Amidst for Minetest, is a very useful Java program that can be used to view a near infinite map of a Minetest world, without actually needing to create the world in Minetest. Draw speed is fairly fast, much faster than actually generating the world in Minetest and there is the ability to highlight specif biomes and load in custom biome definitions, making it great for those of us that are creating new biomes for a mod, or subgame.
In addition to seeing the biomes as a map they can be viewed as Voronoi diagrams which lets you easily see what percentage of each biome a world will have.
The creator of this fork, DrFrankenstone, got in touch with me after watching the video and addressed some points that I brought up and was slightly confused about. I've included his response below in the hopes that it may be helpful to some of you.
Amidst vs Amidstest: Initially Amidstest, but adding -test suffixes to everything probably hasn't been the greatest idea, so I'll drop it - only the game engine need stay stuck with the "test" name. As a fork of Minecraft's Amidst, it wouldn't be polite to take the original project's name, so the full name will be "Amidst for Minetest", but the "for Minetest" can be dropped inside Minetest circles.
Oceans Layer: Most mapgens have a sea-level, usually at altitude 0 or 1, and anywhere the ground level is below the sea level gets filled with water. This is what the oceans layer shows - any ground below sea-level is coloured blue.
Ocean biomes: Biomes determine the type of nodes the land is made of, so to ensure the ocean floor will be made out of sand, people usually create biomes which cover any height below sea-level and give them ocean-ish names. Ocean biomes are usually depicted as yellow in Amidst because they are usually made of sand, but turning the Ocean Layer on will colour the underwater parts of the biome blue. Any above-water parts of an ocean biome will look like a beach, which is why some occean biomes extend up to a height of 5.
Carpathian mapgen and the Rivers Layer: From a player perspective there are rivers in Carpathian. Whether a mapgen has a Rivers Layer in Amidst comes down to whether its rivers are generated in a distinct process that can be easily displayed as a separate layer. Rivers in Carpathian are like oceans - anywhere the ground generates lower than sea-level is filled with water, and the landscape noise is just designed to have river-like shapes in it. Other mapgens like v7 cut rivers into the landscape after the land has been generated, and Amidst can show that as a layer.
The Mountains Layer: This seemed like a good idea when first starting Amidst for Minetest, because v7 was the mapgen being attempted and it switches to using 3D noise in mountainous areas. Finding height is slow with 3D noise and things like overhangs mean it's not always clear what level should count as "ground-level", so showing a Mountains Layer means you can see high altitudes in Amidst, and Amidst doesn't need to calculate them. However the feature often made little sense in later mapgens, and with high altitude biomes becoming increasingly common in Minetest, an accurate height in v7 mountains should really be calculated.
Save game and player icons: Little known trivia - Amidst for Minetest also displays Minecraft worlds. To do so, Minecraft must be installed on the computer, and it won't be able to load the latest 1.13 worlds but it can still display 1.12 or earlier. This is a holdover from an initial idea to add the Minetest support directly into [Minecraft's] Amidst. Adding Minetest support to the Minecraft version is no longer the plan, but idiosyncrasies and limitations remain from Amidst being designed around Minecraft. The "Open Save game" and "Player Icons" features are not yet implemented for Minetest but are available in Minecraft maps. The confusing "UNKNOWN" in the title bar refers to the Minecraft game version, and its origins as a Minecraft app make it difficult to show altitude, humidity, and temperature on the map.
The purpose also shifted. Amidst for Minecraft is entirely player-centric and focuses on things like where treasure and structures can be found, but (as your video notes) Amidst for Minetest can help with the design and testing of mods/games, and it may be a more valuable focus.
The "Show distribution" circles: The button to the left of the distribution slider brings up some text which contains an explanation of the circles.
The Unknown biome problem in the video was land that is higher than 60, as the highest y_max of those biomes was 60. However the video shows me Amidst has some foibles there too - above altitude 60 the Voronoi diagram should also have shown "Unknown", and as mentioned before, Amidst won't be accurate with height in v7 where 3D mountains are found.