diff --git a/examples/results/3dmm/fitted.jpg b/examples/results/3dmm/fitted.jpg
index 062a204..ed1a333 100644
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diff --git a/examples/results/3dmm/generated.jpg b/examples/results/3dmm/generated.jpg
index 1c62205..11e1922 100644
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diff --git a/examples/results/posmap_300WLP/IBUG_image_008_1_0.jpg b/examples/results/posmap_300WLP/IBUG_image_008_1_0.jpg
index b9d8439..f8b442f 100644
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diff --git a/examples/results/posmap_300WLP/IBUG_image_008_1_0_posmap.jpg b/examples/results/posmap_300WLP/IBUG_image_008_1_0_posmap.jpg
index 686d727..d396b5f 100644
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diff --git a/face3d/mesh/transform.py b/face3d/mesh/transform.py
index 93a3267..5eda05b 100644
--- a/face3d/mesh/transform.py
+++ b/face3d/mesh/transform.py
@@ -106,6 +106,7 @@ def similarity_transform(vertices, s, R, t3d):
 
 
 ## -------------- Camera. from world space to camera space
+# Ref: https://cs184.eecs.berkeley.edu/lecture/transforms-2
 def normalize(x):
     epsilon = 1e-12
     norm = np.sqrt(np.sum(x**2, axis = 0))
@@ -198,7 +199,7 @@ def perspective_project(vertices, fovy, aspect_ratio = 1., near = 0.1, far = 100
 def to_image(vertices, h, w, is_perspective = False):
     ''' change vertices to image coord system
     3d system: XYZ, center(0, 0, 0)
-    2d image: X(w), Y(h). center(w/2, h/2), flip Y(h). 
+    2d image: x(u), y(v). center(w/2, h/2), flip y-axis. 
     '''
     image_vertices = vertices.copy()
     if is_perspective: