How to remove shadows and grey background from a scan

Telling the problems apart

• Grey or cream cast. The whole page has the same dull tint. It is a global problem: one setting fixes it everywhere.
• A shadow or gradient. One side, one corner, or the gutter of a book is darker than the rest. It is a local problem: no single global setting can fix it, because whatever rescues the shadow blows out the lit area.
• Show-through. Faint, reversed text from the other side of a thin sheet is visible. It is close in tone to light real ink and is only partly separable.
• Speckle. Isolated dark dots from dust on the glass or a worn original.

Why one slider cannot fix a shadow

A global control applies the same change everywhere. If the left of the page is darker than the right, whatever amount brightens the left overexposes the right. A shadow fix has to measure the paper level in each region and correct each one against its own surroundings. Remove scan shadows does this on a smooth map of the page so it cannot be confused by the text.

The order that works

1. Shadows first. Flatten the lighting with Remove scan shadows, so the paper sits at one even level.
2. Then the background. With the paper even, Whiten PDF background takes it to white. Whiten only moves paper-coloured pixels, so dark ink is untouched.
3. Then speckle. Remove PDF noise clears isolated dots. It leaves strokes and thin lines alone.
4. Last, ink. If the text is still weak, darken it with Darken PDF or contrast.
5. If you want all of this at once, Clean a scanned PDF combines it and darkens the ink as well.

Limits worth knowing

• Shadow removal corrects shadows that darken the paper by about a third or less. Deeper shadows are only partly corrected.
• Sharp-edged shadows, such as a finger or a fold, change too quickly for a smooth estimate and are softened rather than removed.
• Whitening can fade very pale content near the paper level, such as a light highlighter or faint pencil. Start gentle.
• Show-through that is as dark as faint real ink cannot be cleanly separated from it.

Capturing a better scan

• Phone scans: use soft, diffuse light from the side rather than a lamp overhead, keep the page flat, and hold the phone parallel to it.
• Books: a flatbed scanner pressed gently closed gives far more even lighting than a phone, and avoids the gutter shadow.
• Turn on background removal or descreening if the scanner offers it, and scan greyscale rather than black and white.