Mach stripes
In the case of a sequence of surfaces of different shades of gray that do not have any color graduation , we observe Mach stripes along the borders (based on the physicist Ernst Mach , 1865). These are light and dark stripes that increase the contrast between the surfaces.
A basic mechanism for processing visual impressions in humans is the recognition of lines and edges. We often recognize line drawings faster than photos that contain only shades of color. The Martian canals are an example of recognizing line structures where none actually exist, only short line pieces or shadow effects.
The processing in the eye already emphasizes contrasts and lines, because the retina correlates the irritation of neighboring visual cells before it is transmitted to the brain. Although in the figure on the right the upper surfaces only have discrete shades of gray, we can see a brightness gradient at the transitions that increases the contrast: the dark edge appears darker, the light area lighter. The gray wedge at the bottom of the picture appears inhomogeneous, although the gray value changes linearly from left to right.
The cause of this perception phenomenon lies in the interconnection of the receptors in the retina. Approx. 100 million receptors control about 1 million receptive fields that provide the signals for processing. Through the interconnection, the receptors not only amplify signals, but can also weaken them depending on where they originate ( lateral inhibition ).
The principle can best be illustrated by looking at four receptors. Light of higher intensity falls on receptors A and B, in the example image with intensity 30 , on receptors C and D light of lower intensity 10 falls . The excited receptors inhibit the two respective neighboring receptors with a fixed proportion of the amount that they themselves receive in excitation (in the example with a tenth). This results in the following excitation distribution for the downstream neurons :
- Receptor A: 30 -30 * 0.1 -30 * 0.1 = 30 -3 -3 = 24
- Receptor B: 30 -30 * 0.1 -10 * 0.1 = 30 -3 -1 = 26
- Receptor C: 10 -30 * 0.1 -10 * 0.1 = 10 -3 -1 = 6
- Receptor D: 10 -10 * 0.1 -10 * 0.1 = 10 -1 -1 = 8
As a result, the signal is inhibited more or less strongly at the borders of two differently bright areas and the Machian stripes result.
The effect also occurs in computer graphics with calculated, illuminated surfaces (e.g. with Gouraud shading ).
Image processing in the brain also supports edge perception, see for example the Kanizsa triangle .
See also
literature
- E. Mach: About the effect of the spatial distribution of the light stimulus on the retina . In: Session reports of the mathematical and natural science class of the Imperial Academy of Sciences. 52, 1865, pp. 303-322.
Web links
- Interactive demonstration (Java applet)