From 13w02a onwards (for blocks and items, 19w06a for particles, 19w07a for paintings and 19w08a for effect icons), instead of being defined directly in the game's files, the textures are instead automatically stitched together into a single file and saved in VRAM. These generated texture atlases are not accessible normally. However, from 13w02a up to 13w17a, as well as from 14w25a to 1.8, the images for texture atlas files were exported to the .minecraft folder, presumably as a debug feature. 13w18a and 1.8.1-pre1 stopped this practice, resulting in these files no longer being accessible. I propose an addon for 13w18a-14w21b and 1.8.1-pre1-present that, when selected, will create image files for these texture atlases in the game's directory again, for debugging/documentation purposes, as to get a more accurate source for these atlases (while they can be passed to the visual output via a shader, this gives it a black background, therefore forbidding transparency and translucency, and also isn't possible in versions before 1.15).
For the record, here are the known atlases which are generated:
stitched_terrain.png: 13w02a - ? (already output by the game as a file as stitched_terrain.png from 13w02a to 13w07a, and debug.stitched_terrain.png from 13w09a to 13w17a)
stitched_items.png: 13w02a - ? (already output by the game as a file as stitched_items.png from 13w02a to 13w07a, and debug.stitched_items.png from 13w09a to 13w17a. This output version is possibly inaccurate in early iterations as all pixels have full opacity, so I'm not sure if the addon could be made to produce a more accurate version.)
blocks-atlas: ? - 14w21b
items-atlas: ? - 14w21b
blocks.png-atlas: 14w25a - present (merges blocks and items together, has gone through multiple name iterations, and contained the contents of all atlases which were eventually split off of it in 1.15-pre1. Was generated as files from 14w25a to 1.8, but seemingly never afterwards.)
From 13w02a onwards (for blocks and items, 19w06a for particles, 19w07a for paintings and 19w08a for effect icons), instead of being defined directly in the game's files, the textures are instead automatically stitched together into a single file and saved in VRAM. These generated texture atlases are not accessible normally. However, from 13w02a up to 13w17a, as well as from 14w25a to 1.8, the images for texture atlas files were exported to the .minecraft folder, presumably as a debug feature. 13w18a and 1.8.1-pre1 stopped this practice, resulting in these files no longer being accessible. I propose an addon for 13w18a-14w21b and 1.8.1-pre1-present that, when selected, will create image files for these texture atlases in the game's directory again, for debugging/documentation purposes, as to get a more accurate source for these atlases (while they can be passed to the visual output via a shader, this gives it a black background, therefore forbidding transparency and translucency, and also isn't possible in versions before 1.15).