Why Video writing in Computer Vision? - Purpose & Use Cases

The Big Idea

What if you could turn hundreds of pictures into a smooth video with just a few lines of code?

The Scenario

Imagine you have hundreds of images or frames from a video, and you want to save them back as a video file to share or analyze.

Doing this by hand means opening each image, converting formats, and stitching them together frame by frame.

The Problem

Manually combining images into a video is slow and boring.

You might make mistakes like mixing frame order or using wrong settings, which ruins the video.

It's also hard to repeat the process exactly the same way every time.

The Solution

Video writing tools let you automatically take many images and save them as a smooth video file.

This saves time, avoids errors, and makes it easy to create videos from frames with just a few lines of code.

Before vs After

✗ Before

for i in range(len(frames)):
    save_image(frames[i], f'frame_{i}.png')
# Then use another tool to combine images into video

✓ After

video_writer = VideoWriter('output.mp4', fps=30)
for frame in frames:
    video_writer.write(frame)
video_writer.release()

What It Enables

It makes creating videos from images fast, reliable, and easy to automate for any project.

Real Life Example

Imagine a security camera capturing thousands of photos; video writing lets you turn those photos into a watchable video clip quickly.

Key Takeaways

Manual video creation from images is slow and error-prone.

Video writing automates this process with simple code.

This helps create videos quickly for analysis, sharing, or presentations.

Practice

1. What is the main purpose of using cv2.VideoWriter in computer vision?

easy

A. To save a series of images as a video file

B. To read video files frame by frame

C. To apply filters on images

D. To display images on screen

Solution

1. Step 1: Understand the function of cv2.VideoWriter

   cv2.VideoWriter is designed to save multiple images as a continuous video file.

2. Step 2: Differentiate from other OpenCV functions

   Functions like cv2.VideoCapture read videos, while VideoWriter writes videos.

3. Final Answer:

   To save a series of images as a video file -> Option A

4. Quick Check:

   Video writing = save images as video [OK]

Hint: VideoWriter saves frames into video files [OK]

Common Mistakes:

• Confusing VideoWriter with VideoCapture
• Thinking VideoWriter displays images
• Assuming VideoWriter applies image filters

2. Which of the following is the correct way to create a cv2.VideoWriter object for a 640x480 video at 30 fps using the 'XVID' codec?

easy

A. cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc(*'XVID'), 30, (640, 480))

B. cv2.VideoWriter('output.avi', 'XVID', 30, (640, 480))

C. cv2.VideoWriter('output.avi', cv2.VideoCapture_fourcc('XVID'), 30, (640, 480))

D. cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc('X', 'V', 'I', 'D'), 30, 640, 480)

Solution

1. Step 1: Use cv2.VideoWriter_fourcc correctly

   The codec must be passed as cv2.VideoWriter_fourcc(*'XVID') to get the fourcc code.

2. Step 2: Check the parameters order and types

   Parameters are filename, fourcc, fps, and frame size as a tuple (width, height).

3. Final Answer:

   cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc(*'XVID'), 30, (640, 480)) -> Option A

4. Quick Check:

   Correct codec and frame size syntax = cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc(*'XVID'), 30, (640, 480)) [OK]

Hint: Use cv2.VideoWriter_fourcc(*'XVID') for codec [OK]

Common Mistakes:

• Passing codec as string directly
• Using cv2.VideoCapture_fourcc instead of VideoWriter_fourcc
• Incorrect frame size format (not a tuple)

3. What will be the output of this code snippet?

import cv2
fourcc = cv2.VideoWriter_fourcc(*'MJPG')
out = cv2.VideoWriter('test.avi', fourcc, 20.0, (320,240))
frame = 255 * np.ones((240,320,3), dtype=np.uint8)
out.write(frame)
out.release()
print('Video saved')

medium

A. Nothing happens, no file is created

B. A video file named 'test.avi' with a white frame is saved and 'Video saved' is printed

C. RuntimeError because frame size does not match

D. NameError due to missing import of numpy

Solution

1. Step 1: Check imports

   The code uses np.ones but numpy (np) is not imported, causing a NameError.

2. Step 2: Understand error impact

   Without importing numpy, the code will fail before writing or saving video.

3. Final Answer:

   NameError due to missing import of numpy -> Option D

4. Quick Check:

   Missing numpy import = NameError [OK]

Hint: Always import numpy when using np [OK]

Common Mistakes:

• Assuming code runs without numpy import
• Ignoring frame size correctness
• Thinking print means success

4. Identify the error in this video writing code and select the fix:

import cv2
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi', fourcc, 25, (640, 480))
frame = cv2.imread('image.jpg')
out.write(frame)
out.release()

medium

A. Fix: Change codec to 'MJPG'

B. Fix: Resize frame to (640, 480) before writing

C. Fix: Use grayscale image instead of color

D. Fix: Increase fps to 30

Solution

1. Step 1: Check frame size vs VideoWriter size

   VideoWriter expects frames of size (640, 480). If frame size differs, write() fails or produces errors.

2. Step 2: Correct frame size by resizing

   Resize the loaded image to (640, 480) before writing to match VideoWriter settings.

3. Final Answer:

   Fix: Resize frame to (640, 480) before writing -> Option B

4. Quick Check:

   Frame size must match VideoWriter size [OK]

Hint: Match frame size to VideoWriter size [OK]

Common Mistakes:

• Ignoring frame size mismatch
• Changing codec unnecessarily
• Changing fps unrelated to error

5. You want to create a video from a list of images with different sizes. Which approach correctly handles this situation?

hard

A. Use VideoWriter without resizing; it auto-adjusts frame size

B. Write images directly without VideoWriter; just save as separate files

C. Resize all images to the same size before writing to VideoWriter

D. Change codec to 'X264' to support variable frame sizes

Solution

1. Step 1: Understand VideoWriter frame size requirement

   VideoWriter requires all frames to have the same size as specified at creation.

2. Step 2: Handle varying image sizes

   Resize all images to a consistent size before writing to avoid errors and ensure smooth video.

3. Final Answer:

   Resize all images to the same size before writing to VideoWriter -> Option C

4. Quick Check:

   Consistent frame size needed for video writing [OK]

Hint: Resize images to uniform size before video writing [OK]

Common Mistakes:

• Assuming VideoWriter auto-resizes frames
• Trying to write images without VideoWriter
• Changing codec to fix size issues