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10 Experiment design


If the material is known to contain excessive parallax, and thus known to be potentially uncomfortable, then the duration should be limited.

10.1 Inclusion of reference conditions within the experiment


The results of quality assessments often depend not only on the actual video quality, but also on other factors such as the total quality range of the test conditions, the experience and expectations of the assessors, etc. In order to control some of these effects, a number of dummy test conditions can be added and used as references.

Some of the methods listed above include a “reference” sequence, whenever available, as part of the test sequences set. The “reference” is usually a version of the test sequence that has not undergone any processing (i.e. the original source sequence). The experimental plan might include also the monoscopic version of the “reference” (i.e. only one view of the original source sequence); for example in visual comfort studies it might be useful to use the visual comfort of the monoscopic reference as the baseline.


11 Experiment implementation


Viewer instructions must include guidelines on how to react when subjects feel fatigue or discomfort. See [ITU-T Rec. J.3D-fatigue].



11.1 Informed consent


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11.2 Viewer screening


Some populations are less able to perceive 3D content. Additionally, some people are entirely unable to perceive 3D content (e.g., due to blindness in one eye).

In addition to the conventional visual acuity and color vision test, 3D acuity testing should be performed for the viewer. Therefore, acceptable testing procedures should be provided in the new Recommendation.

Other topics: color blind, eye vision, inter-pupillary distance, etc

11.2.1 Eye vision test


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11.2.2 Color blindness test


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11.2.3 Stereoscopic acuity test


Tentatively, a maximum angle of stereopsis of 140 seconds is recommended. [editor’s note: this level can be changed later as more information regarding the necessary threshold is obtained.]

11.2.1 Inter-pupliary distance


When autostereoscopic monitors are used, inter-pupliary distance is a critical factor. This information should be recorded for each subject. Most autostereoscopic monitors are designed for a fixed inter-pupilary distance, and subjects who deviate to a large extent from that fixed inter-pupilary distance may experience increased crosstalk.

11.3 Instructions and training


Instruction should be tailored to dimension (e.g. depth quality, comfort, etc.) under investigation.

Ethical guidelines are critical, since participants might experience visual discomfort. The subjects must be informed of any possible negative resulting from exposure to the stimuli used in the study. The subjects must be told that they can stop the test at any point, without negative consequence (e.g., the subject may leave the test chamber in the middle of the experiment and still be paid in full).


11.4 Voting sessions


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11.5 Questionnaire or interview


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12 Data analysis


The results should be reported along with the details of the experimental set-up. For each combination of the test variables, the mean value and the standard deviation of the statistical distribution of the assessment grades should be given. [ Editor’s note: Some items can be mandatory while others need to be reported whenever possible. The method to calculate these statistical values is described in Recommendation ITU-R BT.500.

Perception of 3D contents depends on the shooting parameters and resulting horizontal disparities as well as on the viewing environment. For instance, the perception of the same 3D content for different visualization conditions (viewing distance, screen size, image definition, etc.) won’t provide necessarily the same subjective results and the same level of visual comfort. In order to provide reliable results analysis, it is essential to provide experimental parameters presented in table 1. These parameters could be taken into account for results comparison between laboratories as well as for publication issues.


Experimental parameter

Parameter unit

Maximum crossed and uncrossed disparities for each 3D scene content

In pixels or percentage of the display width

Image definition of the display

Number of lines x number of rows

3D video format

Side-by-side, Frame Packing, Top-Bottom, etc.

3D rendering technology

Active shutters, line interleaved display using polarized glasses, autostereoscopic display, etc.

Viewing distance

Meter

Display size (diagonal and/or width and height)

Meter

Maximum luminance on the screen through glasses

cd/m2

Crosstalk level

Percentage of the maximum luminance through glasses

Table 1: experimental parameters needed for results presentation]

Appendix I

General considerations on 3D video quality

(This appendix does not form an integral part of this Recommendation)

The following information should be considered in the development of P.3D-sam (Note: when P.3D-sam is finalized, part of this text will be moved to the appropriate normative section when/if deemed appropriate. The rest will be deleted.)



1. Quality requirements for 3D TV systems – assessment factors

Quality factors generally applied to monoscopic television pictures, such as resolution, color rendition, motion portrayal, overall quality, sharpness, depth, etc., could be applied to 3D television systems. In addition, there are many factors peculiar to 3D television systems. Some of them are discussed below.




  • The goal of 3DTV viewing experience should be to create the illusion of a real environment. which can be watched for an indefinite period of time by the audience with normal visual acuity;

  • The quality of the 3DTV service should be established by two principal parameters sensation of reality (SR) and comfort or ease of viewing; (EV). . These need to be established by subjective evaluations.

  • The sensation of reality a viewer sees depends on the combination of quality factors such as resolution, sharpness, color-fidelity, and a group of quality factors termed “depth cues”. The quality factors define the potential for the sensation of reality, but the actual sensation of reality achieved depends on the combination of the quality factors and the scene content itself. The scene content must 'exploit' the quality factors for their effects to be seen.

  • Quality factors generally applied to monoscopic television pictures, of course, could be applied to 3D television systems. In addition, there would be many factors peculiar to 3D television systems. Some of them are listed below

    1. Depth resolution: spatial resolution in depth direction. Coarse resolution in depth direction may reduce picture quality in 3D television.

    2. Depth motion: a factor related to whether motion or movement along depth direction is reproduced smoothly.

    3. Size distortion “Puppet theatre effect”: the reproduction magnification ratio of an object at the shooting distance (the perceived size) varies with the imaging and display conditions. The resulting distortion in size may make an object be perceived as unnaturally small.

    4. Depth distortion “Cardboard effect”: the imaging and display conditions may reduce the reproduction magnification ratio of depth directions and distort the perception of objects with visually imperceptible thickness. The 3-D positions of stereoscopic objects are perceived stereoscopically but they appear unnaturally thin.

    5. The frame effect: 3D pictures appear highly unnatural when objects positioned in front of the screen approach the screen frame. This unnatural effect is called “the frame effect”. The effect is generally reduced with a larger screen, because observers are less conscious of the existence of the frame when the screen is larger.

  • One of the quality factors in the group of depth cues is binocular disparity - the difference between the pictures seen by the left and right eyes. This can be the strongest depth cue for close objects and has a strong influence on the “potency” of the image. Other depth cues include occlusion (objects hidden behind other objects), relative sizes of known objects, vanishing point perception, and others. Depth cues, except for binocular disparity, are provided in monocular planar images (SDTV, HDTV, and UHDTV).Viewers may not feel comfortable viewing left and right images that have size, verticality, inclination, and brightness differences. Cross-talk between the left and right images may also have an impact on viewing comfort.

  • 3DTV systems are based on the (additional) provision of binocular disparity - the simultaneous provision of point source left and right eye pictures. Basic technologies (3D) provide two images only, regardless of viewer's head position. Other more developed systems (multi-view) provide a greater number of images, but with the same purpose of providing each eye with separate point source images, but which can change depending on the viewer's head position

  • All 3DTV systems displayed on a screen in a single plane (such as a television screen) have limitations for a number of reasons. One of them is the potential conflict between convergence (the object that they eyes point themselves towards) and accommodation (the point on which the lens of the eye focuses) which the two signals gives rise to. The human eye focuses on an object according to the distance to that object. At the same time, we also control the convergence point (gaze point) on the object. Therefore, there is no inconsistency between accommodation and convergence in our everyday life. However when viewing 3D images, the focus point (accommodation) must always be fixed on the screen, independent of the convergence point which is derived from the disparity of the signals. Otherwise, the observer cannot focus clearly. Thus, an inconsistency between accommodation and convergence is introduced in 3D systems Optimizing 3D systems is the process of minimizing the effects of the limitations.

  • Attention should be paid to the distribution of parallaxes in the stereoscopic images. From the correlations between psychological factors and the parallax distribution, we can grasp the essential characteristics of stereoscopic images, e.g., the sense of presence they convey and their ease of viewing. The parallax distribution of stereoscopic images is discontinuous during scene-change frames, where the scene depth and perceived convergence distance change. We need to evaluate how these changes affect the visual discomfort experienced during viewing of stereoscopic images.

  • Visual fatigue caused by parallax 3DTV viewing: visual fatigue caused by viewing stereoscopic motion images is a particular safety concern. Viewers’ repeated adaptation to the discrepancy between eye convergence and accommodation causes a decline of their visual functions and results in visual fatigue.

  • Visual functions vary greatly from person to person, so it is essential to understand that there are individual differences before subjective assessment begins. For instance, there are limits to the binocular parallax of left and right images which a person can fuse into one image; when the parallax exceeds these limits, a double image is perceived. In this situation, depth perception collapses and viewing becomes extremely uncomfortable. For this reason, it is necessary to know the range of binocular parallax over which two images can be fused into one. However, individual differences are vast and will necessitate a study of the stereopsis function of many people.

  • Audio systems also play a part in creating a sense of reality for the viewer, and should be arranged so that both vision and sound work together to heighten reality


2. Assessment methods

The methods described in Recommendations ITU-R BT.500 and ITU-T P910 could be applied for the evaluation of the general picture quality of 3D TV systems as well as sharpness and depth.. When a reference image is available, double-stimulus continuous quality-scale or double-stimulus impairment scale methods can be used. When no reference is available, the single-stimulus categorical judgement method can be used, for example, to identify the merits of 3D systems. Evaluation methods for the assessment of particular factors of 3D television systems require further study.


3. Viewing environment and conditions

The effect of the viewing environment is fundamental on the perception of depth and to the quality of the overall viewing experience. The following situations should be considered:



  • Studio/laboratory environment

  • Home environment

In particular, in conjunction with viewing distance, picture size and subtended viewing angle play a role in the three-dimensional effect as perceived by the viewer.

Two major factors peculiar to 3D display should be taken into consideration: the display frame effect and inconsistency between accommodation and convergence,

It is generally said that the minimum value for depth of field of the human eye is ±0.3 D, where diopter (D) is the reciprocal value of distance (m). This means that we can perceive the image without defocusing when the object is located within ±0.3 D. When viewing 3D television, the accommodation point is fixed on the screen, and therefore 3D pictures should preferably be displayed within this range. Since ordinary television programs include images at infinite distance, the desirable range of depth to be displayed with 3D systems is considered to be within 0 to 0.6 D. Therefore, 0.3 D, i.e. 3.3 m, is considered to be the optimum viewing distance.

Camera parameters (camera separation, camera convergence angle, focal length of lens), resolution of the system and the frame effect should be taken into account in determining viewing conditions (screen size). In the case of HDTV when watching at the standard viewing distance of 3 H (H denotes picture height), the viewing distance of 3.3 m corresponds to a 90-inch screen. In the case of standard definition television (SDTV) when watching at the standard viewing distance of 6 H, this distance corresponds to a 36‑inch screen. A subjective assessment of the relationship between screen size and depth perception was carried out with 3D HDTV system, and the results showed that the most natural depth perception was obtained with a screen size of 120 inches, which corresponds to viewing distance of 2.2 H.

The effective viewing angle should allow 20% angular rotation of head movement in the horizontal plane.
4. Observers

Observers should have normal acuity (see Recommendation ITU-R BT.500 or ITU-T P.910). In addition, they should have normal stereopsis, which has to be checked using special binocular vision test materials.


5 Test materials

Test materials should contain still and motion sequences of natural scenes.

The 3-D effects obtained from stereoscopic pictures depend largely on the shooting conditions, such as camera separation, camera convergence angle and focal length of the lens.

Appendix II

General considerations on 3D video quality

(This appendix does not form an integral part of this Recommendation)

The following information should be considered in the development of P.3D-sam (Note: when P.3D-sam is finalized, part of this text will be moved to the appropriate normative section when/if deemed appropriate and the rest will be deleted. )


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