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Hills and crest lines
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 Art De La Guerre Index du Forum > Rules question V4
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elsleyra
Javelinier


Inscrit le: 13 Sep 2022
Messages: 17
Localisation: Newcastle, Australia
MessagePosté le: Mar Nov 08, 2022 5:56 am    Sujet du message: Hills and crest lines Répondre en citant
(Questions from a new player)
‘TERRAIN SIZE AND SHAPE’ p70 lists types of terrain that must be rectangular then states “Other terrain elements are irregular in shape with rounded cornersâ€. So hills are irregular in shape.
‘Gentle or steep hills’ p71 states “Hills can also have a crest line passing through their peak cutting the hill lengthwise into two equal parts.â€
The diagram ‘HILLS AND VISIBILITY’ p71 shows a hill and a crest line that is a straight line and which cuts the hill into two parts, but I can’t see any measure by which the parts would be considered equal (area, shape, symmetry, perimeter ...).

So, I’d like to work with the assumption that we expect players to be sensible and not create terrain pieces contrived to deliver an unreasonable advantage, but still allow some variation in hill shapes. That will work fine for friendly games, but I’m unclear what restrictions tournament umpires would apply.

Can the crest line be bent or curved (as long as the bend or curve is not too extreme)?
How equal must the two parts of the hill separated by the crest line be?
Should I just be avoiding any use of hills that are not approximately rectangular or oval in shape?
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MC_Delicatessen
Auxiliaire


Inscrit le: 30 Juil 2020
Messages: 87
MessagePosté le: Mar Nov 08, 2022 4:25 pm    Sujet du message: Répondre en citant
Have you looked in the errata?

http://www.artdelaguerre.fr/adlg/v3/upload/rules/en/details/Errata_ADG_V4_English.pdf
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KevinD
Centurion


Inscrit le: 23 Aoû 2021
Messages: 499
Localisation: Texas
MessagePosté le: Mar Nov 08, 2022 5:22 pm    Sujet du message: Répondre en citant
Which particular bit of the errata do you think is relevant here?

I assume that crests must be straight (it is a “lineâ€) - to prevent weird gamey mechanics that might otherwise occur.

I assume the crest line should bisect the hill such that (approximately) half the area of the hill is on either side.

I think the rules prohibit having the center point (substantially) nearer one side of the hill than the other side (which given terrain is rarely rotated can give an additional advantage to the player placing it).

(Also, but not directly related to this question, I assume convex shaped terrain (other perhaps than very gently convex terrain like a kidney shaped piece) is prohibited as you can achieve weird visibility effects by using such terrain which can interfere with ambushes and LOS - particularly if such convexity is not symmetrical so the player placing it gains a benefit by having the convexity closer to his army.)
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elsleyra
Javelinier


Inscrit le: 13 Sep 2022
Messages: 17
Localisation: Newcastle, Australia
MessagePosté le: Mer Nov 09, 2022 8:56 am    Sujet du message: Répondre en citant
I too had thought it likely that the rules assume that the crest line will be a straight line and that, for the purpose of visibility of units on the hill, the crest line is assumed to extend beyond the hill (across open plain, fields and any terrain other than hills) ... given the diagram on p71. That would stop people producing terrain contrived to deliver an unreasonable, ’gamey’ setup to hide some units and does make it straightforward to ambush from behind a crest line without needing to consider the impact of horizon lines on visibility, but does come with its own interesting outcomes:
- The LH in the diagram on p71 can draw (otherwise) valid LOS lines to a number of points on the edge of Zone B without crossing any part of the hill. It appears that the visibility to these points is somehow blocked by the extended crest line in the flat field between the LH and those points.
- It is not clear if a unit with the corners of its shooting edge located on a couple of the points noted above would be able to see, and shoot at, the LH. If they can, despite not being visible to the LH, then it would appear that being positioned on a bare hill with a crest line to their side can make them invisible. This example could presumably be extended to include any unit which could draw its LOS lines to the LH without crossing the crest line on the hill or the shooting unit’s horizon line.
- In order for the extended crest line to not block visibility, a unit must pass the (extended) crest line, but I’m not clear what constitutes passing the crest line, at least in the case where the (extended) crest line passes through the unit at the end of its move. Will a single corner that could draw an LOS line to an enemy unit without crossing extended crest line be sufficient? Or should it be based on the centre of the unit (as per determining the horizon line)? I assume that shooting will require both corners of the shooting edge to be on the same side of the extended crest line as the target.
- (Physically) weird outcomes are best minimised by only using hills created so that the crest line runs across the widest part of the hill.
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Za Otlichiye
Signifer


Inscrit le: 07 Sep 2021
Messages: 341
Localisation: Lovecraft country (and you Dan?)
MessagePosté le: Jeu Nov 10, 2022 12:20 am    Sujet du message: Répondre en citant
Yikes! that's a subtle problem!
I don't think it applies to the crestline which is fixed and the same for both units.
But the horizon line needs to be infinite if the LOS passes from, to, or across the hill. Otherwise, yes you can get LOS in one direction and not in the other.
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elsleyra
Javelinier


Inscrit le: 13 Sep 2022
Messages: 17
Localisation: Newcastle, Australia
MessagePosté le: Sam Nov 12, 2022 7:35 am    Sujet du message: Répondre en citant
OK, I now see that, per the first sentence of the second paragraph of ’Gentle or steep hillls’ on p71: “A hill blocks visibility beyond its crest line and beyond its horizon lineâ€, so crest lines and horizon lines should be treated the same way and should block visibility of both the viewer and the target. Also, it appears from the diagram at the top of p71 that both the horizon line and crest line extend beyond the hill onto the surrounding flat terrain and can block visibility across that flat terrain, but only if you’re looking up at or down from a hill. I guess you pay your money and take your choice of “weird gamey mechanicsâ€. The only reason I can see for not adopting the (more intuitively correct) approach of having visibility blocked if the line of sight blocked if it passes through a crest line or horizon line on a hill is to enable more ambushes on hills, by reducing the ability of scouts or troops who are very nearly side on to a hill with a crest line from seeing nearly half of the hill behind the crest line.

Now is probably also a good time to suggest that either the rule or the translation of the second dot point above “Brush†on p71 needs some work. It states that “A unit placed behind the horizon line or the crest line of a hill is only visible if the enemy passes both the horizon line and the crest line or approaches within 1UD of the unit.†As the horizon line changes when a unit moves, passing though the current horizon line of a hill is only possible if the unit passes through the peak. It can pass through a previous/original horizon line any number of ways, some of which may still not leave the target unit visible. It can also potentially move so that it doesn’t pass the original horizon line but still end up in a position where there is no current horizon line between it and its target. Given the rule referenced above “A hill blocks visibility beyond its crest line and beyond its horizon lineâ€, I don’t think this dot point actually needs anything more than “A unit placed behind the horizon line or the crest line of a hill becomes visible if the enemy approaches within 1UD of the unit.â€
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Za Otlichiye
Signifer


Inscrit le: 07 Sep 2021
Messages: 341
Localisation: Lovecraft country (and you Dan?)
MessagePosté le: Sam Nov 12, 2022 5:44 pm    Sujet du message: Répondre en citant
Because crest lines are fixed either both units are visible to each other, or they are not. You can make the crestline extended or terminated at the edge of the hill. Either will work although the former is consistent with what you need to do with horizon lines.
The problem with horizon lines is that they can be different for each element and thus you can have asymmetrical visibility.

As to your second concern, I believe the correct interpretation of "passed" is when the horizon line is no longer between the two elements.

*****

I've looked at this a bit more and diagrammed some examples. The correct solution is different and simpler.
Remember that a LOS is made up of two sight lines, from each corner of the shooting (or viewing) edge to a single point on an enemy edge. This means that unit A might have LOS to unit 1, but not vice versa. However, each sight line must be symmetrical. If you can trace an unobscured line from unit A to unit 1, it should be unobscured when traced from unit 1 to unit A. But because of the way horizon lines work, this is not always true. Thus, when tracing across hills, you must consider both the shooter's and target's horizon lines, and if either is crossed, then the sight line is blocked.
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