Just noticeable gamma differences and acceptability of sRGB images displayed on a CRT monitor

The standard RGB colour space (sRGB) has been proposed as a means for obtaining accurate reproduction of colour and tone for images displayed across the Internet, provided that they are viewed under the reference display and viewing conditions defined in the standard. It has been found, however, that typical display and viewing conditions when accessing online images vary and therefore deviate from the reference sRGB conditions. One of the parameters that may affect the perceived quality of online images is the gamma setting of the display. In this work psychophysical experiments were conducted to determine the imperceptibility and acceptability of gamma differences of sRGB images when they are viewed on cathode ray tube displays. These experiments were carried out under both controlled and uncontrolled display and viewing conditions. The results of these experiments are presented and discussed, including the estimated points of subjective equality and the just noticeable difference of gamma values

Influence of luminance and resolution on the perceived quality of black-and-white images on soft displays

The perceived quality of an image displayed on a computer monitor depends on a number of different factors that can affect viewers' preferences. Two of these are the luminance of the monitor display and the resolution of the image. The effect of luminance is of interest for applications such as on-line access of images where the computer displays used for viewing these images could have different luminance settings. A relationship between cathode-ray tube (CRT) display luminance and resolution has been shown in previous studies. It was therefore interesting to investigate whether there is a relationship between image resolution and CRT display luminance on perceived image quality. Image resolution is related to the image file size, which is an important factor for applications such as on-line access of images. This work used a CRT display to study the effect of the above-mentioned factors on the perceived quality of the displayed image. Three sets of black-and-white images, each set with a different resolution, were presented to observers at three brightness settings of the computer monitor. Results are discussed regarding the effect of monitor display luminance and image resolution on perceived image quality and the interaction between them. Evaluation of results is further extended to the influence of the different backgrounds of the images. The scene content of the images was also shown to affect the viewers' judgement

Comparative performance between human and automated face recognition systems, using CCTV imagery, different compression levels and scene parameters

In this investigation we identify relationships between human and automated face recognition systems with respect to compression. Further, we identify the most influential scene parameters on the performance of each recognition system. The work includes testing of the systems with compressed Closed-Circuit Television (CCTV) footage, consisting of quantified scene (footage) parameters. Parameters describe the content of scenes concerning camera to subject distance, facial angle, scene brightness, and spatio-temporal busyness. These parameters have been previously shown to affect the human visibility of useful facial information, but not much work has been carried out to assess the influence they have on automated recognition systems. In this investigation, the methodology previously employed in the human investigation is adopted, to assess performance of three different automated systems: Principal Component Analysis, Linear Discriminant Analysis, and Kernel Fisher Analysis. Results show that the automated systems are more tolerant to compression than humans. In automated systems, mixed brightness scenes were the most affected and low brightness scenes were the least affected by compression. In contrast for humans, low brightness scenes were the most affected and medium brightness scenes the least affected. Findings have the potential to broaden the methods used for testing imaging systems for security applications

A case study in identifying acceptable bitrates for human face recognition tasks

Face recognition from images or video footage requires a certain level of recorded image quality. This paper derives acceptable bitrates (relating to levels of compression and consequently quality) of footage with human faces, using an industry implementation of the standard H.264/MPEG-4 AVC and the Closed-Circuit Television (CCTV) recording systems on London buses. The London buses application is utilized as a case study for setting up a methodology and implementing suitable data analysis for face recognition from recorded footage, which has been degraded by compression. The majority of CCTV recorders on buses use a proprietary format based on the H.264/MPEG-4 AVC video coding standard, exploiting both spatial and temporal redundancy. Low bitrates are favored in the CCTV industry for saving storage and transmission bandwidth, but they compromise the image usefulness of the recorded imagery. In this context, usefulness is determined by the presence of enough facial information remaining in the compressed image to allow a specialist to recognize a person. The investigation includes four steps: (1) Development of a video dataset representative of typical CCTV bus scenarios. (2) Selection and grouping of video scenes based on local (facial) and global (entire scene) content properties. (3) Psychophysical investigations to identify the key scenes, which are most affected by compression, using an industry implementation of H.264/MPEG-4 AVC. (4) Testing of CCTV recording systems on buses with the key scenes and further psychophysical investigations. The results showed a dependency upon scene content properties. Very dark scenes and scenes with high levels of spatial–temporal busyness were the most challenging to compress, requiring higher bitrates to maintain useful information

Aspect of image quality and the Internet

The colour reproduction of images viewed across the Internet is an area of significant importance due to the recent increase in the levels of on-line shopping and the number of digital imaging libraries accessible over the Internet. The quality of images viewed on-line can be affected by variables such as the display and viewing conditions. This thesis describes a programme of work that investigates existing methods for controlling the colour and tone reproduction of images accessed over the Internet. The reproduced colour of images viewed across the Internet may be affected by transformations applied to provide consistent colour reproduction among different web browsers and operating systems for 8-bit depth graphics card adapter display systems. In this work objective measures (ΔE*ab) are used to determine the range of colours that can be reproduced with perceptible colour differences. It is shown that a 24-bit depth image displayed under the 8-bit depth setting of a graphics card adapter results in lower colour differences than the 216 colour image displayed under the same bit depth setting. The standard RGB colour space (sRGB) has been proposed as a means for obtaining accurate reproduction of colour and tone for images displayed across the Internet provided that they are viewed under the reference display and viewing conditions defined in the standard. A survey has indicated that the typical display and viewing conditions of a sample of Internet users frequently deviate from the reference sRGB standard. Psychophysical experiments to determine the perceptibility and acceptability of differences in CRT monitor gamma under controlled and uncontrolled display and viewing conditions are reported. The point of subjective equality (PSE) and the just noticeable difference values of gamma for the imperceptibility and acceptability of gamma differences in images are estimated. A survey of the gamma settings of a sample of computer monitors is reported. It is shown that, due to deviations in the gamma settings, an sRGB image displayed on 50% of the monitors surveyed will be judged as perceptibly different from a monitor complying with the sRGB standard, while 40% of monitors will be judged as acceptably similar, although observers may detect a difference

The effect of reduced color depth on the color reproduction of web images

Accurate color reproduction of images viewed across the Internet can be a difficult task due to problems,which include variable viewing conditions.The standard RGB color space (sRGB)has been proposed to overcome this problem without the need for special software applications. Different graphics adapter bit-depth settings also affect the number of displayable colors on a web browser.For this reason,the 216 colors "web safe" palette has been introduced. To understand the effect of device characteristics on the process of image reproduction via the web,the characterization of some common devices was carried out. This created a controlled environment whereby experiments could be conducted with reproducible results.Device characterization also enabled the accurate conversion of scanned images to sRGB by using a step-by-step process that followed the specification of the color space. The investigation of color reproduction on a Cathode Ray Tube (CRT)display under different bit-depth settings has been conducted using a suitable test target,which was converted to the sRGB color space and also to the "web safe"(216 colors)palette.These two images were displayed under the 8-bit and 24-bit depth settings of the graphics card adaptor. Colorimetric measurements concerning color differences were performed and evaluated.Experimental investigation on the true color (24-bit color depth)image while displayed under 8-bit setting via different browsers followed.The outcome of the investigations was evaluated and combined to determine the relationship between the different parameters described above and their effect on image quality

Perceptibility and acceptability of gamma differences of displayed sRGB images

The standard RGB (sRGB) colour space was developed to ensure accurate colour reproduction of images when viewed on Cathode Ray Tube (CRT) displays under specified conditions. Typical display and viewing conditions may vary, however, especially when accessing on-line images. Previous work has been carried out on the effects of different display white points and phosphor chromaticities and on modeling gamma tolerances and display conditions for the sRGB colour space. The work described in this paper investigates the effects of different gamma values on viewing sRGB images. Ten images with variable scene content were selected and converted to the sRGB colour space. A set of test images was generated for each scene by adjusting the display gamma of the sRGB image to a value in the range 1.8 to 2.6. Comparative judgments were conducted in which the reference sRGB image (calibrated for displays with gamma equal to 2.2) in each set was compared to each of the images adjusted to different display gammas. Each pair of images was displayed on the same monitor using software specially developed for the purpose. In the first series, the observers' responses concerned the perceptible difference between the reference sRGB image and the images calibrated to the different gamma value. In the second test their response concerned the acceptable gamma difference. The experimental results were evaluated and discussed. Conclusions were drawn regarding the effects of gamma differences on perceived image quality while viewing on-line sRGB images

Acceptable bit-rates for human face identification from CCTV imagery

The objective of this investigation is to produce recommendations for acceptable bit-rates of CCTV footage of people onboard London buses. The majority of CCTV recorders on buses use a proprietary format based on the H.264/AVC video coding standard, exploiting both spatial and temporal redundancy. Low bit-rates are favored in the CCTV industry but they compromise the image usefulness of the recorded imagery. In this context usefulness is defined by the presence of enough facial information remaining in the compressed image to allow a specialist to identify a person. The investigation includes four steps: 1) Collection of representative video footage. 2) The grouping of video scenes based on content attributes. 3) Psychophysical investigations to identify key scenes, which are most affected by compression. 4) Testing of recording systems using the key scenes and further psychophysical investigations. The results are highly dependent upon scene content. For example, very dark and very bright scenes were the most challenging to compress, requiring higher bit-rates to maintain useful information. The acceptable bit-rates are also found to be dependent upon the specific CCTV system used to compress the footage, presenting challenges in drawing conclusions about universal ‘average’ bit-rates