False Contour Research Articles

In-loop selective processing for contour artefact reduction in video coding

Proposed is an in-loop decontouring algorithm that utilises block-based processing for video coding. In order to avoid uniform filtering that brings about unnecessary processing, candidate blocks that may contain noticeable false contours using different kinds of features in a block are sequentially refined. Then, pseudorandom noise masking is applied to them to reduce contour artefacts. It is demonstrated that visual quality could be efficiently improved with a negligible rate-distortion (RD) loss.

Reduction of halftoning errors due to line load in a plasma display panel

The displayed gray level of a plasma display panel (PDP) is often different from the intended target gray level because the displayed level is affected by line load (the number of on-cells in a scan line). Previous attempts to reduce this difference (i.e. error) have been applicable to PDPs with no constraint imposed on the on-off decision for each sub-field. In order to improve the image quality (e.g. dynamic false contour (DFC) reduction), recently-developed PDPs impose constraints on sub-field coding and reduce the number of gray levels. This paper proposes a new sub-field coding algorithm that compensates for level error caused by the load when constraints are imposed on sub-field coding. As the number of gray levels is reduced, halftoning is employed in order to represent intermediate gray levels. In the proposed algorithm, the magnitude of the level error caused by the load effect is predicted and then two levels that are used for the halftoning are modified to compensate for the error. The modified levels are the nearest levels to the input level among the levels that satisfy the constraint of the sub-field coding. In order to reduce errors with moderate computational complexity, error prediction and level update are repeated by twice. A significant error reduction is achieved and shown by simulation and experimentation using a 42-inch PDP, including a 93.5% reduction in the mean absolute error (MAE) in simulations and a 67.2% reduction in MAE in experiments on a PDP.

Novel Digital Driving Method Using Dual Scan for Active Matrix Organic Light-Emitting Diode Displays

A new digital driving method has been developed for low-temperature polycrystalline silicon, transistor-driven, active-matrix organic light-emitting diode (AM-OLED) displays by time-ratio gray-scale expression. This driving method effectively increases the emission ratio and the number of subfields by inserting another subfield set into nondisplay periods in the conventional digital driving method. By employing the proposed modified gravity center coding, this method can be used to effectively compensate for dynamic false contour noise. The operation and performance were verified by current measurement and image simulation. The simulation results using eight test images show that the proposed approach improves the average peak signal-to-noise ratio by 2.61 dB, and the emission ratio by 20.5%, compared with the conventional digital driving method.

Bit Depth Needed for High Image Quality TV-Evaluation Using Color Distribution Index

In this paper, we proposed a new image quality evaluation index-color distribution index (CDI). It considers color gamut, contrast ratio, maximum luminance, gamma characteristics, and just noticeable difference (JND) as human visual system to evaluate the ability for reproducing visually smooth gradation. When the CDI cell was defined, we verified JND from a fundamental matter, such as JND in luminance and chromaticity, using the latest TV sets. According to our investigations, luminance JND was AL/L = 0.01 and chromaticity JND was Au'v' = 0.001. We used these with unit CDI cell and evaluated bit depth needed for high image quality without any visible false contour. We clarified that 12 bit is needed for necessary and sufficient condition to reproduce high image quality with TV set designed based on BT.709. Also, we clarified that color gamut, contrast ratio, maximum luminance, gamma characteristics and bit depth should be balanced to achieve TV sets of high image quality from the viewpoint of the total image quality improvement.

A Study of False Contour Noise in Moving Images through Consideration of the Phosphor Decay Time of AC PDP

The dynamic false contour noise was analyzed with consideration for the phosphor decay time of an ac PDP by computer simulation based on the measurement of the 1/10 phosphor decay times of the primary colors red green and blue at the main wavelengths of each phosphor. The noise level of dynamic false contour is strongly dependent on phosphor decay time. The noise level decreases incrementally with the phosphor decay time whereas the noise width increases. The moving velocity of an object does not affect the noise level. The entire experiment was performed under the condition of 8 subfields ADS driving scheme, 2.5[μsec] scan speed, and 5 [μsec] sustain period with VGA grade panel.

Image Adaptive Incremental Subfield Coding for Plasma Display Panels

In this letter, we propose a new approach to incremental coding of the subfield codes for plasma display panels (PDPs). The proposed approach suppresses the halftone noise of the PDPs, while completely eliminating false contour noise, as do existing incremental subfield codes, by selecting an optimal incremental subfield code adaptively for a given input image. The proposed method maps the problem of selecting the optimal incremental subfield code onto a special-case shortest path problem. Results of experiment using 109 sample images illustrated that the proposed method improved the average peak signal-to-noise ratio by 4.4-6.2 dB in halftone noise compared with existing incremental subfield coding methods.

Rough Sets-Assisted Subfield Optimization for Alternating Current Plasma Display Panel

Plasma display panel technology employs a pulse-number modulation method to generate the gradation of the display. However, this method can create a false contour distortion when moving pictures are reproduced. In this paper, we propose an effective subfield optimization technique which is based on measuring motion picture distortion. To this end, we propose two applications of the measurements. One is optimum selection of the codewords to improve performance, and the other is the application of rough sets theory to subfield optimization, which reduces computational CPU time. Simulation results show that dynamic false contouring can be effectively reduced by this optimization technique, and the complexity can be minimized, using rough sets theory.

대각선 방향 픽셀에 기반한 이방성 확산을 이용한 영상 분할

Anisotropic diffusion is one of the widely used techniques in the field of image segmentation. In the conventional anisotropic diffusion [1]-[6], usually 4-neighborhood directions are used: north, south, west and east, except the image diagonal directions, which results in the loss of image details and causes false contours. Existing methods for image segmentation using conventional anisotroplc diffusion can`t preserve contour information, or noises with high gradients become more salient as the umber of times of the diffusion increases, resulting in over-segmentation when applied to watershed. In this paper, to overcome the shortcoming of the conventional anisotropic diffusion method, a new arusotropic diffusion method based on diagonal edges is proposed. And a method of watershed segmentation is applied to the proposed method. Experimental results show that the process time of the proposed method including diagonal edges over conventional methods can be up to 2 times faster and the Circle image quality improvement can be better up to . Experiments also show that images are segmented very effectively without over segmentation.

Human Perception of Contour in Halftoned Density Step Image

Contour is an important element of an image, usually expressed by the difference in density between two areas of an image. Objects are usually recognized from observation of their contours. In some cases, emergence of a false contour is a problem in image coding and decoding. Halftoning is an essential image process for digital printing of continuous tone. Contour perception is studied experimentally under various halftoning conditions, observation distances, and illumination conditions. The results show that in the human visual system, contour perception becomes easier with a decreasing stimulus of dots, which serve as the component of a halftone image.

Digital Color Image Halftone: Hybrid Error Diffusion Using the Mask Perturbation and Quality Verification

Error diffusion is widely used in digital image halftones. The algorithm is very simple to implement and very fast to calculate. However, it is known that standard error diffusion algorithms, such as the Floyd Steinberg error diffusion, produce undesirable artifacts in the form of structure artifacts, such as worms, checkerboard patterns, diagonal stripes, and other repetitive structures. The boundaries between structural artifacts break the visual continuity in regions of low intensity gradients and therefore may be responsible for false contours. In this paper, we propose a new halftone method to reduce the structural artifacts and to improve the gray expression, called hybrid error diffusion, by using the concept of "error diffusion by perturbing the error coefficient with a mask." The proposed algorithm consists of two steps in each pixel position. In the first step, a perturbation is calculated using the internal pseudorandom number and a selected 4×4 mask, similar to a dither mask. In the second step, error diffusion weights are calculated with the criterion for each pixel value. The proposed hybrid method can reduce the structural artifacts while keeping the advantage of the error diffusion. This paper discusses the performance of the proposed algorithm with experimental results for natural test images. Then, objective assessment results are given using statistical tools and the structural similarity measure for color images.

Image-Dependent Code Optimization to Improve Motion Picture Quality of Plasma Displays

This letter proposes an efficient method to find the optimum subfield code, which minimizes the visual artifacts on the motion pictures of the plasma display panel (PDP). Existing codes were constructed to reduce dynamic false contour (DFC) only, and they are fixed codes used for every image. In contrast, the proposed method aims to minimize the total artifacts by DFC and halftone noise (HN), and it finds the best code for a given image, dynamically. First, this letter presents the novel models to estimate the effect of DFC and HN for given codewords and a given image. Then, it presents an efficient method that finds the optimum code for a given image using the well-known shortest-path algorithm. Experimental results, using 459 HDTV images, illustrated that the proposed approach improved the average PSNR by 0.713 dB and 7.004 dB in DFC and HN, respectively, when compared with Gravity Centre Code [1].

False contour reduction using directional dilation and edge-preserving filtering

As display devices have become larger, false contour artifacts invisible in small displays have appeared visible. False contours are artifacts that appear as step edges in relatively smooth regions of an image, mostly caused by image quantization. In this paper, a new false contour reduction algorithm of still images is proposed using directional dilation and edge-preserving filtering. The proposed algorithm consists of three steps: false contour detection, false contour map expansion, and edge-preserving false contour reduction. In the first step, pixels to be processed are detected and in the second step the processing area is expanded using the proposed directional dilation. Finally, only the pixels selected in the first two steps are filtered, in which edge-preserving filtering using multi-dimensional weight functions is employed. The performance of the proposed algorithm is compared with those of two existing methods. Experimental results in various aspects show that the proposed algorithm efficiently reduces false contours

A Real-time Image Processor with Dynamic Error Diffusion based on Edge Detection for Plasma Display Panel

In the conventional error diffusion method applied to alternating current plasma display panel (AC PDF), all directions' diffusion coefficients are fixed all the time in diffusion process, which results in the loss of image details and causes false contours. To overcome the shortcoming of the conventional error diffusion method, a dynamic error diffusion method based on edge detection is proposed. In the proposed method, error diffusion coefficients are dynamically adjusted according to the edge detection executed along four diffusion directions. The dynamic error diffusion method makes the error diffusion coefficients coincide with the characteristic of image contour, and decreases the accumulated errors generated in the conventional error diffusion process. A real-time image processor with the dynamic error diffusion was implemented and tested on a 42- inch AC PDP. Simulation and experimental results show that both the static false contours caused by lack of gray levels and the loss of image details caused by fixing diffusion coefficients are decreased effectively, which improves image quality of AC PDP greatly.

An adaptive subfield coding method for driving AC PDPs

The image quality suffers from the lack of grayscale expression capability when the simple accumulation of light emission is applied to express grayscale in alternating current plasma display panels (ac PDP). To solve the problem, an adaptive subfield coding (ASC) method for driving ac PDP is proposed, in which subfield vector is adaptively figured out based on the normalized histogram of input image every television field. The calculated subfield vector can express the most grayscale information of input image. And then the number of sustain pulses in each subfield is adjusted according to the calculated subfield vector. Simulation results show that the proposed method can eliminate the dynamic false contours in ac PDPs and overcome the lack of grayscale expression capability caused by the simple-accumulation-lighting-mode. It could provide good grayscale expression in ac PDPs.

Two-stage false contour detection using directional contrast features and its application to adaptive false contour reduction

In the era of digital television, artifacts such as false contours unnoticed in small television sets appear conspicuously as the size of display devices increases. This paper proposes a two-stage false contour detection algorithm and its application to false contour reduction. In stage 1 of false contour detection, smooth regions are first removed by bit-depth reduction or re-quantization. In stage 2, false contours are separated from edges or texture regions using directional contrast features, yielding the binary false contour decision map that shows possible candidate regions of false contours. Also a number of non-directional and directional features for possible false contour detection are considered and their performance is compared. For false contour reduction, variable-size directional smoothing is applied only to candidate regions that are specified by the false contour decision map. We use one-dimensional variable-size directional smoothing masks whose directions are orthogonal to the directions determined by the directional contrast features found in the false contour detection step. Computer simulations with several test images with various types of false contours show the effectiveness of the proposed false contour reduction algorithm in terms of visual quality and the peak signal-to-noise ratio. The proposed algorithm can also be used as a post-processing method for artifact reduction in various display devices.

Contour perception of half-toned density step image

Contour is important elements of image and is usually expressed by the density jump between two areas of image. Recognition of object is usually carried out from the contour. In some cases, emerging of false contour is problem in image coding and de-cording process. Half-toning is essential processing for digital printing of continuous tone. Contour perception conditions are studied experimentally on various half-toning conditions, observation distances, and illumination conditions. It is found that the contour perception become easy as the stimulus of the halftone dot decreases in human visual system.

Hybrid Error Diffusion with Mask Perturbation

The error diffusion algorithm is widely used in digital image halftone. The algorithm is very simple to implement and very fast to calculate. However, it is known that standard error diffusion algorithms such as Floyd-Steinberg produce undesirable artifacts in the form of structure artifacts such as checkerboard patterns, diagonal stripes and other repetitive structures. The boundaries between structure artifacts break the visual continuity in regions of low intensity gradients and may be therefore responsible for false contours. In this paper, we propose a new halftone method to reduce the structural artifacts and to increase the gray expression, called hybrid error diffusion, by the concept of “error diffusion by perturbing the error coefficient with randomly selected mask”. The proposed algorithm consists of two steps in each position. In the first step, a perturbation is calculated using the internal pseudorandom number and randomly selected 4x4 mask, similar to dither mask (R,G,B different). In the second step, error diffusion weights are calculated with the criterion for each pixel values. So, the proposed hybrid method can reduce the structure artifacts with keeping the advantage of the error diffusion. This paper discusses the performances of proposed algorithm with experimental results for natural test images.

A sub‐field rearrangement driving method for reducing dynamic false contour in plasma display panels

A sub‐field rearrangement driving method has been proposed to reduce a DFC (Dynamic False Contour) phenomenon in plasma display panels. The proposed driving method expresses 256 gray levels with 16 sub‐fields, while conventional one uses only 8 sub‐fields. Notwithstanding the increase in the number of sub‐fields, the display time is similar to the conventional 8 sub‐fields driving method by appropriate choosing selective writing and selective erasing for sub‐fields.

31.5: Characterizing Motion Artifacts and Flicker of PDP's

This paper describes a measurement system to characterize the performance of Plasma Displays. The system records the light behavior per grayscale and phosphor. With a simulation model and signal analysis, the system predicts the level of several artifacts, like: dynamic false contour, motion-blur, large area flicker, and fade-in/fade-out flicker.

Improved vibroacoustography imaging for nondestructive inspection of materials

We report a method to improve image quality in the nondestructive investigation and visualization of defects using vibroacoustography (VA). Vibroacoustography is an ultrasound-based imaging technique that uses the dynamic (oscillatory) radiation force of low-frequency excitation (within kilohertz range) to remotely vibrate objects and detect the ensuing acoustic emission. This technique is nondestructive and noncontact and has shown numerous capabilities to produce high-resolution images of different types of materials. However, for reflective materials, ultrasound reflects back and forth between the object and transducer, thus establishing standing waves. This phenomenon produces an artifact in the shape of false contours in the VA images. The goal of this study is to investigate the formation of the standing wave artifacts and develop a process called chirp imaging to improve defect visibility and flaw detection capability. Chirp VA experiments are performed on a flawed fiber-reinforced ceramic composite plate and on an electronic chip. To assess the efficacy of the chirp imaging process in removing the standing-wave artifact, the resulting chirp images are compared to “fixed frequency” VA images. Results show that the false contour can be significantly reduced in the image, thus remarkably improving image quality and flaw detection.