You should be able to jump into this hole or 1 block up and a slab above as shown in the image... However this is not the case. You can not jump into it. In Bedrock Edition, you are able to do this. In Java edition you cannot.
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I have done a lot of testing and found that the exact jump height is 1.2522033. More accurate found by Jarl-Penguin: 1.252203440253726651
How I figured this out:
I used boats with no gravity to figure this out.
You can use these commands to summon boats to test this.
1.
/execute align xyz run summon minecraft:boat ~ ~-0.8147033 ~ {NoGravity:1b}2.
/execute align xyz run summon minecraft:boat ~ ~-0.3147033 ~ {NoGravity:1b}Linked issues
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I was able to reproduce with boats placed at ~-0.(3,8)147034.
Edit: After some trial and error here's a more accurate number: 1.252203440253726651
Actually, the hitbox height in Bedrock Edition is 1.49 blocks, not 1.5 blocks. But in Java Edition it's 1.5 blocks.
So the first jump is impossible in Java Edition.
If the initial height is elevated by 0.01 blocks in Bedrock edition, the first jump will be impossible.
The exact height per game tick when jumping on flat ground is:
0.4199999868869781
0.7531999805212017
1.0013359791121474
1.1661092609382140
1.2491870787446813
1.2522033402537238
1.1767592750642373
1.0244240882136801
0.7967356006686920
0.4952008770059118
0.1212968405391897
0
This is because player's generic.jump_strength is 0.42, and generic.gravity is 0.08.
The height and y velocity of the 1st tick of jump is equal to jump_strength, is 0.42(rounded down to 0.4199999868869781 due to the floating point error), and every tick after that before touching ground is calculated by the formula below (where drag=0.98):
velocity(n)=(velocity(n-1)-gravity)*drag
height(n)=height(n-1)+velocity(n)This formula is not very accurate due to floating point errors, but it could get a height very close to the actual height.
And, you can only jump into a ceiling when the height of one tick is above the ground of it and (the height of that tick)+(Player's hitbox height) is lower than the ceiling height.
For example, if you want to jump into a 1.5 blocks ceiling from 1 block lower when sneaking:
(In Java Edition hitbox height is 1.5 blocks, In Bedrock Edition is 1.49 blocks)
0.7531999805212017<1
1.0013359791121474>1, 1.0013359791121474+1.5=2.5013359791121474>2.5
1.0244240882136801>1, 1.0244240882136801+1.5=2.5244240882136801>2.5
0.7967356006686920<1
So it's impossible in Java Edition.
In Bedrock Edition,
1.0013359791121474+1.49=2.4913359791121474<2.5
So it's possible.
But if you want to jump into it from 0.8125 blocks below, it will be certainly impossible.
{*}{*}This is also the principle of MC-120779.
If you change the jump_strength of the player:
0.419982001 won't let player jump onto 1.25 blocks, only 1.24910 blocks.
0.419982002 will let player jump onto 1.25 blocks, but it jumps 1.25210 blocks high.
Only a 1e-9 change in jump_strength cause 0.003 blocks jump height change. But changing jump_strength from 0.42 to 0.419982002 only cause 0.0001 blocks jump height change.
This is because there's a movement threshold exist, when y velocity is -0.003~0.003 b/t, players won't move on y axis, cause jump height suddenly change.
These are my full analysis, but, actually, I don't think it is a bug.
Thank you for your report!
After consideration, the issue is being closed as Invalid.
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Duplicate of MC-146855, most likely.