Raw bitmap

Revision as of 01:02, 4 November 2017

By a raw bitmap image format, we refer to the loosely-defined class of raster image formats in which the file contains only a list of pixel colors, and nothing else.

Raw bitmap formats are generally not portable, and are best used for temporary files. There is no algorithm to decode an arbitrary raw bitmap image file, but a human expert can often do so, given enough effort.

Disambiguation

This type of "raw" format is not related to the digital negative formats used by many digital cameras. There, the word "raw" refers to the fact that the image has not been post-processed, and does not refer to the structure of the file.

Details

The scope of "raw bitmap" formats cannot be precisely defined, but typical characteristics include:

• No header or footer
• Uncompressed
• Does not use a color palette
• Cannot be decoded without external information, such as:
  • Color type and sample order (RGB, BGR, grayscale, etc.)
  • Image width in pixels
  • Row padding logic
  • Byte order and/or bit order

Variations

It is a matter of opinion whether the types of formats described in the section should be considered to be raw bitmap formats.

Some image formats are structured as raw bitmaps, but can still be reliably decoded just by knowing the name of the format, because all images in that format have the same attributes. Examples include Sony Mavica 411 and PGF (Portfolio Graphics).

Many older formats, especially those associated with home computer systems, are (more or less) just a dump of the computer's video memory. They often use a standard color palette, and can be reliably decoded if you know the name of the format. The pixels and/or the bits corresponding to a particular pixel are often arranged in (what today seems like) an unusual order. Examples include many Apple II graphics formats and Atari graphics formats.

Some formats do have a header, but it is insufficient to reliably decode the image. The PC variant of BSAVE Image is an example.

Some formats contain only one plane of an image, e.g. only the red samples.

Software

Some image editors are capable of importing (some) raw bitmap formats, after prompting the user for information about the image's dimensions, color format, etc.

• ImageMagick
  • Formats named CMYK, CMYKA, GRAY, MONO, PAL, RGB, RGBA, RGBO, UYVY, YCbCr, YCbCrA, YUV (complete image)
  • Formats named A, B, C, G, K, M, O, R, Y (single sample)
• Farbfeld Utilities
  • RGB/RGBA, in various orders and padding, planar, premultiplied-alpha, etc
  • Indexed colour images, with 1, 2, 4, or 8 bits per pixel, various kinds of bit orders and packing