Um, honestly I'm not really good at tutorials, and its very dependent on what editing software you are using. Also, I'm not set up to do recording for YouTube. Personally, I use Photoshop 7.0, which is like 15 yrs old at this point, because its the last version I bought that I can still "own". Most of the features of NitroGen are explained pretty well in either the first post or the readme file included, and I recommend you read that for those. Main post also has links to several tutorials, many of which I used when I started out, so I highly recommend them. But here a few quick things:
1) mask.png is what controls what POIs can/can't be where. Clear is not masked, I use black for masked (nothing spawns in that area), and a single pixel of red is a city seed, while green is a town seed. Yellow is same as clear, but lets you mark your seeds easily. For example, looking at an 8k map, a single red pixel is not visible, so I put a large square around towns, and a large circle around cities. This lets me see what is what quickly when fully zoomed out.
2) To create your mask.png quickly, I copy my import_HM.png, use a magic selection to select all water, and fill it with black, then invert selection and delete to unmask the rest of the world.
3) Biomes are controlled by biomes.png and can only use a few very specific colors. Be sure if you are editing that that you are using a pencil-type tool with no transparency (or I think its called pixel art mode in Kriento). If you get any shading at all, any single pixel outside the 5 predefined colors, it will error on map gen within 7d2d.
4) You don't need a biomes.png when running in NitroGen. It's only used by 7d2d, using the NitroGen output to create the world. In fact, if you have it, NitroGen will overwrite it with its own. So you can do this step last. I recommend you set biomes in NitroGen to "Natural", which will just give you green everywhere. You're going to overwrite it later anyway, but it'll create the base biomes.png that you can then edit.
5) import_HM.png is your main thing here. It needs to be greyscale, and the RGB values will equal the in-game height. You need to use global contrast and brightness so that your lowest point is below sea-level (I use about
40 25 so my seas are kind of deep [Edit: wow, big mistake here, Sea level is 33, oops] ) and you don't have any huge spikes between adjacent pixels, which are what create those huge vertical cliff faces or spires. The blur and smudge tools are your friend here. Photoshop also has a filter called Gaussian Blur, which does a great job of smoothing stuff out quickly. For removing words/roads/labels, I rely heavily on cloning stamp at around 50-70% flow. The trick to removing them effectively is to sample from several areas around it and overlapping those samples, so you're randomizing pixel changes over recognizable things like letters without drastically altering the overall height in that area. This is best when words overlap terrain features like hills/ravines that would be recognizably edited if you weren't careful. Blur afterwards to smooth them out further, and smudge to repair canyons/crevices. Look at pictures below around Mt. Kosciuszo to see what I mean.
6) Remember that unless you have extremely recognizable features, no one is going to know the difference, so feel free to take liberties with terrain features. Trying to preserve, for example, the actual peaks of a given mountain range would be all but impossible, and no one would notice or appreciate it even if you did. You might make sure the tallest mountain is where Everest lies, but no one would make sure that the 2nd and 3rd highest are proportionally accurate in comparison. So have fun, and don't sweat altering terrain to get done.
7) When testing your terrain gen, use all desert for biome, and turn off all POIs. This will make map gen very fast, and desert is the easiest biome to see the terrain in, since it has only small scrubby trees to get in the way. Load it up in 7d2d, turn on god mode, and fly around to see how its looking. Also, in cheat mode (dm in console) you can shift-right-click on the map to teleport straight there (i used to type in coordinates before I figured this out, ugh!), and the lightswitch in the upper right of the map will toggle through things like "see entire map" and "see biomes" etc. Helps you orient quickly if you're trying to find a specific area of your map to critique.
Ex.
Below is a small corner of my last map, which was Australia. The first is part of my source image, and the 2nd is from the final import_HM.png in the same area. In-game, these are all rolling hills, since the scale is obviously way off of real world. Also, I put a massive bomb crater just north of Melbourne. Third is from the mask. You (might) see that I seed 3 cities and 3 towns around the crater, which itself is masked out so there are no POIs in it. The yellow squares and circles are a little hard to differentiate at this scale. End result is that you get a massive single-city-like area with a bomb crater in the center. Finally, the last is a screenshot that shows what it looks like in-game, from the top of a Dishong Tower.
