DarkenMyPDF

PDF contrast enhancer

Contrast is the distance between the darkest and lightest parts of a page. Brightness moves everything up or down together; contrast pulls them apart. For a faint document, contrast is the setting that matters.

This page starts in High Contrast mode, which sets an aggressive black point and white point and applies a steep S-curve between them.

Increase PDF contrast

Starts in High Contrast mode. Switch to Auto if the result is too strong.

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Advanced settings
Processing mode
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LightDark
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LowHigh
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OriginalWhite
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Brightness versus contrast, in one paragraph

Raising brightness on a faint scan makes the ink lighter too — the page gets whiter and less readable. Raising contrast keeps the paper where it is and drives the ink down, which is what you actually want. That is why a generic image editor so often makes a faint scan worse: the obvious slider is the wrong one.

How the curve is built

  1. Sample the page and build a histogram of lightness values.
  2. Take the 95th percentile as the paper level and the mean of everything meaningfully darker than it as the ink level.
  3. Set the black point at or just above the ink level, so ink lands near true black.
  4. Set the white point at or just below the paper level, so paper lands at true white — but never below the ink, which would erase the text instead of darkening it.
  5. Apply a gamma curve plus a smooth S-curve between the two points, so mid-tones separate without clipping.

When high contrast is the wrong choice

High Contrast is built for washed-out documents. On a page containing photographs, charts with subtle shading, or a scanned photo ID, it will flatten the mid-tones you wanted to keep.

For those, Auto Print Enhance is the better starting point: it measures the page and chooses a gentler curve when the page does not need a harsh one.

Questions

What is the difference between contrast and darkness here?

Text Darkness moves the black point and the gamma, so ink gets darker. Contrast controls the steepness of the S-curve between black and white, so the separation between tones increases. In practice you usually want a bit of both.

Will high contrast destroy photos in my document?

It can flatten them. If the document mixes text and photographs, use Auto Print Enhance, which applies a softer curve when the page has a wide tonal range.

Is this the same as adaptive thresholding?

No. Thresholding forces every pixel to pure black or pure white, which is available separately as Black & White mode. Contrast enhancement keeps the greys and simply spreads them out.

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