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Graphic formats

Bitmap and vector graphics

Graphical formats fall into two basic categories, bitmap and vector graphics.

Bitmaps

Bitmap formats (also called raster or pixel maps) use a grid of pixel points to represent an image. The value of each pixel, representing its degree of darkness or colour, is recorded individually. Bitmap formats work well for images with complex variations in colours, shades or shapes. Continuous tone photographs are an example of a graphic that benefits from being stored in a bitmap format.

Bitmap image files require a relatively large amount of storage space and more memory to process and display. The ability to store images in compressed form is important for bitmap formats. To handle this problem, graphic software uses a variety of compression and encoding methods. This variety of methods can cause incompatibility problems among software programs trying to use the same graphic files.

The following file formats are recommended for bitmap images:

  • Photos and scanned images
    • For printed output (PDF): Use TIF (or JPG) with resolution 300 ppi.
    • For online output (CHM, EPUB etc): Use PNG with resolution 72 ppi (or higher)
  • Screen captures:
    • For printed output (PDF): Use TIF (or PNG).
    • For online output (CHM, EPUB etc): Use PNG (or JPG)

The GIF format is not recommended, since it is limited to 256 colours.

Which of the recommended formats to use will always depend on the functionality provided by the editing tool.

Vector graphics

Vector representation describes an image as a series of lines and shapes, with some regions filled with a solid color or a gradient shade. Vector formats work well for images with simple shapes, shading and coloration such as charts and graphs, and line art such as computer-aided design drawings.

The following file formats are recommended for vector images:

  • For printed output (PDF): Use EPS
  • For online output (CHM, EPUB etc): Use PNG

The GIF format is not recommended, since it is limited to 256 colours.

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