So I've been fiddling around with high-res texture packs in the game recently. I use the latest version of Optifine on a 1.9 Vanilla Minecraft. For the most part, I've been using 256x texture packs, I've tried a fair few and they all work fine with not many performance issues (unless I use shaders). My first try at a 512x texture pack was the famous Sphax PureBDCraft texture pack.

This was the result. No crashes, no 'run out of RAM' or anything like that. Just loading in blank blocks.

Thinking that there would be issues anyway due to 1.9 being relatively new and not all the mods being updated for it, I ignored it the first day and thought Optifine just needed to be updated or something.

Today I tried out a different 512x texture pack, since I was interested in the Realistico texture pack which comes at 512x. I installed the S&K Photorealism 512x texturepack (having not changed any other settings) and the result:

It works without any problems.

Obviously, my PC and settings can handle 512x texture packs or that one wouldn't have worked. I've looked up similar issues and most people just brushed it off as 'the PC isn't powerful enough'.

Quick specs for those who want to know:
-OC i7-4790k
-OC GTX970
-16GB RAM (4GB default set to MC via launcher)

I laughed when I found a thread of an individual with even better specs than mine (they had a Fury GPU, which has much better RAM than my 970) and they apparently couldn't run 512x texture packs at all. It's likely that this was the only 512x texture pack they tried and that they were using Optifine. Thinking it's an issue with the pack itself, I delved deeper.

Yet more folks mentioned RAM, so I decided to double up and set it to use 8GB, still no dice. I even tested both texture packs whilst running MSI Afterburner to see RAM/VRAM usage in game. Both texturepacks actually used the same amount of RAM funnily enough. VRAM usage never went above 1.5GB. Some folks mentioned connected textures was an issue, so I turned it off. I also deleted that CTM folder in the .rar of the texture pack itself.

These issues are the only think keeping me from trying out the full version of the Realistico texture pack (also a 512x). Unfortunately there are too few 512x texture packs for 1.9, so I can't test more.

I also tried the KoP Photorealism 512x texture pack and that one also resulted in the same unloaded textures.

I then, according to this thread, I decided to resintall MC and try the texture packs before installing Optifine (the only mod I'm currently using). My reason for using Optifine was for it's support of HD texture packs and shaders.

To my surprise, vanilla minecraft (4-8GB RAM usage) with no optifine ran the 256x texture pack… so I decided to see what would happen if I tried the 512x texture pack on it…

This was the result. I thought, maybe it was fixed so I quickly installed Optifine and rushed back in to find …broken. I switched back out to vanilla and it was back to it's normal working self.

Vanilla MC: 3/3 512x texture packs working
Optifine MC: 1/3 512x texture packs working

Maybe I'm somehow using Optifine wrong, but considering the large amount of folks who have been having trouble with 512x texture packs, I think I'm not exactly wrong.

I guess my initial assumption. that there were issues with Optifine and 512x texture packs, was right. Now my only concern is that bump-mapping on Realistico 512x requires the shadersmod… and that's integrated into optifine for 1.9. Perhaps users of Realistico can attest to it working fine on an Optifine Minecraft 1.9. Now I'm just wondering if I can get a 1.9 shadersmod without Optifine.

It's a real shame that Optifine doesn't seem to run well with these 512x texture packs, the program itself is rather great and the effort put into 512x texture packs is also appreciated. I would have been nice if I could get them working together much better.

Apologies for the wall of text, this is my first time posting on this particular subreddit (I literally got back into Minecraft for the first time in years just yesterday).

TL:DR If you have problems with 512x texture packs, trying them in vanilla Minecraft, ensuring you have some decent RAM allocated.