Image Restoration

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Introduction

Image Restoration is the recovery of an original image x[m,n] from a given degraded image y[m,n] with a priori/posteriori knowledge of the degradation process.

Image Restoration is not the same as Image Enhancement.

Ex. of Image Enhancment

Image Sharpening
Image editing

Ex. of Image Restoration

Correcting a blurred Image caused by a moving camera.

Image restoration is a set of techniques for restoring historic photographs, sketches, and other archival images in digital format.

The primary software used in this module includes Adobe Photoshop CS3 Extended, GIMP, and Paint.NET.

Selecting a suitable image

High resolution
Well composed
If scanned, then scanned in high resolution on a clean scanner
No .jpg artifacting
Damage not too extensive

Image types

Pencil sketch
Pen and ink
Paintings
Engravings
Lithographs
Photography

Obsolete photography methods

Daguerrotype
Salt paper
Colloidon glass
Photochrom
Sheet Kodachrome

More rarely-seen art

Chalk
Coloured pencils
Pastels
Etc.

Graphics information

Raster v. vector graphics

Getting started

Getting started

With image restoration, the first decisions are often the most important ones. It's important to select an image that has good potential and fits your skills.

Clarify goals

Some historic images have the potential to become featured pictures with a good restoration. Other times, because of limitations in the digital file or the scan the original image, a restoration can only go so far.

Here's an image of judge Learned Hand as a student:

A historic and useful image, but one that will never be a visual knockout. The photographer took the shot off center and cut off the building arch, and the file is low resolution.
After restoration: rotated and cropped, with ink marks removed and tears repaired. A small area of trouser leg at lower left was reconstructed. Levels adjusted.

Click on the thumbnails for the student portrait shows that the original version above was 922 × 682 pixel, file size: 136 KB, MIME type: image/jpeg. Compare that to the size below for a restoration that became a featured picture.

2,613 × 3,519 pixels, file size: 3.25 MB, MIME type: image/jpeg
2,613 × 3,519 pixels, file size: 3.52 MB, MIME type: image/jpeg

It may be a humbler subject--just a stonemason at work instead of law students at Harvard--but the photographic composition is pleasing and it serves as a good illustration of stonemasonry using traditional tools. The file size is also more than 20 times greater. Large files take more time to repair, but they also carry more information at high resolution.

One type of restoration isn't necessarily better than the other: both serve useful purposes. Be realistic about what your aims are, and about how much time and effort you intend to devote to a restoration. A skilled restorationist could do the student portrait in an hour, but the stonemason image would take a couple of days.

Be realistic about your skills

A good candidate for a beginning restorationist: moderate amounts of dust, dirt, and fiber to remove.
A tough challenge. Good quality scan and high resolution file, but a severely damaged portrait.

Types of media
File sizes
Copyright
Triage for image editing (easy-impossible-tough)
Best practices
Research
Choosing a vision and following it
Types of problems

Pencil sketch lithograph

This pencil sketch lithograph is a political cartoon from the mid-nineteenth century. It depicts Abraham Lincoln and Andrew Johnson repairing the rift that had caused the American Civil War. Published in 1865, a century and a half of aging has yellowed and faded the paper. Although generally well preserved, a few stains, smudges, and other marks distract from the central subject.

For encyclopedic restoration, the goal here is to remove distracting effects so that the viewer's attention is drawn to the style and content of the sketch itself. The restoration was completed in approximately 3 hours.

The original image

Selecting a suitable image is the most important decision in any restoration. This file was reasonably high resolution (12mb in the original Library of Congress version), well composed, and fairly well drawn.

Other political cartoons of the United States from the mid-nineteenth century often look cluttered and obscure to twenty-first century eyes. Artists often relied on complex allegories and large dialog bubbles. This one had the advantage of two elements well known to modern readers: Abraham Lincoln and the Civil War. The metaphor of sewing for mending the union is well illustrated and easy to understand. So although this is not a particularly famous cartoon, it covers an important subject and is a reasonably good example of a particular cartooning style.

On a technical level, the file doesn't have inherent problems such as .jpg artifacting or scanner streaks and is in adequate focus. The lithograph itself has been well preserved for an item of its age.

First edit

It is generally a bad idea to crop before restoration is complete: If you later realize you cropped out something important, or that you messed up the rotation, it will be very hard to go back. Instead, just note what areas lay outside your planned final crop, and do not focus effort on those.

If the image is very large, a loose crop of things you're certain not to need (such as tabletop around a scan) can be useful to improve your computer's performance with handling large files, however, otherwise, it's best not to.

Remember that in the final crop, the image can be rotated slightly, and, indeed, usually will need to be rotated at least a tenth of a degree or so, since humans aren't that good at positioning things on a scanner, or, indeed, at centering things on a page of hand-cut paper.