Digitization Quality Issues

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Quality and Size

Color Depth, Resolution, and Compression are the main factors that determine the quality and size of a digital image.

Color Depth: The color quality of an image depends on the bit depth, the number of bits used to convey color for each pixel. The options are bitonal (1 bit), grayscale (8 bits), or color (commonly 24 bits, although other values are available). Bitonal produces the smallest files, color the largest. Bitonal is mainly used for the sort of printed text found in books, and sometimes for black-and-white drawings composed of distinct lines with no tonal variation. Color scanning is necessary for color photographs and documents, and grayscale is the best practice recommendation for all other documents. The choice depends on the specific type of item being scanned. For example, field notes written in black ink on index cards might adequately be digitized as grayscale. Even if the notes were written in blue ink, grayscale might suffice if that ink was used consistently. However, if the notes were written in black ink but annotated in blue, color would be the best choice. The question to ask is whether important contextual information will be lost if a lower bit depth is chosen.

Resolution: The clarity of a scanned image is determined by the resolution, the number of dots per square inch. Higher resolution means better quality, but also a larger file size. It is important to make sure that all significant details are captured; for example, diacritics should be clearly visible. Furthermore, if the original, uncompressed image will later be compressed for presentation on the Web, an original image scanned at high resolution will usually provide a better compressed derivative. 300dpi is a good place to start, but it is often best to try initial scans at varying resolutions to find the best choice.

Compression: For the World Wide Web, file sizes must be severely reduced to make them easily downloadable. This further reduction in file size is achieved through "lossy compression" and, depending on the degree of compression, information is lost. Thus, the image cannot be perfectly reconstructed (although in most cases the loss of information is not noticeable). The greater the compression, the greater the information loss. Therefore, you should begin by creating an archivable, uncompressed TIFF file, which can later be converted to a compressed file, either GIF or JPEG, for presentation use. There are also some lossless compression formats, such as LZW (Lempel-Ziv-Welch), but these usually do not result in files small enough for presentation on the Web.

Choosing Formats

Determining which file format is best for your images depends on whether you are using them for archival or presentation purposes. For viewing on the web, images should be either compressed (JPEG) or stored with reduced color (GIF) in order to reduce file sizes and to make loading the images as fast as possible. While GIF is the best choice for thumbnail images, and excellent for computer-generated images with limited palettes, JPEG is far better for photographs, providing better quality images for the same file size. PNG will later be a good alternative to GIF; it has the advantage of being free and open source, using a lossless compression algorithm that creates smaller files than GIF, but at this time, it is not supported by all browsers. TIFF should not be used for online delivery. However, if you want your images to have any future use (either for archiving, enlargement, manipulation, or printing) or if you'd prefer to have a master copy, then uncompressed TIFF is the ideal format to store the images. BMP (bitmap) is also acceptable; it is a simple, uncompressed, open source format.

The Biao Min notecards were scanned for archival purposes at 300dpi and archived in TIFF format. For illustration purposes, presentation images were made in both JPEG and GIF format. The results are shown below. (Click on thumbnail for a larger view)

JPEG image format of a Digitized Biao Min Notecard

Link to Biao Min JPEG Notecard

Image Details: Width: 1480 pixels; Height: 876 pixels; Bit Depth: 24 bits per pixel; Compression: Compressed; Size: 638 KB

TIFF image format of a Digitized Biao Min Notecard

Link to Biao Min TIFF Notecard

Image Details: Width: 1480 pixels; Height: 876 pixels; Bit Depth: 24 bits per pixel; Color Representation: True color RGB; Compression: Uncompressed; Size: 3.73 MB

GIF image format of a Digitized Biao Min Notecard

Link to Biao Min GIF Notecard

Image Details: Width: 1484 pixels; Height: 878 pixels; Bit Depth: 8 bits per pixel; Color Representation: Palettized; Compression: Lempel-Ziv; Size: 580 KB

For long-term storage, choose an archival format that offers LOTS:

FORMAT FILE EXT. L O T S ACCEPTABLE BEST PRACTICE?
 TIFF
.tif
+
+
+
+
YES
GIF
.gif
-
+
-
+
NO
JPEG
.jpg
-
+
-
+
NO
PNG
.png
-
+
-
-
NO
 BMP
.bmp
+
+
+
+
YES

The content of this page was developed following the recommendations of the E-MELD working groups and the Electronic Text Center.


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