01 Fundamental Image Aspects
question Questionsobjectives Objectives
- What do I need to know about images?
- 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
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.