How does the player understand which quality to choose?

The player chooses video quality based on several factors:

• Internet connection speed: The player measures the current network bandwidth (e.g., through the download speed of video segments) and selects a quality that can be played without buffering.
• Video buffer: If the buffer is full, the player may switch to a higher quality; if the buffer is depleting, it may reduce the quality.
• Device and screen resolution: The player may consider the screen size and device capabilities to avoid loading too heavy a video.
• User preferences: Sometimes, the user can manually select the quality.

Example: adaptive streaming (HLS, DASH) divides the video into segments of different qualities. The player analyzes the download speed of segments and dynamically switches between them to ensure smooth playback.