Digital signal and image processing
Myth vs fact
Does JPEG keep every pixel? Myth or fact
You save a photo as JPEG and think nothing was lost. Think again.
Quick check · tap an answer to continue
Lesson notes
Key points
- JPEG splits the image into 8 by 8 blocks and converts each block into frequency coefficients using the discrete cosine transform.
- Quantisation rounds away fine high-frequency detail your eye barely notices, and rounding can never be undone, so the data is gone.
Myth: Saving as JPEG keeps every pixel value exactly as it was.
Fact: JPEG is lossy: quantisation permanently discards image detail.
Takeaway: JPEG saves space by throwing away detail you can't get back.
Quick check: Myth or fact: re-saving a JPEG many times can steadily degrade it.
- Myth
- Fact
- Depends
Each lossy save re-quantises the data, so errors can accumulate.
DCT blocksLossy quantisation
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