ISO 20462-3:2012 写真—画質を推定するための精神物理学的実験方法—パート3：品質定規法 | ページ 6

3.1

artefactual attribute

attribute of image quality that, when evident in an image, nearly always leads to a loss of overall image quality

Note 1 to entry: The commonly used terms"defect" and"impairment" are similar in meaning.

3.2

attribute

aspect, dimension, or component of overall image quality

3.3

digital reference stimuli

DRS

set of digital images used in the softcopy ruler, which vary in sharpness and are calibrated against the standard quality scale (SQS) when suitably displayed and viewed

Note 1 to entry: The DRS will be available at the Standards Resources link at www.imaging.org.

3.4

image quality

impression of the overall merit or excellence of an image, as perceived by an observer neither associated with the act of photography nor closely involved with the subject matter depicted

Note 1 to entry: The purpose of defining image quality in terms of third-party (uninvolved) observers is to eliminate sources of variability that arise from more idiosyncratic aspects of image perception and pertain to attributes outside the control of imaging system designers.

3.5

instructions

set of directions given to the observer for performing the psychophysical evaluation task

3.6

just noticeable difference

JND

stimulus difference that leads to a 75:25 proportion of responses in a paired comparison task

3.7

magnitude estimation method

psychophysical method involving the assignment of a numerical value to each test stimulus that is proportional to image quality; typically, a reference stimulus with an assigned numerical value is present to anchor the rating scale

Note 1 to entry: The numerical scale resulting from a magnitude estimation experiment is usually assumed to constitute a ratio scale which, ideally, is a scale in which a constant percentage change in value corresponds with one JND. In practice, modest deviations from this behaviour occur, complicating the transformation of the rating scale into units of JNDs without inclusion of unidentified reference stimuli (having known quality) among the test stimuli.

3.8

multivariate

varying in multiple attributes of image quality

3.9

observer

individual performing the subjective evaluation task in a psychophysical method

3.10

paired comparison method

psychophysical method involving the choice of which of two simultaneously presented stimuli exhibits greater or lesser image quality or an attribute thereof, in accordance with a set of instructions given to the observer

Note 2 to entry: However, if a series of stimuli having no large gaps are assessed, the differences between more widely separated stimuli may be deduced indirectly by summing smaller, reliably determined (unsaturated) stimulus differences. The standard methods for transformation of paired comparison data to an interval scale (a scale linearly related to JNDs) perform statistically optimized procedures for inferring the stimulus differences, but they may yield unreliable results when saturated responses are included in the analysis.

3.11

preferential attribute

attribute of image quality that is invariably evident in an image, and for which the preferred degree is a matter of opinion, depending upon both the observer and the image content

Note 1 to entry: Because the perceived quality associated with a preferential attribute is dependent upon both the observer and image content, in studies involving variations of preferential attributes, particular care is needed in the selection of representative sets of stimuli and groups of observers.

Note 2 to entry: The term"noticeable" in"just noticeable difference" is not linguistically strictly correct when applied to a preferential attribute, but is nonetheless retained in this part of ISO 20462 for convenience. For example, the higher contrast stimulus of a pair differing only in contrast might be readily identified by all observers, whereas there might be a lack of consensus regarding which of the two images was higher in overall image quality. Nonetheless, if the responses from the paired comparison for quality were in the proportion of 75:25, the image chosen more frequently would be said to be one JND higher in quality. The JND is best regarded as a measurement unit tied to the predicted or measured outcome of a paired comparison.

3.12

psychophysical method

experimental technique for subjective evaluation of image quality or attributes thereof, from which stimulus differences in units of JNDs may be estimated

3.13

quality just noticeable difference

quality JND

measure of the significance or importance of quality variations, corresponding to a stimulus difference that leads to a 75:25 proportion of responses in a paired comparison task in which multivariate stimuli pairs are assessed in terms of overall image quality

Note 1 to entry: See attribute JND (3.3) and quality JND (3.14) in ISO 20462-1:2005 for greater detail.

3.14

quality ruler method

psychophysical method that involves quality or attribute assessment of a test stimulus against a series of ordered, univariate reference stimuli that differ by known numbers of JNDs

3.15

reference stimulus

image provided to the observer for the purpose of anchoring or calibrating the perceptual assessments of test stimuli in such a manner that the given ratings may be converted to JND units

Note 1 to entry: The plural is reference stimuli.

3.16

scene

content or subject matter of an image, or a starting image from which multiple stimuli may be produced through different experimental treatments

Note 1 to entry: Typically, stimuli depicting the same scene are compared in a psychophysical experiment because it is the effect of the treatment that is of interest, and differences in image content could cause spurious effects. In cases where scene content is not matched, a number of scenes should be used so that scene effects may be expected to average out.

3.17

standard quality scale

SQS

Note 1 to entry: SQS₁ (primary SQS) denotes values obtained through assessments traceable to the standard reference stimuli (SRS). SQS₂ (secondary SQS) denotes values obtained through assessments traceable to the digital reference stimuli (DRS) or the average scene relationship (see 7.2).

3.18

standard reference stimuli

SRS

set of reflection prints used in the hardcopy quality ruler, which vary in sharpness and are calibrated against the standard quality scale (SQS)

Note 1 to entry: The SRS will be available at the Standards Resources link at www.imaging.org.

3.19

stimulus

image presented or provided to the observer either for the purpose of anchoring a perceptual assessment (a reference stimulus) or for the purpose of subjective evaluation (a test stimulus)

Note 1 to entry: The plural is stimuli.

3.20

suppression

perceptual effect in which one attribute is present in a degree that seriously degrades image quality and thereby reduces the impact that other attributes have on overall quality, compared to the impact they would have had in the absence of the dominant attribute

Note 1 to entry: To generate reference stimuli that are separated by a specified number of JNDs based on variations in one attribute, it will be necessary to ensure that other attributes do not significantly suppress the impact of the varied attribute.

3.21

test stimulus

image presented to the observer for subjective evaluation

Note 1 to entry: The plural is test stimuli.

3.22

treatment

controlled or characterized source of the variations between test stimuli (excluding scene content) that are to be investigated in a psychophysical experiment

Note 1 to entry: Different treatments may be achieved through hardware or software changes, or may be numerical simulations of such effects. Typically, a series of treatments is applied to multiple scenes, each generating a series of test stimuli. The effect of the treatment may then be determined by averaging the results over scene and observer to improve signal-to-noise and reduce the likelihood of systematic bias.

3.23

triplet comparison

psychophysical method that involves the simultaneous scaling of three test stimuli with respect to image quality or an attribute thereof, in accordance with a set of instructions given to the observer

Note 1 to entry: The triplet comparison method is described in more detail in ISO 20462-2.

3.24

univariate

varying only in a single attribute of image quality