Grayscale images means photographs and illustrations that do not have any colours. Illustrations may have a number of areas in different "shades of gray". Updated: 2014-06-09
Grayscale images means photographs (See Figure 2) and illustrations that do not have any colours. Illustrations may have a number of areas in different "shades of gray". When processing print text books there may be no colour photos available so you have to compromise with grayscale photos in the e-book.
These are perhaps the easiest images to handle and those with print production experience will have handled many in their career. Greyscale images are similar to indexed colour images in that there are only 256 shades of gray in the palette. Grayscale PNG images will typically be one-third the size of colour equivalents.
If the images are detailed photographs always use JPEG/JPG and keep the image quality high in the production process.
IGP:Digital Publisher will provide a standard PIL compression of 25 when images are uploaded. If you want more or less compression on specific images they will have to be Replace loaded.
This setting will reduce an average JPG image to 25% of its file size with no appreciable compression artifacts.
When assessing JPG images for compression artifacts ALWAYS do the assessment at 100% view. Never zoom in.
Do not be too conservative on compression (as you would be if processing for print). Squeeze that image.
Different images will need different compression applied depending on the content. You especially do not want to see JPG compression artifacts in portraits, but they could be acceptable in a land or treescape type image.
If the image has strong contrast colour or lighting lines the compression artifacts will be mostly seen at those junctions. If necessary move to a PNG to avoid those pixel-splattered edges.
If you only have grayscale images consider applying a suitable duotone for the digital edition. This can be done in a book colour-theme and relieves the tedium of grayscale images.
Obviously you will probably not apply duotones if the content of the photographs is of historical importance or similar.
Ideally you should be considering coloration of illustrations for use in a textbook product as this is relatively easy to do. You have 16 million colours available on the screen and don't have the print colour cost overhead.
Grayscale illustrations should be saved as indexed images and palettized to the number of grays in the image, plus an allowance of 5-10 colours for antialiased edges. 32 palette colours is a good starting point.
If you are using an image that can be explored in a reading systems image viewer remember the size limit defined by Apple for the iBooks reader is two million pixels (2,000,000 px). That is 1000 px X 2000 px or some similar multiple.
While we can exceed this in AZARDI this is a fairly sensible guideline for maximum image size on any standard tablet reading device.
Posted by Richard Pipe