Video Playback: Difference between revisions

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This page outlines some best practices; however, we advise to always consult a TSG member if you plan to run a video experiment in the labs.
This page outlines some best practices; however, we advise to always consult a TSG member if you plan to run a video experiment in the labs.
==Video playback==
Note that the Lab Computer displays are typically set to 1920×1080 at 120Hz. We found that this is sufficient for most applications. There are possibilities to go higher. Later in this wiki we will explain how to build audio and video. We will start with playing video, both with and without audio.
=== Python psychopy 2024.2.4===
Example demonstrating how to play a video with audio:
<syntaxhighlight lang="python" line>
from psychopy import logging, prefs
prefs.hardware['audioLib'] = ['PTB']
prefs.hardware['audioLatencyMode'] = 2
from psychopy import visual, core, event
from psychopy.hardware import keyboard
# File paths for video and audio
video_file = "tick_rhythm_combined_30min.mp4"
win = visual.Window(size=(1024, 768), fullscr=False, color=(0, 0, 0))
movie = visual.MovieStim(
    win, filename=video_file,
    autoStart= False
)
kb = keyboard.Keyboard()
# Play the video
win.flip()
core.wait(3.0)
video.play()
video_start_time = core.getTime()
# Main loop for video playback
while not movie.isFinished:
    # Draw the current video frame
    movie.draw()
    win.flip()
    keys = kb.getKeys(['q'], waitRelease=True)
    if 'q' in keys:
        break
movie.stop()    # stop playback
del movie
gc.collect()
win.close()
core.quit()
</syntaxhighlight>
Example demonstrating how to play a video with audio disconnected:
<syntaxhighlight lang="python" line>
from psychopy import logging, prefs
from psychopy import visual, core, sound, event
import time
prefs.hardware['audioLib'] = ['PTB']
prefs.hardware['audioLatencyMode'] = 2
# File paths for video and audio
video_file = "tick_rhythm_30min.mp4"
audio_file = "tick_rhythm_30min.wav"
win = visual.Window(size=(1280, 720), fullscr=False, color=(0, 0, 0), units="pix")
movie = visual.MovieStim(
    win, filename=video_file,
    size=None,  # Use the native video size
    pos=[0, 0],
    flipVert=False,
    flipHoriz=False,
    loop=False,
    autoStart=False,
    noAudio=True,
    volume=100,
    name='myMovie'
)
# Load the audio
audio = sound.Sound(audio_file, -1)
# Synchronize audio and video playback
win.flip()
time.sleep(5)
audio.play()
time.sleep(0.04)
movie.play()
movie_start_time = core.getTime()
while not movie.isFinished:
    # Draw the current video frame
    movie.draw()
    win.flip()
    # Check for keypress to quit
    if "q" in event.getKeys():
        audio.stop()
        break
movie.stop()    # stop playback
del movie
gc.collect()
# Close the PsychoPy window
win.close()
core.quit()
</syntaxhighlight>
Example demonstrating if video and audio encoding are correct:
<syntaxhighlight lang="python" line>
import subprocess
import json
file_path = "tick_rhythm_combined_1min.mp4"
def check_video_file(file_path):
    try:
        # Run ffprobe to get file metadata in JSON format
        result = subprocess.run(
            [
                "ffprobe",
                "-v", "error",
                "-show_streams",
                "-show_format",
                "-print_format", "json",
                file_path
            ],
            stdout=subprocess.PIPE,
            stderr=subprocess.PIPE,
            text=True
        )
        metadata = json.loads(result.stdout)
    except Exception as e:
        print(f"Error running ffprobe: {e}")
        return
   
    # Check for video stream
    video_stream = next((stream for stream in metadata['streams'] if stream['codec_type'] == 'video'), None)
    if video_stream:
        # Check video codec
        video_codec = video_stream.get('codec_name')
        if video_codec == 'h264':
            print("Video codec: H.264")
        else:
            print(f"ERROR: Video codec is NOT H.264 (Found: {video_codec})")
        # Extract and report frame rate
        if 'r_frame_rate' in video_stream:
            raw_frame_rate = video_stream['r_frame_rate']
            calculated_frame_rate = eval(raw_frame_rate)  # Convert string like "30/1" to float
            print(f"Frame rate: {calculated_frame_rate:.2f} FPS (raw: {raw_frame_rate})")
        else:
            print("ERROR: Could not determine raw frame rate from metadata.")
        # Check for constant frame rate
        if video_stream.get('avg_frame_rate'):
            avg_frame_rate = eval(video_stream['avg_frame_rate'])
            if abs(avg_frame_rate - calculated_frame_rate) < 0.01:
                print("Frame rate: Constant")
            else:
                print(f"ERROR: Frame rate is NOT constant (avg_frame_rate: {avg_frame_rate:.2f} FPS)")
        else:
            print("ERROR: Could not determine average frame rate consistency.")
       
        # Check for frame drops
        try:
            frame_info_result = subprocess.run(
                [
                    "ffprobe",
                    "-v", "error",
                    "-select_streams", "v:0",
                    "-show_entries", "frame=pkt_pts_time",
                    "-of", "csv=p=0",
                    file_path
                ],
                stdout=subprocess.PIPE,
                stderr=subprocess.PIPE,
                text=True
            )
            # Filter out empty or invalid lines
            frame_times = [
                float(line.strip()) for line in frame_info_result.stdout.splitlines()
                if line.strip()  # Exclude empty lines
            ]
            expected_interval = 1.0 / calculated_frame_rate  # Expected time between frames
            frame_drops = [
                i for i, (t1, t2) in enumerate(zip(frame_times, frame_times[1:]))
                if abs(t2 - t1 - expected_interval) > 0.01  # Tolerance for irregularity
            ]
            if frame_drops:
                print(f"ERROR: Detected frame drops at frames: {frame_drops}")
            else:
                print("No frame drops detected.")
        except Exception as e:
            print(f"Error analyzing frames for drops: {e}")
    else:
        print("ERROR: No video stream found")
   
    # Check for audio stream
    audio_stream = next((stream for stream in metadata['streams'] if stream['codec_type'] == 'audio'), None)
    if audio_stream:
        # Check audio codec
        audio_codec = audio_stream.get('codec_name')
        if audio_codec == 'pcm_s16le':
            print("Audio codec: WAV (PCM)")
        else:
            print(f"ERROR: Audio codec is NOT WAV (PCM) (Found: {audio_codec})")
       
        # Check sample rate
        sample_rate = audio_stream.get('sample_rate')
        if sample_rate == "44100":
            print("Audio sample rate: 44.1 kHz")
        else:
            print(f"ERROR: Audio sample rate is NOT 44.1 kHz (Found: {sample_rate} Hz)")
    else:
        print("ERROR: No audio stream found")
   
    # Check synchronization
    if video_stream and audio_stream:
        video_start_pts = float(video_stream.get('start_time', 0))
        audio_start_pts = float(audio_stream.get('start_time', 0))
        if abs(video_start_pts - audio_start_pts) < 0.01:  # Tolerance for synchronization
            print("Video and audio are synchronized.")
        else:
            print(f"ERROR: Video and audio are NOT synchronized. Start difference: {abs(video_start_pts - audio_start_pts):.3f} seconds")
    else:
        print("ERROR: Could not determine synchronization (missing video or audio streams).")
# Example usage
if __name__ == "__main__":
    check_video_file(file_path)
</syntaxhighlight>
Example demonstrating how to disconnect audio from video:
<syntaxhighlight lang="python" line>
import os
import subprocess
input_file = 'tick_rhythm_combined_1min.mp4'
directory = os.path.dirname(input_file)
base_name = os.path.splitext(os.path.basename(input_file))[0]
output_video = os.path.join(directory, f"{base_name}_video_only.mp4")
output_audio = os.path.join(directory, f"{base_name}_audio_only.wav")
subprocess.run(['ffmpeg', '-i', input_file, '-an', output_video])
subprocess.run(['ffmpeg', '-i', input_file, '-vn', '-acodec', 'pcm_s16le', '-ar', '48000', output_audio])
print(f"Video saved to: {output_video}")
print(f"Audio saved to: {output_audio}")
</syntaxhighlight>
Example demonstrating how to combine audio and video:
<syntaxhighlight lang="python" line>
import os
import subprocess
# --- Inputs
video_file = 'tick_rhythm_combined_1min_video_only.mp4'  # Your video-only file
audio_file = 'mic_segment.wav'                            # Your trimmed audio
output_file = 'final_synced_output.mp4'                  # Output file name
# --- FFmpeg command to combine
subprocess.run([
    'ffmpeg',
    '-i', video_file,
    '-i', audio_file,
    '-c:v', 'copy',              # Copy video stream as-is
    '-c:a', 'aac',                # Encode audio with AAC (widely compatible)
    '-shortest',                # Trim to the shortest stream (prevents overhang)
    output_file
])
print(f"Synchronized video saved to: {output_file}")
</syntaxhighlight>


==Video encoding==
==Video encoding==
Line 9: Line 287:
Stabilize the camera and avoid automatic exposure, white balance, or focus during recording to prevent inconsistencies.
Stabilize the camera and avoid automatic exposure, white balance, or focus during recording to prevent inconsistencies.
Record in a controlled environment with consistent lighting and minimal background distractions.
Record in a controlled environment with consistent lighting and minimal background distractions.
You can use the '''facecam''' for high quality video recording.
The TSG recomends to use the [[Elgato Facecam]] for high quality video recording.


===Video Settings===
===Video Settings===
Line 16: Line 294:
|-
|-
!File format  
!File format  
|.mp4 (H.264 codec(libx264)) ik wil hier een link naar de dll?
|.mp4 (H.264 [[media:Openh264-1.8.0-win64_.zip | codec(libx264)]])  
|-
|-
!Frame rate  
!Frame rate  
Line 31: Line 309:
|-
|-
|}
|}
===Windows Settings===
Windows 10 has a habit of automatically enabling '''video enhancements''' or unnecessary processing features, which can interfere with smooth playback. Therefore, please make sure these are disabled:
right click background → Display settings → Graphics Settings.
If available, disable "Hardware-accelerated GPU scheduling" for critical timing experiments.
For specific applications (e.g., PsychoPy), under "Graphics Performance Preference," set them to "High Performance" to ensure they use the dedicated GPU.


=== Python ===
=== Python ===
Line 119: Line 405:
<pre>
<pre>
   ffmpeg -i input.wav -ar 48000 -ac 2 -sample_fmt s16 output_fixed.wav
   ffmpeg -i input.wav -ar 48000 -ac 2 -sample_fmt s16 output_fixed.wav
  Explanation:
  -ar 48000 → Set sample rate to 48000 Hz (standard for ASIO/Windows audio, matches most soundcards)
  -ac 2 → Set 2 channels (stereo)
  -sample_fmt s16 → Use 16-bit signed integer samples
</pre>
===Windows Settings===
Windows 10 Settings to check
<pre>
sound → Playback → right-click → Properties → Advanced Tab:
  - Set Default Format to 48000 Hz, 16 bit, Studio Quality.
  - Disable sound enhancements.
  - In the same properties window, go to Enhancements tab → Disable all enhancements.
  - Exclusive Mode:
  - In the same Advanced tab.
  - Allow applications to take exclusive control of this device → CHECKED
  - Give exclusive mode applications priority → CHECKED
</pre>
</pre>


=== Python ===
=== Python ===
Example demonstrating how to check and play your audio:
Example demonstrating how to check your os settings, audio file and play your audio:
<syntaxhighlight lang="python" line>
<syntaxhighlight lang="python" line>
#!/usr/bin/env python3.10
#!/usr/bin/env python3.10
Line 179: Line 491:


==FFmpeg==
==FFmpeg==
{{See also|FFmpeg}}
===Synchronization===
===Synchronization===
Ensure the audio and video streams have consistent timestamps:  
Ensure the audio and video streams have consistent timestamps:  
Line 191: Line 504:
       -map 0:v:0 and -map 0:a:0: Explicitly map video and audio streams to avoid accidental mismatches.
       -map 0:v:0 and -map 0:a:0: Explicitly map video and audio streams to avoid accidental mismatches.
</pre>
</pre>
=== Python ===
Example demonstrating how to use ffmpeg:
<syntaxhighlight lang="python" line>
#!/usr/bin/env python3.10
# -*- coding: utf-8 -*-
</syntaxhighlight>


===Recommended FFmpeg Command===
===Recommended FFmpeg Command===
Here’s a command that encodes video and audio while maintaining high time accuracy:
Here’s a command that encodes video and audio while maintaining high time accuracy:
ffmpeg -i input.mp4 \
<pre>
      -c:v libx264 -preset slow -crf 18 -vsync cfr -g 30 \
ffmpeg -i input.mp4 -c:v libx264 -preset slow -crf 18 -vsync cfr -g 30 -c:a pcm_s16le -ar 48000 -fflags +genpts -async 1 output.mp4
      -c:a pcm_s16le -ar 44100 \
-c:v libx264: Encode video using H.264.
      -fflags +genpts -async 1 \
-preset slow: Optimize for quality and compression efficiency.
      output.mp4
-crf 18: Adjusts quality (lower = better; range: 0–51).
• -c:v libx264: Encode video using H.264.
-vsync cfr: Enforces constant frame rate.
• -preset slow: Optimize for quality and compression efficiency.
-c:a pcm_s16le: Encodes audio in uncompressed WAV format.
• -crf 18: Adjusts quality (lower = better; range: 0–51).
-ar 48000: Sets audio sample rate to 48.0 kHz.
• -vsync cfr: Enforces constant frame rate.
-fflags +genpts: Ensures accurate timestamps.
• -c:a pcm_s16le: Encodes audio in uncompressed WAV format.
-async 1: Synchronizes audio and video streams.
• -ar 44100: Sets audio sample rate to 44.1 kHz.
</pre>
• -fflags +genpts: Ensures accurate timestamps.
• -async 1: Synchronizes audio and video streams.
 
===Tips===
• Ensure Low Latency: If you're processing video/audio in real time, use low-latency settings (e.g., -tune zerolatency for H.264).
• Avoid Resampling: If possible, use the original frame rate and sample rate to avoid timing mismatches.
• Testing: Always test playback on different devices or players to confirm synchronization.


Alternatively, you can use Shotcut, a simple open-source editor, available here: https://shotcut.org/
===Enumeration===
- '''Ensure Low Latency''': If you're processing video/audio in real time, use low-latency settings (e.g., -tune zerolatency for H.264).


- '''Avoid Resampling''': If possible, use the original frame rate and sample rate to avoid timing mismatches.


The [[Lab Computer]] displays are typically set to 1920×1080 at 120Hz. We found that this is sufficient for most applications. There are possibilities to go higher.
- '''Testing''': Always test playback on different devices or players to confirm synchronization.


==Editing==
==Editing==
We recommend using DaVinci Resolve for editing and converting video files. DaVinci Resolve is a free, professional-grade editing program, available here: https://www.blackmagicdesign.com/products/davinciresolve
==Windows Settings==
Windows 10 has a habit of automatically enabling '''video enhancements''' or unnecessary processing features, which can interfere with smooth playback. Therefore, please make sure these are disabled:
Open Settings → System → Display → Graphics Settings.
If available, disable "Hardware-accelerated GPU scheduling" for critical timing experiments.
For specific applications (e.g., PsychoPy), under "Graphics Performance Preference," set them to "High Performance" to ensure they use the dedicated GPU.
==Playback==
=== PsychoPy ===
Example demonstrating how to play a video:
<syntaxhighlight lang="python" line>
#!/usr/bin/env python3.10
# -*- coding: utf-8 -*-
import time
import keyboard
from psychopy import visual
from psychopy import core
## Setup Section
win = visual.Window([720,720], fullscr=False, monitor="testMonitor", units='cm')
# append this stimulus to the list of prepared stimuli
vlc_movies = []
my_movies = ['YourMovie.mp4']#path to your movies from this directory
for movie in my_movies:
    mov = visual.VlcMovieStim(win, movie,
    size=600,  # set as `None` to use the native video size
    pos=[0, 0],  # pos specifies the /center/ of the movie stim location
    flipVert=False,  # flip the video picture vertically
    flipHoriz=False,  # flip the video picture horizontally
    loop=False,  # replay the video when it reaches the end
    autoStart=True)  # start the video automatically when first drawn
    vlc_movies.append(mov)


print("playing video....")
Alternatively, you can use '''Shotcut''', a simple open-source editor, available here: https://shotcut.org/
while not(keyboard.is_pressed('q')) and vlc_movies[0].status != visual.FINISHED:
    vlc_movies[0].draw()
    win.flip()
    buffer_in = vlc_movies[0].frameIndex
    print(vlc_movies[0].status)


print("Stop")
Another one is DaVinci Resolve for editing and converting video files. DaVinci Resolve is a free, professional-grade editing program, available here: https://www.blackmagicdesign.com/products/davinciresolve
 
## Closing Section
core.quit()
</syntaxhighlight>
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