diff --git a/tinyrender4/obj.go b/tinyrender4/obj.go
index 2ba8970..18c5713 100644
--- a/tinyrender4/obj.go
+++ b/tinyrender4/obj.go
@@ -51,6 +51,8 @@ func (m *Mat44f) SetProjection(fov float32, near float32, far float32) {
 	// 0    0 -f/(f-n) -1
 	// 0    0 -fn/(f-n) 0
 	// where S (scale) is 1 / tan(fov / 2) (assuming fov is radians)
+	// NOTE: -1 there is actually -1/c, where c is distance from viewer to
+	// projection plane. We fix it at 1 for now but...
 	m.ZeroFill()
 	scale := float32(1 / math.Tan(float64(fov)*0.5*math.Pi/180))
 	m.Set(0, 0, scale)