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01 Fundamental Image Aspects

question Questions
  • What do I need to know about images?
objectives Objectives
  • Learn difference between bitmap and vactor files
  • What image file type to use
  • When do I have to use file compression

time Time estimation: 15 minutes

Bitmap vs Vector images


  • Pixels in a grid/map
  • Resolution dependent
  • Restricted to rectangle
  • Resizing reduces visual quality
  • Easily converted
  • Minimal support for transparency
  • Popular file formats: BMP, GIF, JPEG, JPG, PNG, TIFF

Bit depth or color depth is the amount of data assigned to every pixel (e.g. 1-bit = black/white, 4-bit = 16 colors/shades of grey, etc.) The more data, the more realistic your image will be. More data per pixel also means larger files.


  • Scalable
  • Resolution independent
  • No background
  • Inappropriate for photo-realistic images
  • XML based text format
  • Popular file formats: SVG, AI, CGM, DXF, WMF, EMF


Resolution = number of pixels = how much detail an image holds PPI: pixel per inch

  • Screen pixel density (monitor/smartphone)
  • Tells you how large an image is

DPI: dots per inch

  • Print-out dots density (inkjet/laser printer)
  • Printer settings

An image at 300 PPI will look fine on a monitor, but printing is another matter! Print it on paper and you will notice the difference between 72 DPI and 300 DPI

File formats and compression


  • Supports 26 million colours (24 bit)
  • Lossy compression (information is lost from original file)
  • Small file size (compressed)
  • Photographs


  • Supports 8/16/24-bit
  • Uncompressed file format
  • Large file size


  • Tagged Image File Format
  • All colour and data information is stored
  • Uncompressed (lossy and lossless compression is possible)
  • Very large file size


  • Graphics Interchange Format
  • Only 256 colours possible (8-bit)
  • Replace multiple occuring patterns into one
  • Small file size
  • Animation


  • Portable Network Graphics
  • 256 / 16M colours
  • 8-bit transparancy
  • Lossless compression


  • Scalable Vector Graphics
  • XML-based format
  • Lossless data compression
  • Creatable and editable with a text editor
  • Can contain both bitmap and vector data


  • Portable Document Format
  • Can contain both bitmap and vector data


  • Digital Negative (DNG) is a universal RAW file format
  • Raw image file (without white balance, color saturation, contrast settings, …)
  • RAW files can be camera brand specific
  • Large file size
  • Multiple options without taking the picture again

    Publication vs Presentation

    Key features for publications:

  • Raw/uncompressed image file (e.g. TIFF)
  • High quality image (300 PPI) and resolution
  • Lossless compression (e.g. PNG)
  • Compression is sometimes allowed (check journal website!)

Key features for presentation:

  • Normal quality image (72 PPI) and smaller resolution (max width: 1920 pixels)
  • Compression is allowed (e.g. JPEG)
  • Smaller file size

Guidelines on image editing

Scientific accepted image manipulations are described in guidelines. VIB also has a document to guide you in what is and what isn’t acceptible when adjusting your images. Some examples are:

  • No specific feature within an image may be enhanced, obscured, moved, removed or introduced
  • Adjustments of brightness, contrast or color balance are acceptable if they are applies to the whole image as long as they do not misrepresent information in the original
  • Grouping of images from different parts of the same or different gel, fields or exposures must be made explicit by the arrangement of the figure (dividing lines)
  • The original data must be available by the author when asked to provide it, otherwise acceptance of the publications may be revoked

you can find all the VIB guidelines here.

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