diff --git a/imperative-shell/file-thread.sml b/imperative-shell/file-thread.sml
index b26a2c6..f62ac89 100644
--- a/imperative-shell/file-thread.sml
+++ b/imperative-shell/file-thread.sml
@@ -68,9 +68,9 @@ struct
            \    let\n\
            \       val startX = Real32.fromInt startX\n\
            \       val startY = Real32.fromInt startY\n\
-           \       val startY = windowHeight - (startY + drawHeight)\n\
+           \       val endY = windowHeight - startY\n\
            \       val endX = startX + drawWidth\n\
-           \       val endY = startY + drawHeight\n\
+           \       val startY = startY - drawHeight\n\
            \       val windowHeight = windowHeight / 2.0\n\
            \       val windowWidth = windowWidth / 2.0\n\
            \    in\n\