Digital signal and image processing
Why?
Why median filters beat averaging on speckles
Why does a median filter erase specks that averaging only smears?
Quick check · tap an answer to continue
Lesson notes
Key points
- Because salt-and-pepper noise is a few isolated pixels stuck at full white or full black.
- Because a mean filter adds that extreme value into every average, spreading it into a grey blotch.
- Because a median sorts the window and picks the middle value, so one outlier sits at the end and is ignored.
Definitions
- Salt-and-pepper noise
- Random isolated pixels forced to extreme values, 0 or 255, while neighbours stay normal.
Takeaway: So median filters remove impulse noise while keeping edges sharp.
Quick check: In a 3×3 median filter, which sorted position gives the output value?
- The 1st value
- The 5th value
- The 9th value
Nine sorted values have their middle at position five.
Impulse noiseMean spreadsRank ordering
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