Herramientas del Kaizen implantadas

When combined, the four contributing terms allow evaluation of a colour scheme in a manner that should correlate with the appeal of a colour scheme to an average human observer, but there are circumstances in which it will fail.

Minor reductions in the colour strength balance and wireframe alignment contri- butions to a colour scheme’s fitness will not significantly impact the quality of the colour scheme as a whole. However, the same argument does not apply to textual readability or distinguishability: if any item of text is unreadable, or any control is indistinguishable, the impact on the overall score may not be great, but the inter- face will be unusable13. Therefore, if any item of text is unreadable, or any pair of items that should be distinguishable have colours that are not sufficiently different,

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The readability constraint returns true if item i & j must be sufficiently distinct to be easily readable.

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There are exceptions: not all broken distinguishability constraints will render an interface unusable. The developer may have defined some distinguishability constraints for aesthetic reasons, and if these are not enforced, appearance, but not usability, will be affected. However, as there is no indication of the reason for any constraint, all distinguishability constraints are enforced.

the overall readability or distinguishability score for the colour molecule is multiplied by a penalty factor. The value of this multiplier is progressively reduced from one to its minimum value over a range of readability or distinguishability, to model the fact that text becomes progressively less readable (or distinguishable) over a small range of colour differences. Unacceptable separation Acceptable separation Marginally acceptable Penalty 1

Colour space separation

0

l

u

q

Figure 4.10: The penalty function applied to the overall scores for both distinguishability and readability to ensure all items are easily readable or distinguishable

Although the threshold values differ, the same method is used to calculate the penalty (if any) to be applied to the distinguishability scores (for user-defined pairs of items) and readability scores (for textual items against their backgrounds). The method of calculating the penalty factor will be detailed using readability as the example.

The raw score (R0 defined above) is not penalised if all the readability constraints are satisfied: that is, if the lightness differences between all items and their backgrounds – when translated from the abstract colour space to real colours – are sufficient for good readability. If the readability of any of the item pairs is unacceptable, the raw score is scaled down to a fraction of its initial value, significantly penalising the fitness score for the molecule. For the intermediate cases, where a text item is discernable but not yet sufficiently different from its background to be easily readable, the raw score is a linear interpolation between penalised and raw values.

The readability score for each item-background pair is translated to an acceptabil- ity rating by a function that returns 0 if the readability score (v) is less than a lower threshold (l), returns 1 if v is above an upper threshold (u), or returns an interpo- lated value if l < v < u. Figure 4.10 shows the form of this function. The lower (l) and upper (u) thresholds indicate the thresholds of marginal and acceptable readability.

The rating of acceptability Ra is given by:

Ra(v, l, u) =      0 if v < l — contrast is unacceptable 1 if v ≥u — contrast is acceptable

(v−l)/(u−l) if l≤v < u — item is discernable but not acceptable

(4.19)

4.20. This function finds the acceptability rating for all pairs of items that should be readable or distinguishable. All of these values will be between 0 and 1. As it is desired for any single item being unacceptable (0) to affect all values, the values are multiplied together (the multiplicative product

k

Q

i=1

Ra(vi, l, u) in the equation), so that:

• any single unacceptable readability or distinguishability value will force a 0 result.

• if items are acceptably readable or distinguishable, the product is 1.

• if all items are discernable but not yet all acceptable, the result is between 0 and 1.

This multiplicative product is then combined with the fully penalised value (q), so that: if the multiplicative product returns 0, the original readability or distinguishability score is scaled down by q (i.e. qx); if the product is 1, the original score x is returned unchanged; and otherwise the result is an interpolation betweenqx and x:

P(x) =x q+ (1−q) k Y i=1 Ra(Liv, l, u) ! (4.20) where:

P(x) − returns the final value ofx , the raw readability/distinguishability score

x − R0 orD0— the molecule’s overall Readability/Distinguishability score

Ra(·) − the rating of acceptability interpolation function – equation 4.19

q − 0< q≤1 – the penalty factor for unacceptable scores (e.g. 0.8)

Lm − list of Readability/Distinguishability constraint pairs for the molecule

k − the number of item pairs with a Readability/Distinguishability constraint

vi − the Readability/Distinguishabilty score of a specific item/background pair

l − discernability threshold value for Readability/Distinguishability

u − acceptability threshold value for Readability/Distinguishability

Using the threshold values for discernability and acceptability derived from human per- ception, this function modifies the raw score for the readability (or distinguishability) term of a molecule. It is not intended as an exact analog of a human evaluation, but one in which the raw score is unchanged if all required elements are readable (or distin- guishable), but a single unacceptable element penalises the overall score. The penalty is reduced once all elements are at least discernable, and removed completely when all are acceptable.

The threshold values used in the Colour Harmoniser prototype are 10 & 20 ∆E for distinguishability (were determined empirically, and are discussed further below), and the 24 & 48 ∆L14 for readability. The penalty factor q is 0.8, which penalises the raw

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scores by 20% if any constraint is unacceptable. The 0.8 is arbitrary, but sufficient to ensure that flawed schemes are significantly downgraded.

The ∆E limits for calculating the separation required for visually distinguishable elements are based on the assumption that the size of the elements (e.g. buttons, panels, and backgrounds) is substantial. However, if the target interface includes semantically important finely detailed graphical elements (e.g. the thin lines and symbols used in a CAD package), the distinguishability term would need to use the lightness difference (∆L) in its evaluation, not the colour space difference (∆E) (Klassen et al., 1998).