quick question, why are you trying to use DXT instead of just using a background image?
Because a sprite or tiled background is not compressed on the GPU. So a single 4K texture would take up like 90MB. This only makes sense for high resolution games where all the backgrounds are unique from screen to screen or level to level. Which means no repeated elements. I.e. you create an entire level background in a single photoshop project and export the layers as is. So tiled backgrounds and tilemaps are out of the question.
while unity can compress the hell out of them, you can use png compression on your exported spritesheet once exported like tinypng etc... just make sure the size of the spritesheet doesn't change, you can bring down a 5mb to 100kb or less if done right.
I'm not talking about compression on disk. Yes png files are great for low disk size. I'm talking about the texture being compressed on the GPU so the memory size at runetime is lower.
having a 18mb 4k resolution background image sure is cool if you run it on a 4k monitor and im not talking about the 1920x1080 monitors that claim to be 4k... but an actual 4k monitor... that is 160cm wide or larger (also if you don't have a 4k gpu card, and a monitor/tv to be able to run 4k the detailing is pointless to have), if you do your game for majority of players a 1080p texture should be sharp enough to offer the quality and the detail needed. Mobile wise, you don't need textures greater than 480p.
I'm not talking about screen resolution. I'm talking about the size of the texture. If your screen resolution is 1920x1080 and a level background is 4096px wide, then you would obviously have to scroll the screen to see it all.
i just don't get the convoluted way of doing a simple replica of the game you mentioned. it looks 2d to me, no need for 3d stuff, but just the illusion of 3d, the player won't care that much if your gameplay is great.
Also I'm not talking about making a 3d game. DXT has nothing to do with 3D games. Sure it was developed around the time 3D games got super popular so artists could continue pushing the limits. But DXT is simply a compression algorithm. It can compress a 4x4px texture or a 8192x8192px texture. The point is that the GPU can efficiently unpack them at runtime, regardless of 2D or 3D game.