Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

ImagesOriginal

Quantized

2-dimensional sampled

Sampled and quantized

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

RED

GREEN

BLUE

Colour

Original

Colour Separation

Eye

Computer

2-dimensional sampling

Quantising(A/D-Converter)

Storage inRAM or HD

Output(Graphic adapter

and monitor, printer)

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

Size of Images FilesDefinition: Each sample in images is called Pixel (from picture element)

N x M N=300 pixel

M=200 pixel

here: 300x200 = 600 000 pixel

Quantisation: 8bit (256 grey levels)

File size: (N x M x 8) bit = (N x M) Byte = 600kB per colour 1.8 MByte totally

No of pixels:(per colour R,G and B)

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

Sampling of Images

Zoneplate testpattern

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

Subsampling of Images

Throw away every 2nd pixel

Alias

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

Solution of the Subsampling Problem

Before subsampling: lowpass filter puts away high frequencies

(corresponds to fine structures in images)

lowpass filter subsampling

Reduction of alias!

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

Scan 100dpiOriginal

as less pixel (resolution) as possible, because of small image size (fast download for all interested people)

Our choise: adjustment of the scanner to 100 dpi

Scan 100dpi

Example

condition:

first result:

Original

We want to put a nice scan from a map of your home country to your homepage. Title: „Hello World, here I live!“

Alias

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

A closer look...

Original

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

The wrong way...

Original Scan 100dpi

Prof. Dr. W.-P. BuchwaldFachhochschule Braunschweig/Wolfenbüttel University of Applied Sciences

Scan 400dpiLowpass filtered

The right way...

OriginalDirect scan 100dpi Subsampled to 100dpi