Anvil/Chest Manipulation

Anvils don't have a fixed collision box. Instead there is a single collision box for all variants (North, East, South, West), which is updated whenever the player looks at one (or by doing other actions which are listed later). Normal anvils, slightly damaged anvils, and very damaged anvils are not independent.

Chests behave in the same way, though only Double Chests have a different collision box. Chests and Trapped Chests are independent.

By standing next to an anvil/chest, and by looking at another variant, the player can update the collision box in such a way that they are now considered "inside the block". The same trick can be used to artificially extend the length of the anvil/chest, and manage to stand one or two pixels further than intended.

You must be careful not to look back at the original block, as that will reset the collision box to normal.

Top-view comparison of collision boxes

Anvils are a simple 1×0.75 bounding box (16×12 pixels), that can be oriented along the X or Z axis.

Single chests are a simple 0.875×0.875 bounding box (14×14 pixels). The "double chest" variant extends one side by 1 pixel.

In the graphic, darker colors indicate collision areas that are common to all variants : you should stand there in order not to fall during manipulation.

Lighter colors indicate collision areas that are specific to one variant: you should stand there in order to clip inside the block during manipulation.

Explanation

The BlockAnvil and BlockChest classes define the setBlockBoundsBasedOnState method like so:

/* Simplified code */

/* In BlockAnvil.java */
public void setBlockBoundsBasedOnState(BlockPos pos)
{
    EnumFacing enumfacing = getBlockState(pos).getValue(FACING);

    if (enumfacing.getAxis() == EnumFacing.Axis.X)
    {
        this.setBlockBounds(0.0, 0.0, 0.125, 1.0, 1.0, 0.875);
    }
    else
    {
        this.setBlockBounds(0.125, 0.0, 0.0, 0.875, 1.0, 1.0);
    }
}


/* In BlockChest.java */
public void setBlockBoundsBasedOnState(BlockPos pos)
{
    if (getBlockState(pos.north()).getBlock() == this)
    {
        this.setBlockBounds(0.0625, 0.0, 0.0, 0.9375, 0.875, 0.9375);
    }
    else if (getBlockState(pos.south()).getBlock() == this)
    {
        this.setBlockBounds(0.0625, 0.0, 0.0625, 0.9375, 0.875, 1.0);
    }
    else if (getBlockState(pos.west()).getBlock() == this)
    {
        this.setBlockBounds(0.0, 0.0, 0.0625, 0.9375, 0.875, 0.9375);
    }
    else if (getBlockState(pos.east()).getBlock() == this)
    {
        this.setBlockBounds(0.0625, 0.0, 0.0625, 1.0, 0.875, 0.9375);
    }
    else //single chest
    {
        this.setBlockBounds(0.0625, 0.0, 0.0625, 0.9375, 0.875, 0.9375);
    }
}

This method is called from these other places (same for BlockChest):

Notably, the following actions can be used for manipulation:

• Simply looking at the block (without necessarily drawing its selection box)
• Shooting an arrow at the block (has a persistent effect).
• Causing the block to re-render, which can be done by updating the surroundings.
• Using rain to update the block (randomly).

Limitations

On Singleplayer, the player can usually walk through an anvil or chest once they clip into one.

On Multiplayer, the anticheat will typically prevent the player from walking through an anvil or chest completely.

In 1.9, this mechanic was patched (each variant now has its own collision box).