Contrast (vision)

About: Contrast (vision) is a research topic. Over the lifetime, 10379 publications have been published within this topic receiving 221480 citations.

A Comparative Analysis of Histogram Equalization based Techniques for Contrast Enhancement and Brightness Preserving

Abstract: Histogram Equalization (HE) is a simple and effective image enhancement technique.But, it tends to change the mean brightness of the image to the middle level of the permitted range, and hence is not a very suitable for consumer product. While preserving the original brightness is essential to avoid annoying artefacts. To preserve brightness and to enhance contrast of images, numerous methods are introduced, but many of them present unwanted artefacts such as intensity saturation, over-enhancement and noise amplification. In the present paper, available histogram equalization based methods are reviewed and compared with image quality measurement (IQM)tools such as Absolute Mean Brightness Error (AMBE) to assess brightness preserving and Peak Signal-to-Noise Ratio (PSNR) to evaluate contrast enhancement.

Pattern Modulation and Pattern Detection in the Visual System of Hymenoptera

Abstract: In the bee, Apis mellifica, and the ant, Cataglyphis bicolor, pattern modulation and pattern detection are studied by optical methods (1) as well as by pattern discrimination tests using the training procedure (2). (1) Pattern modulation; The directional-intensity function measured by means of the light flux in a single rhabdom is characterized by a half-width of Δ ϱ = 2.6o (Apis) resp. 7.8o (Cataglyphis). In both insects, a better correlation between the directional-intensity function and the divergence angle can be found in the horizontal direction (z-axis) than in the x- and y-axes, respectively, meaning contrast transfer to be more complete in the horizontal direction. Some evaluations of contrast modulation of an array of ommatidia are given for light spots and stripe patterns by means of the optical data. (2) Pattern detection; (a) Using a simple antisymmetric intensity distribution, a vertically presented black-and-white disk with a specifically inclined contrast line, a topological central nervous representation of the visual field of the eye can be proved. (b) That system is shown not to be isotropic in function in so far as the middle lower part of the frontal visual field is most decisive for detecting contrast changes. Therefore, position-dependent weighting factors have to be introduced in any model of pattern recognition in bees. (c) The head or eye position is nearly kept constant in the flying bee, as shown by high-speed cinematography during pattern discrimination tests. Therefore, the receptor coordinates are stabilized with respect to a space-constant system. (d) The mechanism of correlating two patterns by means of overlapping and non-overlapping areas in a space-constant system, where the patterns are not allowed to rotate, cannot be the only principle in data processing due to pattern recognition: for the direction of a stripe is also detected irrespective of the sign of contrast changes (as during training or completely reversed). Therefore reduction of information may be caused by visual generalization processes.

Contrast sensitivity in evaluation of visual impairment due to macular degeneration and optic nerve lesions

Abstract: Spatial contrast sensitivity was investigated in a group of 17 patients with macular degeneration or optic atrophy. It has been reported earlier that patients with optic atrophy may have reduced contrast sensitivity at low spatial frequencies without decrease of visual acuity, a phenomenon called 'hidden visual loss'. The present study shows that also the opposite may be true: patients with macular degeneration or optic atrophy may have greatly decreased visual acuity and yet normal or nearly normal contrast sensitivity at low spatial frequencies and reasonably good sensitivity at intermediate spatial frequencies. This phenomenon could be called 'hidden vision'. In normal individuals and many visually impaired patients the spatial contrast sensitivity at high and intermediate spatial frequencies is equal when measured using grating fields of different size. In patients with central scotoma due to macular degeneration or optic atrophy contrast sensitivity is dependent on the size of the grating field: the maximal contrast sensitivity is higher when larger grating fields are used. The size of the smallest grating field that is needed for normal contrast sensitivity values at low spatial frequencies is one measure of visual impairment in this group of patients. Another measure of the function of the eccentric viewing area is the grating acuity which may also vary as a function of the grating field size.

Neurovisual abnormalities preceding the retinopathy in patients with long-term type 1 diabetes mellitus.

Abstract: Background: Changes in the retina caused by diabetes may lead to visual impairment in dim light, even with good visual acuity and visual fields. To evaluate the visual abnormalities preceding the retinopathy in patients with type 1 diabetes mellitus (DM), we applied electrophysiological methods. Methods: The visual evoked responses were recorded with sinusoidally modulated vertical gratings at 10 spatial frequencies presented sequentially on a high-resolution monitor in patients with type 1 DM and in normal volunteers. Similarly, the contrast visual evoked responses of 10 contrast levels were recorded at five spatial frequencies. Both amplitudes at the second harmonic were calculated by discrete Fourier transform. The visual acuity and contrast thresholds were determined objectively. Results: There was dissociation between the Snellen and the estimated visual evoked response acuity measurements in patients with diabetes (r2=0.077, P=0.44). The saturation phenomena were observed at lower levels of contrast stimuli than in normal individuals at. 1.0, 2.0, 4.0 and 8.0 cycles per degree (P=0.0001). The contrast sensitivity function was deeply abnormal in all tested patients despite the metabolic control. The values of the area under the curve of the visual evoked response amplitude–contrast level function at five spatial frequencies were smaller in patients with DM than in the control group (P<0.05) at all spatial frequencies tested. Conclusions: Patients with type 1 DM without retinopathy showed significant lower amplitude of the visual evoked responses at all spatial frequencies tested, with the saturation phenomena observed at lower level of contrast stimuli. In addition, there was a dissociation between the sweep visual evoked responses and the Snellen acuity measurements. A significant and nonselective neuronal visual loss involving the visual pathway precedes the ophthalmoscopically detectable retinopathy in patients with type 1 DM.