Some strips of chunks generate completely dark

The bugged tree generation is a separate unrelated issue: MC-125007

Chunk loading issues in water: https://www.youtube.com/watch?v=LhCd__Lr8Rs

@unknown, that's a perhaps related but different issue – MC-214808

@unknown Thank you, will post it there then.

I've seen this is several places in this world, both above ground and underground.
Generation seed "Mandatory" XYZ 416/112/233.

Those dark areas can appear in various places, and they can go from both North-South as well as East-West-axis.

So far, I couldn't reproduce this with a specific seed, as in: "Dark areas always appear in this seed at this location." Furthermore, when I re-entered the world, those dark areas were gone. Likewise, opening the world to LAN and having a second account look at the affected area resulted in not seeing the dark area, while the first/host account still saw the dark area. For the secondary account, Sky light level was 15, for the first/host account 0.
So this issue seems to be client-side.

If you apply a night vision effect, those dark areas cannot be seen anymore; however, if you clear that effect, you'll see them again.

They are actually, really dark; Hostiles can spawn during daylight, and F3 shows 0 for Sky at Client Light.
The dark areas also remain visible during night/are darker than the surrounding areas, as opposed to the normal sections (chunk parts).

After just quick testing (unsure if what I write now is for certain), the dark areas can be removed if you place (or change or update already-existing blocks, e.g. via running water) at maximum 2 away from a chunk border, however, it seems that this only succeeds if the block of a neighbouring chunk has a certain light level, presumably just higher than 0 (can be probably better seen if you turn off Smooth Lighting).

It seems the Skylight is normal again at a certain height above the dark area; it appears to be the 2nd chunk above the highest block of affected chunk, e.g.: Dark area, highest block in that chunk at Y=69, Skylight 0; Skylight 15 would be then at Y=96. (69/16=4etc.; 4+2=6, 6x16 = 96). Dark area, highest block in that chunk at Y=105, Skylight 0, Skylight 15 at Y=128 (105/16=6etc. 6+2=8, 8x16=128).

Quick testing video (Hostiles spawn, block placing, night vision effect): https://www.youtube.com/watch?v=959q91bRXmg
Demo Client Light Sky 0/15 dependent on Y-level, and relog fixing dark areas: https://www.youtube.com/watch?v=KjFJOOAkQww

Render distance 16.

Relates to MC-214898

Observed in current 1.17 snapshot at Z=-2225.  Hostile mobs spawn on the line in midday.  Save and Quit and Reopen causes the chunks to be lit again.

Placing a block at y319 causes a few chunks around it to be restored to normal.
The tree issue has been marked as "unrelated and separate iusse MC-125007)
but I find it unsuall that most of the areas with bugged lighting also contain these cut off trees

ive had the same problem but yours is on a much bigger scale.  maybe it was because i was close to spawn and it made it smaller but i got rid of some of it with torches if you place it on the edge and get rid of the torch.  and i dont know if it was just me, but i saw way more trees and mountains cut off then usual, right at the border of the chunk.

Still happening in 21w08a

still 21w08b

Noticed this too, they are gone when reloading the world though!

This happend to me on 1.16.5 but they just didnt load. at ALL.

still 21w10a

Is this going to be fixed?

Of course it's going to be fixed, but these are snapshots and large bugs are often pushed until all the features are in. 

I was doing some tests and noticed that while the lighting updates visually when you put blocks, or, in my case, torches, on the edge, that only affects the rendering, and not things like mob spawning. Then I went into a nether portal and came back, and the area looked almost normal, but the mobs were still spawning in daylight.

Happening still in 21w10a

Cannot reproduce with intel graphics 3000

Even with a light update at the border of one such chunk, like with a jack o'lantern, the darkness is never fully lifted. It comes back when the light block is removed again although the darkness near the chunk's edges might have smoothed out a bit.

When I recreated the world with the same seed, the glitch was gone in that particular place. Seems to be at either random times or random places.

Is the bug similar to MC-214808?

Confirmed for 21w13a

Not sure if this is a direct cause from the issue but it looks like it also might break tree generation

Can confirm NVIDIA GeForce GTX 1060 6GB

21w14a

Definitely confirmed for 21w14a. This all occurs in singleplayer, and can confirm it also happens when running a server at least in 21w13a.  The lighting issue does not disappear with F3+a chunk reloading, but fixed when saving and quitting to title, and rejoining the world.

The lighting glitch is more than just client-side visual, because mob spawning is able to occur in it midday.

Furthermore, the glitch can be more severe sometimes where the chunk entirely fails to render.  When inside the chunk space, F3 is missing chunk info and states "waiting for chunk" indefinitely.  This cannot be solved with F3+a either.

Just made a new report for the missing chunk thing, MC-222574, same bug?

That is probably MC-214808

Maybe relates to MC-142134?

Can confirm for 21w14a

If at y=95, it will think you are standing out in the light. If at y=94, it will think you are in the shade.

Setting a block above y=80 will cause the lighting to update. If you set a block below y=80 it will not update.

A similar thing has happened to me where I the /fill command and the lighting acts as if it didn't have any blocks above it, making it impossible to turn back to darkness, and this is annoying in my mob farm.

Confirmed for 21w15a

Does this issue seem to coincide with other unusual chunk glitches, such as cliffs stopping sharply on the borders, structures that are partially missing, etc.? I have a similar (but also quite different) issue to this, but I noticed that there were all kinds of peculiarities in the area, such as a messed up ruined portal, a glitched out cave, a half a tree, etc.
Also, does this issue resolve itself upon relogging? Or does it persist?

Yeah, this frequently coincides with other worldgen bugs like half generated trees- this should be marked as relates to MC-125007 IMO, almost every time I see this bug there are half-trees at the edge of it

Any chance someone could post a screenshot of this in a version before 21w06a/after 21w14a (with the debug screen)? I can't seem to reproduce it in a standard-height world and would be interested to see if it looks any different from what I ran into during the extended-height snapshots.

Cannot reproduce in 21w15a, probably due to the reverted world generation changes, so this is likely not fixed yet.

I've attached a screenshot of this issue in 21w15a.

Can also confirm for 21w15a.

Does reloading the world fix this, or does the issue persist?

I can reproduce it now again too, maybe I didn't cover enough distance before. This time I've covered over 40k blocks outwards and circled around an entire (huge) desert.

It appears that the lighting glitches occur in chunks that were newly generated in 21w15a. All these lighting glitch occurrences seem to disappear upon logging out and back in, but always come back in different places (so it's not just a first-load problem but doesn't persist either).

Lastly I also experienced this glitch in a 1.16.5 world, HOWEVER that was in a modded game so I still need to check that version back in vanilla.

Doesn't seem to be an issue in 1.16.5, though we can't be sure.

Do they sometimes appear to be lit up despite being dark? Like even without night vision or high gamma values?

Found some dark chunks again in 21w16a (along with what I believe is MC-125007). Definitely seems less common after the height revert.

I would like to bump this again because I am finding a pattern and probable cause for its occurrence, as well as evidence for linkage to another significant bug.

I've done extensive testing for hours trying different things, and it actually started with bug MC-166005 - "Stuttering when crossing chunk borders/loading chunks", in which I discovered that villages in 21w16a with the Caves & Cliffs datapack, created enormous lag spikes in the region, up to 30 chunks away from the village boundary.  It turns out that in the same lag spike region, the dark/missing chunk issues were often found.  I would repeat it over and over-- fly to a new village, fly around it a bit, and I would get both lag spikes, and dark chunks.  Repeat with a new village.  While I found that severe lag spikes were unique to having the Caves & Cliffs datapack running, the dark chunks are still repeatable under the same circumstances even in prior versions before datapack usage, i.e. 21w14a.

There are 2 good ways to predictably reproduce the dark/missing chunks, and anticipate where they will occur.  Start in spectator, and go full speed with the mouse wheel + sprinting in either case.

1. Villages - Fly to a village and past it along an axis, X or Z.  The fly-through generates the village and surrounding chunks in an easier straight-line pattern.  Keep flying away to completely unload the village and surrounding area (I go 1000 blocks away).  Turn around and fly back to the village.  There will be a chance the dark chunks will appear near the village, usually arranged in lines, if you flew along an axis.  If the village has a certain size bounding box surrounding it, the dark chunks will usually be straight lines on the perimeter of a larger box around the village.

2. Teleportation - Teleport to a completely new area away from any already generated chunks (I suggest doing nicer numbers like 2000, 2000 so its easy to remember and stay aligned with).  Immediately upon teleporting, fly around in a spiral (increasingly larger radius circle) surrounding your teleportation location in such a way to create as many chunks as quick as possible for up to a minute or so.  Align yourself with the teleportation location, and fly directly along the X or Z axis far enough away to completely unload the area (i.e. 1000 blocks), and return.  Check the area surrounding your teleportation location.  You can try different things each time you teleport to a new location to see different patterns of dark chunks.  While the shapes can vary, you can still find straight line and 'corner' shaped dark chunks.

In either case, here is what I think is happening:

• During chunk generation, the game is demanding a certain amount of resources which can't be met as quickly for whatever the reason is, so let's say we call this "stress".  Based on my findings in bug MC-166055, villages are somehow creating more stress (especially in the C&C datapack to the point of severe lag spikes), so the likelyhood of finding dark chunks around it are higher.  Creating lots of new chunks generally creates more stress, but especially teleporting to a new area that's surrounded by completely ungenerated terrain and flying around-- more chunks have to be generated around the player rapidly, so it's more likely to find dark chunks.  In both cases, revisiting terrain you generated under high-stress conditions, more likely gives dark chunks where it had issues generating.  The dark chunks usually seem to take a shape based on the way you stress-loaded the generation.

Here's a few examples to illustrate what commonly happens in each method:

Case 1 Examples: Dark chunk line perpendicular to village line of sight

Case 2 Examples: Dark chunk "box" surrounding a teleportation location & Other irregular shapes

I'm heavily inclined to believe that the dark/missing chunk issues are caused by anything that causes excess stress during the chunk generation process, and some chunk data is getting at least temporarily corrupted.  Example: the game is generating a chunk, something interrupts the process, causing data to be incomplete.  Let's say we roll with that idea.  Sky lighting maybe doesn't get the chance to have data populated in the sub-chunk, so the default value is 0 (dark) unless otherwise given the opportunity to be filled with a "15".  As some have noticed, the dark chunks can also coincide with trees being chopped at the boundary, which seems like it would be consistent with chunk generation being interrupted as well.  Maybe there's other generation features that can be interrupted too.  They seem like different issues at face value, but might have the same root cause.

note: On a server, some dark chunks will disappear after relogging, some won't.  For the ones that won't disappear, even completely shutting down the server and restarting it multiple times won't fix it, and remain 'permanently' mob-spawnable.

And maybe the reason why it's more difficult to reproduce this issue with the non-C&C generation, is because it's harder to achieve the amount of stress needed to start corrupting chunk data-- less chunk height means less to generate, and a smaller window of opportunity for something to interrupt the chunk generation process?  So something would need to be optimized/fixed with chunk generation so that it's much harder or impossible to interrupt chunk data creation?  And something about villages would need to be potentially optimized?

Just my thoughts based on the patterns I've observed, hope this helps!

What about 1.16.5?

I seem to have this glitch too, but sometimes it's not a strip of chunks, but entire areas. 

Signing in and out of the server sometimes fixes the light but the mobs keep spawning. 

Anything you build in these zones does not save if you sign in and out. 

Dark zones above villages causes the villagers to run in and out of their houses repeatedly, they don't seem to understand whether it's day or night. (Which is strange, I thought they went by the day cycle, not light.)

I've tried running my world through the "Optimize world" function but it causes the launcher to crash about halfway through on a total of three different computers, both MacOS and Windows. 

Found a very broken seed where this occurs extremely frequently: -2171582531736847681. About 1/10 of the world is lighting errors. Occurring in areas that were previously lit. Occurred in spectator mode, which was thought to be impossible. Without teleportation, just flying around, huge lighting errors everywhere. They clearly do not always generate completely dark, they just fail to load the proper lighting for some reason.