How Video Compression Works (Simple Explanation)
Ever wondered how a 1GB video becomes 10MB? Here's the magic behind video compression, explained without the technical jargon.
The Basic Idea
Video compression works by removing information that humans don't notice. Think of it like this: if you're describing a blue sky, you don't need to describe every shade of blue — "it's blue" is usually enough.
Uncompressed 1080p video at 30fps = ~150 GB per minute
(That's why we compress!)
Two Types of Compression
Spatial Compression
Compresses each frame individually. Like JPEG for photos — instead of storing "blue, blue, blue, blue" for the sky, it stores "blue x 1000".
Also called: Intra-frame compression
Temporal Compression
Stores only changes between frames. If the sky stays the same for 30 frames, why store it 30 times?
Also called: Inter-frame compression
What Are Keyframes?
Video compression doesn't store every frame completely. Instead, it uses three types of frames:
I-frames (Keyframes)
Complete images. Like taking a photo. Larger in size but essential for seeking.
P-frames (Predicted)
Stores only what changed since the previous frame. Much smaller.
B-frames (Bi-directional)
Uses both previous AND future frames. Smallest but slowest to decode.
What's a Codec?
A codec (coder-decoder) is the algorithm that compresses and decompresses video. Different codecs have different trade-offs:
| Codec | Year | Efficiency | Compatibility |
|---|---|---|---|
| H.264 / AVC | 2003 | Good | Universal |
| H.265 / HEVC | 2013 | Better | Most devices |
| VP9 | 2013 | Better | Most browsers |
| AV1 | 2018 | Best | Limited |
Understanding Bitrate
Bitrate is how much data is used per second of video, measured in Mbps (megabits per second). More bitrate = better quality = larger file.
File size formula:
File Size (MB) = Bitrate (Mbps) × Duration (seconds) ÷ 8
1 Mbps
Low quality
5 Mbps
Good for 1080p
15 Mbps
High quality
Why Some Compressed Videos Look Bad
Compression artifacts appear when you push compression too far. Common issues:
Blocking
Visible squares, especially in smooth gradients like the sky. Caused by too low bitrate.
Banding
Visible stripes in gradients instead of smooth transitions.
Mosquito Noise
Fuzzy edges around moving objects, like a swarm of tiny dots.
Motion Blur
Fast-moving scenes become smeared when bitrate is too low.
How VidCompressor Handles This
VidCompressor uses two-pass encoding to achieve the best possible quality at your target file size:
- 1Pass 1: Analyzes your video to find complex scenes that need more bitrate
- 2Pass 2: Encodes with optimal bitrate distribution — more data for action scenes, less for static moments
This is the same technique Netflix and YouTube use, running entirely in your browser.
Try It Yourself
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