Tuesday 11 April 2017


The 10-point rating system

According to the announcement, Pixiv removed the 10 point rating system and instead put a "Like" button up there. The reason is that "95% of the evaluation function are currently evaluated at 10 points. Also, some problems rose due to the negative usage of the evaluation function".

Well the reason is not very surprising especially to those Osu! players. The average rating rises with time due to multiple reasons (the actual improvement in quality, fan-based community, points became popularity contests etc) and it's occasionally abused like spammed by 1-pointers (this is less common in pixiv because we don't have a in-built critic circle there, but for Osu! this is really obvious).

Two questions arose: should we modify the 10-point rating system to avoid those problems? Is the "Like" button serves as a better indicator than the rating system?

One important observation is that there's almost zero qualification required for someone to vote in both system. You just need to be a member (and played once, for Osu!) then you can rate the map/picture. That makes the source of voters very suspicious and very vulnerable against manipulation. Whether those manipulated accounts are alts or "followers" from the same community are not important -- it's out of the control of the system itself. The system can do nothing but to serve as what the sources wanted. In Pixiv, people spam 10-points for drawings they like/appreciate and that becomes a popularity contest, so why not? A "Like" system is better as for a popularity contest.

As for Osu! the in-build communication platform allows us to manually adhere some metric on the rating system - one may provide a guideline on giving rating and perhaps some suggested examples. Assuming that players (or at least the officials) reached an agreement on the ratings and players are willing to rate according to the guidelines, it is possible to introduce weighting on how valuable a vote is from a player. Players are still free to rate by their free will but votes that are overwhemingly random will receive a relatively lower weight. Ratings may also be scaled by approximating players' voting curve and make it close to the supposed guideline.

It is not possible to eliminate popularity contest because (1) first of all popularity-ish rating would still be the dominating voters after all (2) the minimal weight can't really be too low because at the end of the still this is "players' rating" not "players' rating on quality" - popularity remains as a non-negligible factor, but what we did is to reduce the power of pure haters/lovers and that gives a more representative overall rating.

A last note: although "my favourite" function exists in both Pixiv and Osu!, I would still prefer the "Like" button for Pixiv and the rating system to Osu! because Osu! system provide possibility of improving the rating system and we really have so many metric that has high correlation to pure popularity - instantaneous and long term play count etc that we don't really need another useless button to show that explicitly.

3D Touhou

Blender these days are powerful to simulate these nasty bullet hell screens and this is one of my favourite. But my question comes from the perspective of the characters themselves.

It's extremely interesting to see how the authors blend a 2-dimensional spell card into a 3-dimensional space so that the bullets (at least apparently) fill the space. But what if we really want to fill the space and wipe those characters off the screen? What kind of spellcards should we use?

- For spherical bombs you would like to pack the space so that no escape route exists. That's essentially the sphere packing problem...that clearly gets harder at higher dimensions.

- If you want to shoot radial danmaku that floods the character without allowing it to move towards you, the thing you will need is to increase the bullet density on the manifold as the tangent plane of the character per Hausdorff measure in the space (?!?) -- in simple terms, it grows in the volume of (n-1)-sphere for Euclidean spaces, which is not looking good either.

- Laser type weapons. Using spherical coordinates it is also clear that aiming with the correct angles in all those angle-based coordinates has exponential difficulty as well.

The best weapon might be Sakuya's ability of freezing time because time as an extra dimension is independent of the space dimension so it works equally well despite the change in space...but well I am not a resident of Gensokyo so I'd better leave the problem to them...

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