Overview
The CrossFade transition creates a smooth blend between two clips by fading out the first clip while simultaneously fading in the second clip. This applies to both video (opacity) and audio (volume) if the clips are VideoClip instances with audio tracks.
Requires overlapping clips: The clips must have overlapping time ranges with at least duration seconds of overlap.
Constructor
from movielite import vtx
transition = vtx.CrossFade(duration)
Parameters
Duration of the crossfade effect in seconds. The clips must overlap by at least this amount.
Methods
apply()
Apply the crossfade transition between two clips.
transition.apply(clip1, clip2)
Parameters
The outgoing clip that will fade out at the end. The fade-out occurs during the last duration seconds of this clip.
The incoming clip that will fade in at the beginning. The fade-in occurs during the first duration seconds of this clip.
Raises
- ValueError: If clips don’t have sufficient overlap for the transition duration
- ValueError: If clips are not in proper sequence (clip2 must start after clip1)
How It Works
Video Crossfade
- Clip1 fade-out: During the last
duration seconds, opacity gradually decreases from 1.0 to 0.0
- Clip2 fade-in: During the first
duration seconds, opacity gradually increases from 0.0 to 1.0
- The overlapping region shows both clips blended together
Audio Crossfade
If both clips are VideoClip instances with audio:
- Clip1 audio fade-out: Volume decreases at the end
- Clip2 audio fade-in: Volume increases at the beginning
- Creates smooth audio transition without abrupt cuts
Usage Examples
Basic Crossfade
from movielite import VideoClip, VideoWriter, vtx
# Create clips with 0.5 second overlap
clip1 = VideoClip("scene1.mp4", start=0, duration=5)
clip2 = VideoClip("scene2.mp4", start=4.5, duration=5) # Starts 0.5s before clip1 ends
# Apply 0.5 second crossfade
transition = vtx.CrossFade(duration=0.5)
clip1.add_transition(clip2, transition)
# Calculate total duration accounting for overlap
total_duration = clip1.duration + clip2.duration - 0.5 # 9.5 seconds
writer = VideoWriter("output.mp4", fps=30, size=clip1.size, duration=total_duration)
writer.add_clips([clip1, clip2])
writer.write()
clip1.close()
clip2.close()
Longer Crossfade for Smoother Transition
# Create clips with 2 second overlap for a slower blend
clip1 = VideoClip("scene1.mp4", start=0, duration=8)
clip2 = VideoClip("scene2.mp4", start=6.0, duration=8) # 2 second overlap
# Longer crossfade creates very smooth transition
transition = vtx.CrossFade(duration=2.0)
clip1.add_transition(clip2, transition)
total_duration = clip1.duration + clip2.duration - 2.0 # 14 seconds
writer = VideoWriter("output.mp4", fps=30, size=clip1.size, duration=total_duration)
writer.add_clips([clip1, clip2])
writer.write()
Multiple Sequential Crossfades
# Create three clips with overlaps
clip1 = VideoClip("scene1.mp4", start=0, duration=4)
clip2 = VideoClip("scene2.mp4", start=3.5, duration=4) # 0.5s overlap
clip3 = VideoClip("scene3.mp4", start=7.0, duration=4) # 0.5s overlap with clip2
# Apply crossfades between each pair
clip1.add_transition(clip2, vtx.CrossFade(duration=0.5))
clip2.add_transition(clip3, vtx.CrossFade(duration=0.5))
total_duration = 11.0 # 4 + 4 + 4 - 0.5 - 0.5
writer = VideoWriter("output.mp4", fps=30, size=clip1.size, duration=total_duration)
writer.add_clips([clip1, clip2, clip3])
writer.write()
Image Slideshow with Crossfades
from movielite import ImageClip, VideoWriter, vtx
# Create image clips with overlaps
images = [
ImageClip("photo1.jpg", start=0, duration=3),
ImageClip("photo2.jpg", start=2.5, duration=3), # 0.5s overlap
ImageClip("photo3.jpg", start=5.0, duration=3), # 0.5s overlap
ImageClip("photo4.jpg", start=7.5, duration=3), # 0.5s overlap
]
# Apply crossfade between consecutive images
for i in range(len(images) - 1):
images[i].add_transition(images[i + 1], vtx.CrossFade(duration=0.5))
total_duration = 9.5 # 3 + 3 + 3 + 3 - 0.5 - 0.5 - 0.5
writer = VideoWriter("slideshow.mp4", fps=30, size=images[0].size, duration=total_duration)
writer.add_clips(images)
writer.write()
Combining Crossfade with Effects
from movielite import VideoClip, VideoWriter, vtx, vfx
clip1 = VideoClip("scene1.mp4", start=0, duration=5)
clip2 = VideoClip("scene2.mp4", start=4.5, duration=5)
# Add visual effects to individual clips
clip1.add_effect(vfx.Saturation(1.3)) # Boost saturation
clip2.add_effect(vfx.Brightness(1.1)) # Increase brightness
# Apply crossfade transition
clip1.add_transition(clip2, vtx.CrossFade(duration=0.5))
total_duration = clip1.duration + clip2.duration - 0.5
writer = VideoWriter("output.mp4", fps=30, size=clip1.size, duration=total_duration)
writer.add_clips([clip1, clip2])
writer.write()
Common Errors
Insufficient overlap errorValueError: Insufficient overlap for transition.
Required overlap: 1.0s, actual overlap: 0.5s.
No overlap errorValueError: Clips do not overlap.
Clip1 ends at 5.0s, but Clip2 starts at 5.0s.
Implementation Details
The crossfade is implemented by:
- Modifying clip1’s opacity function: At time
t in the last duration seconds, opacity = original_opacity × (1 - progress)
- Modifying clip2’s opacity function: At time
t in the first duration seconds, opacity = original_opacity × progress
- Audio transforms (if applicable): Similar fade applied to audio samples using per-sample multiplication
Source code reference: /home/daytona/workspace/source/src/movielite/vtx/crossfade.py:24
See Also
