diff --git a/qoi.h b/qoi.h
index 7addd70e..58d14ae5 100644
--- a/qoi.h
+++ b/qoi.h
@@ -173,8 +173,8 @@ The alpha value remains unchanged from the previous pixel.
 The green channel is used to indicate the general direction of change and is
 encoded in 6 bits. The red and blue channels (dr and db) base their diffs off
 of the green channel difference and are encoded in 4 bits. I.e.:
-	dr_dg = (last_px.r - cur_px.r) - (last_px.g - cur_px.g)
-	db_dg = (last_px.b - cur_px.b) - (last_px.g - cur_px.g)
+	dr_dg = (cur_px.r - prev_px.r) - (cur_px.g - prev_px.g)
+	db_dg = (cur_px.b - prev_px.b) - (cur_px.g - prev_px.g)
 
 The difference to the current channel values are using a wraparound operation,
 so "10 - 13" will result in 253, while "250 + 7" will result in 1.