Digital signal and image processing
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3 histogram equalisation mistakes that cost marks
3 mistakes that cost marks in histogram equalisation
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Lesson notes
Key points
- Using the PDF, not CDF: Students map with raw pixel probabilities; the fix is mapping each level using the cumulative distribution.
- Forgetting to scale by L−1: Leaving CDF values between 0 and 1 is wrong; multiply by L minus one, like 255.
- Not rounding output levels: Output grey levels must be integers, so round each scaled value before building the new histogram.
Takeaway: Equalise with the CDF, scale by L−1, then round.
Quick check: For an 8-bit image, what do you multiply the CDF by in equalisation?
- 256
- 255
- 1
Levels run 0 to 255, so scale by L−1 = 255.
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