Stackable Sorting

Stackable item sorters separate items by type are the very base of of storage tech. Nearly every storage tech contraption uses an item filter or a variation on the concept.

How Item Filters Work

Most item filters utilize the property of items that only allows them to stack with items of the same item type and name. Hoppers are the main method for checking this. To sort a single item type, each slot of a hopper must be filled, as to only allow the item that will be sorted into the hopper. The last 4 slots are filled with a different item type/name. These are called "filler items". It is recommended that the filler items be named in an anvil to avoid accidental interference. The first slot contains the item that should be filtered. 

For example, the following image shows the inventory of a filter hopper that will sort sticks. Note that the barrier items can be replaced with any other item.

A second hopper is placed beneath the filter hopper. The filter hopper cannot face into any other inventories as it is always unlocked. The hopper below it however is locked by default. This second hopper is used to remove any new items that enter the filter hopper. If an item enters the filter hopper, it means that the item type matches what is being sorted.

To know if an item has entered the hopper, a comparator is used to read the inventory fill level of the filter hopper. The hopper is filled in a way that if a single new item enters the filter hopper, the signal strength from the comparator will increase. This is used to unlock the second hopper and remove any new items until the item count is dropped below the threshold to lock the second hopper again.

The absolute simplest and cheapest item filter possible. The hopper on the top contains the inventory shown above.

Although the above filter is very cheap, it has some issues. Namely that it is not compact. The design cannot be repeated in a compact manner. The following filters are a solution to this.

Variations on the Item Filter

Signal Strength 3 Item Filters

The easiest way to make a compact. repeatable and overflow-proof (will not break if its output inventory is full) item filter is to use signal strength 3 (SS3) filters. These filters are named after the signal strength threshold they must meet to move items. To enter signal strength 3, a hopper requires at least 46 (64 stackable) items inside its inventory. Because of this, SS3 filters top hoppers have a different prefill than other filter types.

The filter hopper inventory on an overflow-proof SS3 item filter.

The following two images are common versions of SS3 filters.

The TechRock filter.

The ImpulseSV Item Filter.

The above filters are versatile and easy to use because of their ease of tiling. As long as the filter hopper prefill is done correctly, they can be placed directly next to each other without any interference.

5 TechRock item filters tiled. All 5 slices work independently of the others.

Double Hopper Speed Item Filters

A major limitation of the basic item filter is speed. A single hopper can only move 2.5 items per second. This is refered to as Hopper Speed, or HS for short. A single hopper is 1xHS (one times hopper speed) or 9,000 per hour. Despite how fast 9,000 items per hour sounds, many farm exceed this number easily. To avoid needing more filters, it is possible to speed up the rate that a single filter can move items. It is usually easiest to speed up a filter to double the rate of a single hopper by having the filter hopper push items at the same time the second hopper is pulling from them. Note that these 2xHS filters are not practical for main storage systems and are best used as sorting in farm outputs.

The following images are common versions of 2xHS item filters. Special prefill is marked as required per hopper. The output is split into 2 different 1xHS streams.

Piston-based 2xHS item filter. Trading low prefill for a piston.

A variation on the ImpulseSV filter, works at 2xHS. Note that the shovels may be replaced with named items, as long as they are a different name than the filler items in the filter hopper.

Signal Strength Independence

A major weakness of the simple ImpulseSV or TechRock item filters is the amount of the sorted item that needs to be held inside the filter hopper. Not only is this expensive, it also limits the amount of items that the filter can pick up at once. 

A common first idea is to increase the amount of filler items in the filter hopper so that less sorted items are needed. This works in theory for solving our problem, but it also increases the maximum amount of items that can be in the hopper at once. This number is outside the range for signal strength 3 output, so the filter reads signal strength 4. Due to the method for tiling these filters, the adjacent filters will also activate, causing them to lose their prefill.

To combat this, the concept of Signal Strength Independence (SSI) was developed. As the name states, it separates the signal for each filter so that if the filter hopper contains an amount of items beyond the range for its desired signal strength, adjacent filters will not break.

The inventory of a SSI filter hopper

However, this idea is not as simple as it sounds to implement. Due to the nature of redstone dust, it is impossible for SSI filters to be one-wide-tileable with every slice remaining the same. SSI filters are always one-wide-AB-tilable, where every other slice changes.

Below is an example of SSI filters. There are too many versions of these filters to include them all here.