Summary
A digitised image requires careful preparation before it is suitable for distribution.
This document describes a workflow for improving the quality of scanned images
by correcting faults and avoiding common errors.
Preparing your master image
The sequence in which modifications are made will have a significant contribution
to the quality of the final image. Although conformance to a strict sequence is
not always necessary, inconsistencies may be introduced if the order varies
dramatically between images. The Technical Advisory Service for Images (TASI)
recommends the following order:
- Does the image require rotation or cropping?
In many circumstances, the digitiser will not require the entire image.
Cropping an image to a specific size, shape or orientation will reduce the time
required for the computer to manipulate the image and prioritise errors to those considered important.
- Are shades and colours difficult to distinguish?
Scanners and digital cameras often group colours into a specific density range.
This makes it difficult to differentiate shades of the same colour. Use the
Histogram function with Photoshop (or other software) and adjust the different
levels to best use the range of available tones.
- Is the colour balance accurate in comparison to the original?
Some colours may change when digitised, e.g. bright orange may change to pink.
Adjust the colour balance by modifying the Red, Green & Blue settings. Decreasing one colour increases its opposite.
- Are there faults or artefacts on the image?
Visual checks should be performed on each image, or a selection of images, to
identify faults, such as dust specks or scratches on the image.
Once you are satisfied with the results, the master image should be saved
in a lossless image format - RGB Baseline TIFF Rev 6 or PNG are acceptable for this purpose.
Improving image quality
Subsequent improvements by resizing or sharpening the image should be performed on a derivative.
- Store work-in-progress images in a lossless format
Digitisers often get into the habit of making modifications to a derivative
image saved in a 'lossy' format, i.e. a format that simplifies detail to reduce
file size. This is considered bad practice, will reduce quality and cause
compression 'artefacts' to appear over subsequent edits. When repeatedly altering
an image it is advisable to save the image in a lossless format (e.g. TIFF, PNG)
until the image is ready for dissemination. Once all changes have been made it
can be output in a lossy format.
- Filter the image
Digitised images often appear 'noisy' or contain dust and scratches.
Professional graphic manipulation (Photoshop, PaintShop Pro, etc.) possesses
graphic processors that can be useful in removing these effects. Common filters
include 'Despeckle' that subtly blurs an image to reduce the amount of 'noise'
in an image and 'median' that blends the brightness of pixels and discards pixels
that are radically different from adjacent pixels.
- Remove distracting effect
If you are funded to digitise printed works, moiré (pronounced more-ray)
effects may be a problem. Magazine or newspaper illustrations that print an
image as thousands of small coloured dots produce a noticeable repeating pattern
when scanned. Blur effects, such as the Gaussian blur, are an effective method of
reducing noticeable moiré effects, however these also reduce image quality.
Resizing the image is also an effective strategy that forces the image-processing
tool to re-interpolate colours, which will soften the image slightly.
Although these effects will degrade image to an extent, the results are often
better than a moiré.
Further Information
Improving The Quality Of Digitised Images
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