While testing a new mapper, I discovered distinct banding artifacts in some underwater terrain.
The confirmed bands run in an east-west direction and are always at the same y coordinates modulo 4. This causes them to run in straight lines across the map. In bulk, the bands show up as clear straight lines running across the map if the map is shaded to highlight differences in underwater terrain elevation. Up close, the bands can be seen as distinctive flat underwater cliffs. These bands are most obvious underwater near deep ocean islands.
North-south bands are strongly suspected to occur with similar behaviour. Consider these to exist as well.
The screenshots show the issue. The terrain is shaded so that terrain is darkened as if lit from the north. It is implemented by reducing the brightness of a pixel by about 5% for every block the north block is higher than the south block.
Map seed is 13.
Screenshots:
(1) Mapper output showing the terrain banding. Top left has coordinates -1024, -1024 and the centre is at the origin.
(2) Closeup of underwater terrain at (174, -764).
(3) Closeup of underwater terrain at (238, -748).
This banding has also been confirmed in 1.12.2 and in 1.8.9.
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Yes, the underwater screenshots were taken at that location (selected locations on the south side of the triple island near the top of the map). I have uploaded a cropped version of the top of the map with the approximate coordinates marked with crosses. Examination of the map shows a number of other instances, mostly near deep ocean islands. I intend to perform a statistical analysis later.
The impression I get after examining several instances is that the terrain is compressing five blocks of horizontal generation into four by omitting every fifth block. This is mostly apparent near deep ocean islands, which appear to be a special case in terrain generation.
Confirmed: the banding occurs in both directions. See the animation "TestSeed13 Animation.gif" for a demonstration centred near the triple island at the top of the original map.
Similar banding has been observed in Extreme Hills biomes. See the animation "TestSeed13 Animation Extreme Hills.gif". This Extreme Hills island can be found near the centre of the map at approximate coordinates 500,400.
This is probably a duplicate of MC-125034.
In the map, I assume you mean the horizontal line going past the bottom of the left-most island in the group island south of the top-right ocean monument?