Thumbnail Logo Scanning Guideline
Digital images for use as logos are not required in the OASIS system, but if they are provided, they must meet the specifications listed below. For advice about how to create thumbnails or other derivatives from master images, refer to the readings listed among the LDI Scanning Resources or contact an LDI consultant for additional information.
Requirements
Images must be viewer-ready
Repositories/museums are responsible for image orientation, and all elements related to pictorial quality: amount of acceptable skew, color, brightness, contrast, and overall legibility of images. No changes to images will take place after delivery to OIS.
Bit depth
For color images, 24 bits per pixel is highly recommended, although 8-bit is also acceptable. (Note that GIF supports a maximum of 8 bits per pixel.)
Image format
The file format must display as inline images on standard WWW browsers (i.e., without plug-ins). At present, JPEG and GIF are the two formats that meet this specification, although PNG may soon become more widely supported. In general, JPEG/JFIF is recommended for continuous tone materials and GIF for line art and other less complex source materials.
Image size
Thumbnails must fit within the boundaries of 150 x150 pixels. Rectangular images should not be resized to this dimension as the proportions will be distorted. Instead, size the long dimension of the image to 150 pixels and let the shorter one scale to the appropriate size. Note that all image content must fit within the 150 pixels. If you want to exclude excess borders, labels, rules, grayscales or other targets that were photographed with the image, then this information must be cropped ahead of time. Image quality of thumbnails is always better if the high resolution image is cropped first, then the image is resized.
File size and compression
The maximum allowable file size for OASIS thumbnails is 20KB. However, make every effort to minimize file sizes by the judicious use of bit depth, cropping, and compression. For grayscale images, create thumbnails of no more than 8-bits per pixel. Crop excess pixels (such as borders) whenever feasible. If you save images to GIF, LZW compression is automatically applied. With JPEG, you may apply varying amounts of compression. Start with "best quality" — in other words, the lowest amount of compression — and apply a bit more if image quality does not degrade.