Day 7 - Lossy Data Compression

Day 7: Lossy Data Compression

DAT-1.D: Compare data compression algorithms to determine which is best in a particular context.

Review and Connection (3 minutes)
- Review lossless compression from previous lesson
- Connect to today's focus on lossy compression techniques
Warm-up Discussion (5 minutes)
- Ask: "When might it be acceptable to lose some data during compression?"
- Discuss: "How much quality loss can humans detect in images and sound?"

Lecture: Principles of Lossy Compression (12 minutes)
- Define lossy compression: reducing file size by discarding some data
- Explain that lossy compression works by:
  - Removing data that humans are less likely to perceive
  - Approximating similar values
  - Prioritizing perceptually important information
- Discuss key properties:
  - Original data cannot be perfectly reconstructed
  - Generally achieves higher compression ratios than lossless
  - Quality-size trade-off can be adjusted
- Explain when lossy compression is appropriate (photos, music, video)
- Introduce common lossy formats (JPEG, MP3, MP4) and their uses
Demo: Image and Audio Compression Examples (10 minutes)
- Show the same image at different JPEG quality levels
- Point out compression artifacts (blocking, ringing, blurring)
- Explain how JPEG works (color space conversion, DCT, quantization)
- Play the same audio clip at different MP3 bitrates
- Explain how MP3 works (psychoacoustic model, frequency masking)
- Demonstrate how file size decreases as quality decreases
Activity: Compare Original and Compressed Media Files (10 minutes)
- Students work in pairs with computers
- Provide access to original and compressed versions of media files
- Students compare:
  - Visual/audio quality at different compression levels
  - File sizes
  - Noticeable artifacts or quality issues
- Students document their observations and preferences

Analysis of Compression Trade-offs (7 minutes)
- Students complete a compression trade-off analysis worksheet:
  - For each use case (web images, professional photography, music streaming, archival storage), recommend a compression approach
  - Justify each recommendation based on the context
  - Explain the trade-offs involved
Preview Next Lesson (3 minutes)
- Explain that next class will shift focus from data representation to data analysis
- Ask students to think about how large datasets might be analyzed

Formative: Quality of media comparison observations
Compression Trade-off Analysis: Appropriateness of recommendations and justifications

Compress the same image or sound using different quality settings and document the results
Research a specific lossy compression format and create a one-page summary
Find examples of compression artifacts in everyday media

Have clear examples that demonstrate compression artifacts
Make connections to students' experiences with streaming media and downloads
Emphasize that the "best" compression depends on the specific use case
Consider discussing ethical implications of using lossy compression in certain contexts (medical imaging, legal evidence, etc.)