improve hw composer documentation
Change-Id: I39363e37417991b9853309154d6691240e706acf
diff --git a/include/hardware/hwcomposer.h b/include/hardware/hwcomposer.h
index 0fa3512..a17f3e4 100644
--- a/include/hardware/hwcomposer.h
+++ b/include/hardware/hwcomposer.h
@@ -287,6 +287,11 @@
* entire composition has been handled by SurfaceFlinger with OpenGL ES.
* In this case, (*set)() behaves just like eglSwapBuffers().
*
+ * IMPORTANT NOTE: there is an implicit layer containing opaque black
+ * pixels behind all the layers in the list.
+ * It is the responsibility of the hwcomposer module to make
+ * sure black pixels are output (or blended from).
+ *
* returns: 0 on success. An negative error code on error:
* HWC_EGL_ERROR: eglGetError() will provide the proper error code
* Another code for non EGL errors.