TL;DR this doesn't make your game look any different. It's meant to make it easier to make your own vanilla-like lightmap, or if you only want to edit the torch light or something.

I spent the last few months looking for a decent lightmap to use in my personal resource pack. Couldn't find one. They all seemed to have problems for me: either the nights are too black, the daylight too yellow, the evenings too pink or the shadows too dark.

That's when I decided to make my own! But... I had no idea how bright I should make all these pixels. "Default brightness looks fine I guess, I just want to tweak some colors," I thought. Problem is, vanilla Minecraft doesn't use a lightmap texture, it generates all these colors from code. You couldn't find an identical-to-vanilla lightmap on the entire internet! (Trust me, I looked. A lot.) So, I made a quick resource pack with a solid white texture, made a quick test setup, and started making my own.

I made a 16 block long strip of solid white blocks, covered most of it so each block had a different level of sky light exposure, figured out through trial and error by comparing dozens of screenshots at which time values night starts and stops transitioning into day and divided the time between those by 14.

Then, I took a screenshot of the strip at the start of the transition, turned each light level into a pixel on a column of 16 pixels, increased the time by the divided difference from earlier, took another screenshot, repeat.

For 15 screenshots.

Stitched all of them together, and done.

Except...

Lightning.

Optifine made the 16th column of their lightmaps the lightning strike lighting color for some reason. In vanilla, the light from a lightning strike is added to the light of the time of day. This is impossible in Optifine's system, so I took a screenshot during a lightning strike at midnight, since this gave the most vibrant color. It's not perfect, but it's good enough. This entire thing is only for reference purposes anyway.

I hope this helps someone. Nobody should have to go through all of that just to get a vanilla game asset ever again.