Bitmap vs Vector images

Bitmap

• 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.

Vector

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

Pixels

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

JPG/JPEG

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

  BMP

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

  TIFF

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

  GIF

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

  PNG

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

  SVG

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

  PDF

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

  RAW/DNG

• 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.