBA, because you need
+to have transparency for masking to show or will end up with only a black area.
 \begin{enumerate}
     \item To define a mask, in the Compositor window click on the \textit{Edit mask} icon to get the popup Mask menu.  If the menu does not come up, click on the \textit{Show tool info}.
     \item  On the video, LMB click on the place where you want to start a mask.