Guidelines for Derived JPEG Images

Summary:
These Guidelines indicate recommended requirements for web-deliverable JPEG image files derived from master TIFF image files created for PALMM projects.

Specifications:

File Format:

JPEG (Joint Photographic Expert Group)
Specification: ISO 10918-1/2
Reference: http://www.jpeg.org/public/jpeghomepage.htm 

Encoding:

Standard

Compression:

Minimum (highest image quality),
Not greater than 15% (image quality not less than 85% of source file)

Bit-depth:

Same as source file

DPI:

Same as source file
Mitigation of scaling may require down-sampling or Gaussian blur.  These processes require skill and experience.

Image Dimensions:

Width = 630 pixels; Height = variable
Maintain original image aspect ratio
Note: PALMM currently provides an area 630 pixels wide for document image display.  Wider images will scroll horizontally.  Horizontal scroll, generally, should be avoided for documents with portrait orientation

Interpolation: Resample
File Header: Append structured information about the derivation process as appropriate to the SUSDL XML Metadata DTD (forthcoming)
A structured information template will be added in a future revision of these Guidelines

Process:
No single method of processing is recommended.  However, whether converting images individually or by batch process, a visual inspection together with a query of the file header should be completed by spot check.  Spot check requires inspection of every image in thumbnail view and of no less than 10% of the images in full-image view.
A variety of factors effecting on-screen and print legibility should be examined.  For additional guidance, we  recommend reading Cornell University's Digital Imaging Tutorial: Quality Control.