Research Paper Confirms 8K Visibility
At the recent SID Display Week event in San Jose, one of the papers (P-29) caught our attention as the authors had investigated the “Perceptual Visual Quality of UHD-2/8K on Consumer Display”.
The authors of the paper were Hosub Lee and Bill Mandel of Samsung Research America and image quality maven, Pawel Achtel, the developer of a very high resolution camera and a familiar name to those that read the 8K Association website. One of the challenges of testing the visual effect of 8K is to ensure that the content shown actually contains enough detail to allow a difference to be seen.
The Project
The paper reported a research project performed in the US to check if viewers could see the difference between 4K and 8K images when they were both visible at the same time. As the paper points out, previous studies had tended to use a sequential method, where images were shown at one resolution and then another. The researchers found that viewers struggled to remember the differences between clips when they tested the sequential method. (This coincides with the experience of the writer over many years that to understand the difference between displays, you have to see them side by side).
In the project, two 85” TVs were stacked vertically, one above the other with care being paid to the viewing angles. Both TVs were set into Movie mode as this minimizes the use of processing features such as AI upscaling. Marks were made on the floor at different viewing distances. 40 subjects were tested in the experiment, half of them who had some professional knowledge or experience in video image quality and half of whom did not. Subjects were tested for vision and 87.5% had 20/20 (normal) vision. The participants were not told the purpose of the investigation although they were informed that it was about ‘visual quality’.
When is 8K not 8K?
As we have previously reported, sometimes, content delivered as ‘8K’ i.e. in an 8K format or container does not have the level of detail in it that would allow it to be differentiated from 4K. To ensure this was not the case, the team performed MTF testing on the content. The content was encoded using HEVC (52.2 Mbps for 4K video files and 74.8 Mbps for 8K) as this is a widely used compression system for content delivery in the home. The content was captured using the Achtel Digital Cinema camera which has a horizontal resolution of 9.3K. The content was cropped to 8K for the higher resolution and downscaled in DaVinci Resolve for the 4K.
The MTF analysis showed that there could be distinct sharpness differences in 8K content, even when captured by the same camera. The tests seemed to show that where objects had clearly defined edges (e.g. a ‘mountain’ test video), it led to a higher perception of 8K clarity.
The Results
The tests were performed at different viewing distances and found that the observers could see the difference between the 4K and 8K images both from scoring and from qualitative feedback.
The results of the tests gave the conclusion that from the user’s perspective, 8K is immediately discernable and perceived to present higher clarity levels than 4K in a living room situation. 8K resolution can increase people’s perception of clarity in images, possibly enhancing their overall quality of experience (e.g., realism).
One of the factors that the researchers wanted to understand was the effect of the age of the viewers on the result. Younger subjects (25-34) generally had better visual acuity, but it turned out that the group in the 45-54 age group spotted the improvement in the 8K more easily, which backs up some previous research that showed that older viewers tended to be more sensitive to screen sharpness.
Viewing distance also showed interesting results, with the conclusion being that although there was generally some reduction in the perception of clarity at further viewing distances, this was very content dependent, with one video of a tiger being seen as better more often with 8K in the medium and far viewing distances.
Other Comments
One of the joys for this writer of the Display Week event is the chance to meet with so many people from the industry, on the supplier side and the user side. We were able to have a brief word with Lee about the project.
He was keen to emphasize the importance of the type of content on the impact of 8K. For example, a video that had a lot of detail of animal fur, but with relatively low contrast between adjacent pixels, as the fur (meerkat) was similar in color (gray on gray), was hard to pick out as having higher clarity*.
During the poster presentation, Lee also received the comment that it would be interesting to see the result if content was shown side by side, but on the same display (i.e., butterfly-cut) or if it was shown on an OLED display rather than an LCD. This might be a topic for a further project on this subject. He also heard the comment that screen size could be a factor as in some regions, home space can be too limited for many to enjoy an 85” display.
The full paper will be downloadable by SID members or subscribers from July.
* The writer was for some years interested in black and white photography. Although some of the films and chemicals used produced higher resolution, he often found that viewers of prints with larger grain, but where the edge contrast of the grain was pronounced – so called ‘high acutance development’ – often thought that the images were sharper. This is a not dissimilar effect to the ‘unsharp mask’ filters available in many digital image processing software applications.
