Amos Tversky works have shown that human similarity judgement is not a metric (in a mathematic sens). In particular the following properties are not always verified:

1. Minimality :
2. Symmetry : .
3. Triangular inequality : .

Computer vision shows more and more interests in Tversky’s results. In their introduction, many publications try to illustrate the non-verification of the previous properties. My point is that the examples of the non-verification of the triangular inequality are not correct (for images).

By example, in a projet report (Graphem – in french) on the visual similarity, the authors propose these images:

A                               B                               C

1. left (A) and right (C) images can be judged quite dissimilars.
2. But middle image (B) can be judged similar to (A) et à (C).
3. The distance d(A, C) would thus be greater than the sum d(A,B)+d(B,C), contradicting the triangle inequality.

1. Ok for the first point, one can imagine an experience reinforcing these facts. The distance d(A, C) is thus obtained “high”. It could be discussed on the precise measure. Or if the measure is d(A, C) or d(C, A). But this can be admitted.
2. If B is judged similar to A and C, the measures d(B, A) and d(B, C) must be “low”, in  any case smaller than d(A, C).

For the last point, a conclusion on the triangle inequality can only be given from a comparison between d(A, C) and the sum d(B, A) + d(B, C). But the numerical values are unknown, or not given by the authors. It is thus impossible to compute the sum and thus impossible to conclude. More formally, we have:

and

.

The only possible conclusion is , but not .

According to the author: with respect to a partial registration the triangle inequality is not validated d(a, b) + d(b, c) <= d(a, c). My previous commentaries still apply, i don’t understand how the author produces its conclusion.

In order to be complete, in E. Baudrier PhD thesis (p. 67) (that i supervised), the same kind of examples is reproduced:

There, the author doesn’t try to compute any sum and its argument is close to Tversky one’s in “Features of Similarity” :

Here Tversky makes a confusion between transitivity and triangular inequality. Does the former implies the latter ? It is not proved! Elsewhere in its publication, Tversky points out that visual stimulus are different in nature of others stimulus (verbal ones by example). Can visual stimulus lead to a verification of the triangular inequality? I don’t think so but still can’t find indisputable example of this in the case of visual objects.

FMN.