Laboratory for Image & Video Engineering

Human Visual Foveation Model

The photoreceptors (cones and rods) and ganglion cells are non-uniformly distributed in the retina in the human eye as shown in the left figure below. The density of cone receptors and ganglion cells play important roles in determining the ability of our eyes to resolve what we see. Spatially, the resolution has the highest value at the point of the fovea and drops rapidly away from that point as a function of eccentricity. When a human observer gazes at a point, a variable resolution image is transmitted through the front visual channel into the high level processing units in the human brain. The region around the point of fixation (or foveation point) is projected into the fovea, sampled with the highest density, and perceived with the highest sensitivity. The right image below simulates the foveation process of the human eye.

The human visual system's foveation feature has two different effects:

At a certain eccentricity, the highest spatial frequency that can be resolved by the human eye without aliasing is limited by the corresponding sampling density in the human visual system. This feature frequency is usually called the local cutoff frequency.
Within the frequency band that can be resolved by the human eye, the visual error sensitivity varies as a function of eccentricity.
The following figures demonstrate our visual sensitivity model, where brightness indicates the normalized strength of visual error sensitivity. The top-left, top-right, bottom-left and bottom-right figures are for image size 512 by 512 and viewing distance v = 1, 3, 6 and 10 times of the image width, respectively. The white curves show the foveated cutoff frequency.

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