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

Image Quality Evaluation in Lossy Compressed Images

This research focuses on the quantification of image quality in lossy compressed images, exploring the impact of digital artefacts and scene characteristics upon image quality evaluation. A subjective paired comparison test was implemented to assess perceived quality of JPEG 2000 against baseline JPEG over a range of different scene types. Interval scales were generated for both algorithms, which indicated a subjective preference for JPEG 2000, particularly at low bit rates, and these were confirmed by an objective distortion measure. The subjective results did not follow this trend for some scenes however, and both algorithms were found to be scene dependent as a result of the artefacts produced at high compression rates. The scene dependencies were explored from the interval scale results, which allowed scenes to be grouped according to their susceptibilities to each of the algorithms. Groupings were correlated with scene measures applied in a linked study. A pilot study was undertaken to explore perceptibility thresholds of JPEG 2000 of the same set of images. This work was developed with a further experiment to investigate the thresholds of perceptibility and acceptability of higher resolution JPEG 2000 compressed images. A set of images was captured using a professional level full-frame Digital Single Lens Reflex camera, using a raw workflow and carefully controlled image-processing pipeline. The scenes were quantified using a set of simple scene metrics to classify them according to whether they were average, higher than, or lower than average, for a number of scene properties known to affect image compression and perceived image quality; these were used to make a final selection of test images. Image fidelity was investigated using the method of constant stimuli to quantify perceptibility thresholds and just noticeable differences (JNDs) of perceptibility. Thresholds and JNDs of acceptability were also quantified to explore suprathreshold quality evaluation. The relationships between the two thresholds were examined and correlated with the results from the scene measures, to identify more or less susceptible scenes. It was found that the level and differences between the two thresholds was an indicator of scene dependency and could be predicted by certain types of scene characteristics. A third study implemented the soft copy quality ruler as an alternative psychophysical method, by matching the quality of compressed images to a set of images varying in a single attribute, separated by known JND increments of quality. The imaging chain and image processing workflow were evaluated using objective measures of tone reproduction and spatial frequency response. An alternative approach to the creation of ruler images was implemented and tested, and the resulting quality rulers were used to evaluate a subset of the images from the previous study. The quality ruler was found to be successful in identifying scene susceptibilities and observer sensitivity. The fourth investigation explored the implementation of four different image quality metrics. These were the Modular Image Difference Metric, the Structural Similarity Metric, The Multi-scale Structural Similarity Metric and the Weighted Structural Similarity Metric. The metrics were tested against the subjective results and all were found to have linear correlation in terms of predictability of image quality