Image Restoration for

Halftone Pattern Printed Pictures

in Old Books

Adrian Ciobanu, Tudor Barbu, Mihaela Luca

Institute of Computer Science, Romanian Academy – Iasi Branch

Conventional-dot halftoningused in Romanian old books dated arround 1935

for printing images

Digital halftoningintroduced in 1970, based on

printing technology advances

Scanned pictures and detailsat 600 dots per inch

Pictures and detailsin a PDF available version

Proposed restoring procedure

• Scan a picture from an original book, preferably with the highest resolution

possible and in grayscale.

• Find a global optimum threshold and transform the scanned image in black and

white. The result is a collection of black spots on a white background or white

spots on a black background. These spots are centered on a grid specific to each

halftone pattern method.

• Preprocess the thresholded image by detecting and deleting isolated black pixels

on white background.

• Detect all the black spots on white background with less than 20 pixels in

composition and transform them in black spots with regular shape (close to a

disk) depending on their size. In this way the analogue halftoning is converted in a

digital halftoning. Black spots with more than 19 pixels will be addressed at a later

step.

• Detect all the white spots on black background with less than 20 pixels in

composition and transform them in white spots with regular shape depending on

their size. White spots with more than 19 pixels will be addressed at a later step.

Proposed restoring procedure

• Based on the obtained regular black spots with less than 20 pixels in composition,

detect the grid points of the original halftone pattern as their center of mass or

centroid. Apply a recurrent procedure to fill as much as possible the missing grid

points if three already known neighbor grid points are available.

• For black spots with more than 19 pixels apply cleaning techniques to divide them

in several black spots with less than 20 pixels in composition, using as context

information the already detected grid points. Then apply a regularization of shape

also for them.

• A similar technique should be applied for white spots with more than 19 pixels

(rarer and not so important for the quality of the restored image).

• The final restored halftone pattern image will contain only regular black and white

digital spots, with leaks between them almost completely reduced, so the restored

image will look very clean.

• The last step will consist of an inverse halftoning procedure that will produce a

gray level image with superior quality, ready to be used in common everyday

tasks.

Implementationgraylevel image → black&white image

optimal Otsu threshold

Implementationblack&white image → mostly black & mostly white regions

Implementationmostly white regions mostly black regions

ImplementationPreprocessing of mostly white regions

initial black&white image → eliminating pixels on edges

→ eliminating several types of connecting pixels

ImplementationReplacing analogue halftoning with digital halftoning

Black spots with 3 to 6 pixels

ImplementationReplacing analogue halftoning with digital halftoning

Black spots with 7 to 10 pixels

ImplementationReplacing analogue halftoning with digital halftoning

Black spots with 11 to 14 pixels

ImplementationReplacing analogue halftoning with digital halftoning

Black spots with 15 to 19 pixels

ImplementationReplacing analogue halftoning with digital halftoning

Final cleaning of isolated 1 to 3 pixels spots

ImplementationPreprocessing of mostly black regions

black&white image after processing mostly white regions

→ eliminating isolated pixels

ImplementationReplacing analogue halftoning with digital halftoning

White spots with 3 to 6 pixels – formation I

ImplementationReplacing analogue halftoning with digital halftoning

White spots with 3 to 6 pixels – formation II

ImplementationReplacing analogue halftoning with digital halftoning

White spots with 7 to 10 pixels

ImplementationReplacing analogue halftoning with digital halftoning

White spots with 11 to 14 pixels

ImplementationReplacing analogue halftoning with digital halftoning

White spots with 15 to 19 pixels

ImplementationReplacing analogue halftoning with digital halftoning

Final cleaning of isolated 1 to 3 pixels spots

Implementationgraylevel image → 50,35% processed black&white image

Other resultsgraylevel image → 46,14% processed black&white image

Other resultsgraylevel image → 37,25% processed black&white image

Other resultsgraylevel image → 42,43% processed black&white image

Other resultsgraylevel image → 36,22% processed black&white image

Other resultsgraylevel image → 44,97% processed black&white image

Other resultsgraylevel image → 38,16% processed black&white image

Other resultsgraylevel image → 45,62% processed black&white image

Other resultsgraylevel image → 33,18% processed black&white image

Other resultsgraylevel image → 40,59% processed black&white image

Other resultsgraylevel image → 39,13% processed black&white image

Conclusions and further work

• A novel halftoning image restoration algorithm for pictures printed in old books

has been proposed.

• Its implementation is now in the stage of completely and successfully processing

black and white spots with less than 20 pixels in composition, replacing them with

digital regular shapes, which makes the image clearer and cleaner.

• Further work will imply implementing the cleaning of black and white spots with

more than 20 pixels in composition, based on information extracted about the grid

of halftoning points.

• This will completely clean the image and restore it almost perfectly to the halftone

image that the publishing house wanted to print in the book.

• The last stage should implement an inverse halftoning technique meant to give

the image the feeling of nowadays graylevel images and make it ready to be used

in any computer environment.

Thank you very much!