Scribe can accept a wide variety of images. All formats have their strengths and their weaknesses. This page touches on some basic aspects to guide publishers and authors when preparing image files as part of the Well-formed Document Workflow.
The two types of image formats are vector and bitmap.
Some common file types used in typesetting (.eps, .pdf, .ai) cannot be used in ePub files. Scribe will convert these files to .jpg files for use in electronic publications.
For more information about converting images locally for use in ebooks, see the (Image Processing)[/wfdw/docs/resources/image-processing.html] documentation page.
Vector Images
Vector images are best for images with smooth curves that must be crisp and clean.
Vector File Formats
- .eps (Encapsulated PostScript)
- .ai (Adobe Illustrator)
- .svg (Scalable Vector Graphics)
Recommended Use Cases
- logos
- images that contain text that might need to be edited or revised
- data-intensive images, like charts and graphs
Vector Image Resolution and Limitations
True vector images have functionally infinite resolution, so they are never low resolution.
Most vector file formats allow bitmap images to be embedded inside vector images. This limits the resolution to that of the internal bitmap.
Bitmap Images
Bitmap images are best for images with subtle variations of color and shade.
They are the more common type of image used for digital photographs. Bitmap images are composed of a series of dots (also called “pixels” when on-screen). The amount of dots in a given space is called the “resolution.”
The measurement unit of resolution is “dpi,” or “dots per inch” (also called “ppi,” or “pixels per inch”).
High-quality printing requires a resolution of 300 dpi or higher. For most trade books, the text width is 4.5 inches or less. Therefore, images that are 1350 pixels wide are generally acceptable.
Bitmap File Formats
- .tif or .tiff (Tag Image File Format)
- .jpg or .jpeg (Joint Photographic Experts Group)
- .png (Portable Network Graphics)
- .psd (Photoshop Document)
Bitmap Image Resolution and Limitations
Properly formatted, high resolution TIFF images are the preferred format for bitmap images.
JPEG images are more common, with smaller file sizes due to compression. JPEG is one of the most popular image formats on the internet and is supported by most web browsers and image editing software.
Portable Network Graphics is a raster-graphics file format that supports lossless data compression. A raster is a two-dimensional image made up of a grid of pixels. Rasters are used in digital photography and computer graphics. These are often suitable for the web or ePub formats but too low resolution for print books.
Photoshop documents are best for images that need to be editable, such as photos that have text added to them.
PDF Files
PDF File Format
- .pdf (Portable Document Format)
PDF Limitations
PDF files are always derivative of other file formats, thus are not recommended. A PDF can be either a vector or a raster (bitmap) file, depending on how it was created. It is generally considered a vector image file. However, a PDF can also contain raster elements or be entirely raster based.
PDFs can be included in print books when necessary. However, they must be converted for ebooks. The conversion process usually involves data loss, so it is best to provide the original file from which the PDF was derived.