Steps to reproduce

1. Open a world with many maps, or create many maps by using empty maps.
2. Place a map in an item frame
3. Close and re-enter the world to clear the map cache
4. Use the following command to change the ID of the map in the item frame:

   /data modify entity @n[type=item_frame] Item.components."minecraft:map_id" set value <id>
   

5. If the item frame is briefly blank, the map is loading. Try to change the map ID during that time frame.

Observed behaviour

The item frame is blank for a some amount of time, then it shows up. The more complex the map, the longer it takes to show up.

Afterwards, it loads immediately, unless the map ID is changed again while loading.

Expected behaviour

Either the map always immediately shows up, or at least always shows up immediately after being loaded once.

Notes

This especially impacts data pack creations, especially those using maps to display videos in-game.

For example, this data pack was generated using McMovieMaker. There, the game only manages to load one frame out of 5 every second, and this is constant regardless of the frame size of the video. The video needs to run for a long time for the video to play properly.

This is likely due to an abnormally long delay when reading the maps, which may be due to the somewhat obscure format used to save the maps, causing the game to take almost a second to read them. Additionally, the game starts reading/extracting the current image, and waits for it to finish before starting again, which makes caching extremely time-consuming.

Videos

• Minecraft 1.21 - Singleplayer 2024-07-18 15-05-52.mp4 - reproduction steps
• Minecraft 1.21 - Singleplayer 2024-07-18 15-24-04.mp4 - changing map ID while the map is still loading
• start caching.mp4 - video data pack, initial video play
• after 45min of caching.mp4 - video data pack, after playing the video for 45 minutes