May 27, 2004

Contents

Image Aspect Ratio

Aspect Ratio here refers to the height/width ratio of the area represented by a pixel. Digital images used for measurements (lengths, areas, aspect ratios, angles, etc.) should have an aspect ratio of 1:1, in other words the pixels should be 'square'.

This may not always be the case. Some older frame grabbers were designed for commercial TV systems, which have a 3:4 aspect ratio. When a round object was 'digitized' with such a system, it looked round on the screen of the frame grabber (a normal TV screen where the pixels had a 3:4 aspect ratio), but when the image was viewed on a Mac which has square pixels, the round object looked like an ellipse. This is because the round object, as taken with the TV frame grabber was, say, x pixels high, but 4x/3 pixels wide.

The same situation can exist for scanning microscopes, but not to such a great degree. The instrument should be adjusted to give a 1:1 aspect ratio to the scan. If it isn't, it can be several percent off.

Example of altering the aspect ratio of an image:

 This is an electron diffraction pattern of gold. (courtesy Eric Steel, NIST)  

 

Now the circles are ellipses.

The blocky apparance (vertical edginess seen in the brighter rings) is due to the Mach Band effect (also seen in the ramp example). Bilinear Interpolation, below, gets rid of this effect by doing some pixel averaging.

 

Expanded image has been smoothed a bit due to Bilinear Interpolation.