Frame Reduction Script — Understanding the Packages¶

This notebook processes all video files in a given directory and creates new videos where every other frame is skipped, while preserving the original video duration.

Packages Used¶

1. os

Lets us interact with the file system
We use it to:
- Loop through files in a directory
- Build input/output file paths
- Check file extensions

2. cv2 (OpenCV)

A computer vision library for reading, writing, and manipulating videos and images
We use it to:
- Open each video file
- Read frames one by one
- Resize frames
- Write output videos with modified frames

3. tqdm (optional but recommended)

Creates progress bars
Makes it easier to see the processing progress for each video

Output Format¶

For an input file like: walking.mp4 The output file will be: walking_FrameReduced.mp4 in the same directory, and the video will keep its original duration even though half the frames are removed.

How We Keep the Original Duration¶

If we skip every other frame, we effectively output half the frames.
To keep the video length the same, we also halve the FPS when writing the output video.

Example:

Input video: 120 FPS, 600 frames → 20 seconds
Output video: 60 FPS, 300 frames → still 20 seconds

This preserves video timing.

Make the Python Function¶

import os
import cv2
from tqdm import tqdm

def reduce_frames_in_directory(directory_path: str):
    # Supported video extensions
    VIDEO_EXTENSIONS = ('.mp4', '.mov', '.avi', '.mkv', '.flv')

    # List all video files in the directory
    files = [f for f in os.listdir(directory_path) if f.lower().endswith(VIDEO_EXTENSIONS)]

    if not files:
        print("No video files found in the directory.")
        return

    for file in files:
        input_path = os.path.join(directory_path, file)
        basename, ext = os.path.splitext(file)

        output_filename = f"{basename}_FrameReduced{ext}"
        output_path = os.path.join(directory_path, output_filename)

        print(f"\nProcessing: {file}")
        
        cap = cv2.VideoCapture(input_path)
        if not cap.isOpened():
            print(f"Could not open {file}. Skipping.")
            continue
        
        # Read input video properties
        input_fps = cap.get(cv2.CAP_PROP_FPS)
        width  = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
        height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))

        # New FPS = half the original
        output_fps = input_fps / 2

        # Video writer
        fourcc = cv2.VideoWriter_fourcc(*"mp4v")
        out = cv2.VideoWriter(output_path, fourcc, output_fps, (width, height))

        frame_count = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
        
        # Loop through frames
        for i in tqdm(range(frame_count), desc=f"Writing {output_filename}"):
            ret, frame = cap.read()
            if not ret:
                break
            
            # Skip every other frame → keep even-numbered frames only
            if i % 2 == 0:
                out.write(frame)

        cap.release()
        out.release()

        print(f"Saved: {output_filename}")

RUN THE FUNCTION BELOW¶

# Example:
directory_path = "path/to/yours/videos"
reduce_frames_in_directory(directory_path)