Partially Hidden credits

Before seeing the code for this video, here is a tutorial video that explains the different steps (also made with MoviePy):

And here is the code:

from moviepy.editor import *
from import credits1

# Load the mountains clip, cut it, slow it down, make it look darker
clip = (VideoFileClip('../../videos/', audio=False)
           .speedx( 0.4)
           .fx( vfx.colorx, 0.7))

# Save the first frame to later make a mask with GIMP (only once)
#~ clip.save_frame('../../credits/mountainMask2.png')

# Load the mountain mask made with GIMP
mountainmask = ImageClip('../../credits/mountainMask2.png',ismask=True)

# Generate the credits from a text file
credits = credits1('../../credits/credits.txt',3*clip.w/4)
scrolling_credits = credits.set_pos(lambda t:('center',-10*t))

# Make the credits scroll. Here, 10 pixels per second
final = CompositeVideoClip([clip,