U1Ch1L10_Lossy Compression

Purpose: Students will understand what Lossy Compression is and when it is best to use.

Vocabulary: Lossy, Lossless

Activity: Go the Lossy Text Compression App Lab
In the upper screen, type in any two sentences of your choosing, or copy and paste the following portion of a poem found on the Statue of Liberty: "Not like the brazen giants of greek fame, with conquering limbs astride from land to land, here at our sea washed sunset gates shall stand, a mighty woman. Her name, Mother of Exiles." In the lower screen, view how the text was compressed using Lossy Compression Scheme.

Journal: Please explain what you see is the difference between Lossless that you used to compress the poem/songs and Lossy? Which is more beneficial when compressing text and Why? Share with your partner.

Activity: Go to the Lossy Image Compression App Lab (Compress the Bus / Compress your image.)
Journal: Now go on the internet and in no more than a sentence: 1. Explain why it is necessary to compress files? 2. Define Lossy? 3. Define Lossless? 4. Explain the difference.

Video:  Lossy Video (lossy in second half – go to 00:35 -1:22)

Understanding Lossy & Lossless:

• Lossless: in doing the compression, and in reconstructing the original text or image, nothing was lost; every character that was part of the original text could be recovered.
• Lossy compression schemes are ones in which “useless” or less-than-totally-necessary information is thrown out in order to reduce the size of the data. Problem: there’s no real way to know what the original word was. The original word is lost. Positive: human brains are good at filling the gaps when information is missing.
  • Lossy compression can greatly reduce the file size, but it can also greatly reduce the quality and it’s important to find that balance between quality and file size. The real challenge here is finding where that line is - how much can we compress but still keep it recognizable?

So when does a program choose to use Lossy and when does it choose to use Lossless?

• Text should use Lossless / uncompressed because it already has a small file size and it’s important to be able to reconstruct the original message for communication purposes.
• Video should use lossy compression because it usually has a large file size which we need to reduce so we don’t use up all our data
• Image & Audio can have various answers, but what’s most important is how the answer is justified. Correct answers should emphasize lossy compression when they want to save data or transmit quickly, and lossless when it’s important to perfectly recreate the original file.

Activity: Guess which Compression Scheme is used for the following files... Handout! (5pts.)