Mysterious blinking pixels when in a dark area

The pixels are cracks between 16x16x16 sections. The cracks can hardly be seen before but became much worse since 1.13.

It's noticeable that the flickering pixels aren't white. Their colors are probably the color of the sky.

Can confirm in 21w03a. I used the following command to reproduce this issue and this made it more noticeable.

/fill ~ ~ ~ ~20 ~20 ~20 minecraft:black_concrete hollow

Can confirm in 21w05b. Not sure about anyone else, but for myself, it can be seen a lot more often in this snapshot/version.

Can confirm in 21w07a.

Can confirm in 21w13a.

Even tho they're still exist in the newer snapshots, they're way less visible.

This is cause by precision issues in some of the floating point calculations. Or maybe better said: too much precision. If we have 2 chunks offset by the camera by 0.0000001f and 16.0000001f the second chunk offset get's rounded to 16.0f
which means if we now look at the vertices at the border with coordinate offsets 0 and -16 respectively, we get the coordinates 0.0000001f and 0.0f. As these are not the same coordinates, they will have slightly different edges which means these pixels can occur.

In my pr to Sodium to fixed this I round the camera part to 1/2^14, I think something similar could work here.

When pushing the ChunkOffset uniform

// old
chunkOffsetUniform.set(
  (float)(blockPos.getX() - cameraX),
  (float)(blockPos.getY() - cameraY),
  (float)(blockPos.getZ() - cameraZ)
);

// new
private double fix(double pos){
  // reduce to 14 bits of precision
  // pos should have about 25 bits for the block part + 14 bits for the fractional which is 
  // 39 bits which fits inside a `long` which `round` returns, it does not fit in a float
  // so it needs to stay a double
  return Math.round(pos * 0x1p14) * 0x1p-14;
}

{

double fixedCameraX = fix(cameraX);
double fixedCameraY = fix(cameraY);
double fixedCameraZ = fix(cameraZ);

// in loop
  chunkOffsetUniform.set(
    (float) (blockPos.getX() - fixedCameraX),
    (float) (blockPos.getY() - fixedCameraY),
    (float) (blockPos.getZ() - fixedCameraZ)
  );

}

Note:

I haven't tested this, I have only tested some equivalent code in Sodium.

Can confirm in 1.18.1. To more easily reproduce this, you can run the following command and observe the issue occurring.

/fill ~10 ~-1 ~10 ~-10 ~9 ~-10 minecraft:black_concrete hollow

Can confirm in 1.18.2.