Making Normal Channels in GIMP (with njob)
Hello everyone! I was explaining how to do this in the Sims of History discord server and realized how much of this process I learned through trial and error. There isn’t really a good step-by-step tutorial about how to do this in GIMP, only in Photoshop (at least no text-based tutorials). While I use both Photoshop and GIMP for various things, I prefer to make my normal channels in GIMP. This tutorial will walk you through the process and hopefully demystify normal channels in GIMP.
Normal channels (bump maps) add additional depth beyond your mesh, which is useful for things like folds, painted on pockets, and buttons.
This tutorial is particularly for how to make Create-a-Sim items, not objects, but a lot of steps should be transferable.
You will need (all free programs):
GIMP
njob
Sims4Studio and/or CAS Tools
First, open your diffuse texture in GIMP. I often try to use the light/base texture rather than one I have colored already, but if you have already colored it and didn’t save a base, don’t worry, it doesn’t make much of a difference. Don’t use an image which has a pattern applied to it, as that will create a bump on the pattern and appear like applique or something along those lines (unless that is your goal).
I recommend doing this step after you have tested your diffuse texture image and mesh (if applicable) in the game. If you have any last minute changes to either of these, you will probably need to re-do your normal from scratch.
If your image is in layers for recoloring, or not, choose to flatten the image. Transparency isn’t helpful in making normal channels, so get rid of it so you won’t have to worry about it later.
Once you have flattened it, the background usually turns either black or white. It doesn’t really matter which one, it won’t make much of a difference in njob.
Now, select the crop tool. You can see the current dimensions of the image in the “aspect ratio” box. For a CAS items, the original will be 1024:2048, which is 1 x 2. We will need to crop the image into a 1 x 1, or perfect square.
To have your normal channel as high quality as possible, you should use 1024:1024. This will work for clothing items like full body outfits, tops, bottoms, gloves, socks, and tights (basically anything mapped in this bottom portion of the UV map). For all of these items, you must use a square and cannot crop it to be smaller. For shoes, this means a very large blank area.
For accessory items like jewelry and hats, the cropping is different. For instance, a hat would be 512:256. If you are unsure of the dimensions to use, export the normal channel on the maxis item and copy its dimensions. The following instructions will assume you are making a clothing item and not an accessory item.
Click anywhere on the image while you have the crop tool active, then adjust your dimensions in the box to the left. You can manually type it in or drag the box and follow the size in the box as you drag.
The box should be perfectly aligned with the bottom and have no space below. If you have space below, just drag it as far to the bottom as you can. GIMP will stop you from dragging it outside of your current dimensions. If a little is sticking out at the top, that is okay. Nothing can be mapped outside of the 1024:1024 dimensions, so it is probably just bleed over or space filler that you are cropping off.
Press “Enter” on your keyboard to complete the crop.
Optional Step: If you desire, you may want to decrease the brightness and increase the contrast on your image so there will be more for njob to pick up. If you already have a lot of contrast, you may not need to do this.
Next, export your image as a PNG or BMP. Be sure to not overwrite your original diffuse texture.
You can now close GIMP, though you will need to open it again later.
Open njob, then your saved, cropped image.
Maximize your screen so you can see what you’re doing.
Go to Filter > Diffusemap > Heightmap and select that option.
The screen will pause to load for a bit before opening up a new box.
Your image will convert to black and white and may look a bit strange. The first step is to change your “Course Detail” setting to the lowest (or close to the lowest) setting and your “Fine Detail” to the highest setting. I generally play around with the “Mid Detail” and “Scale” until I get what I want. Try to have what you want to be visible stand out, while folds should be soft and fuzzy but still somewhat distinguishable.
Once you have what you need, click OK and go to Filter > Heightmap > Normalmap
There are two settings, “Scale” and “Blur Radius.” Neither of them have “ideal” settings, so you will need to adjust as you need. “Scale” controls the depth of the contrast and “Blur Radius” impacts the softness of the image. If your edges are too harsh, your normal map may look odd in game.
You will probably also have lines in areas in no texture. This is normal, and I will go over how to remove those later.
Once you are satisfied with how things look, save the image as a bitmap.
Now, open your bitmap image in GIMP. It is time to clean up the image and get rid of the artifacts. Unfortunately, unlike a specular, a normal map doesn’t have a mask to prevent bleed over onto skin or other textures. The unused areas need to be a midrange, solid grey. It is easier to edit at this step before you create your transparency.
Select a midrange blue color from one of the blank areas with your color selector and make it your background color by using the arrow button between the foreground and background colors.
Select the areas that should be blank and delete them, which will replace the lines with a solid blue color. This would be areas around the neck, wrists and ankles, and also places like the filler beneath skirts and tops that doesn’t need texturing. Be sure to select the odd lines around the image, which are usually a bright teal or hot pink color. Those can be very visible.
Sometimes, you may need to take your paintbrush and clean up the artifacts if they are in curved or very small areas. If anything looks too sharp, you can also use the smudge brush to smooth it out (very lightly). But don’t move anything around too much.
Once you have cleaned up your image, export it again as a bitmap. This is just so you can go back to it if you make a mistake later or need to modify it. Usually I save it as a new image, but you can overwrite the old one if you are feeling confident.
I have to point out that sometimes you can get away with not cleaning the artifacts from your image. But I have had too many issues with it in the past to skip this step.
Now it is time to make the normal map. Finally!
In the layers area, right click on your single layer and add an alpha channel to it. There is also a small button at the bottom you can use to add an alpha channel. You will need this transparency for the next step.
Next, go to Colors > Components > Decompose.
A small box will open up. Change your color model from RGB to RGBA to enable the alpha channel as a separate channel (layer).
Now, a new image will open that is your bitmap but greyscale. You will see four layers on the side called red, green, blue and alpha.
Select the red layer and click CTRL + A on your keyboard to select the entire layer. Then, click CTRL + C to copy the layer. (If you don’t have a keyboard, you can do “Select all” and “Copy” but this takes longer).
Now, go to your layer named alpha and press CTRL + V (paste) and CTRL + H (to anchor the layer). Now, you have replaced the alpha channel with the red channel.
Next, go to the green layer and select and copy it. Paste and anchor it into the red and blue layers, just as you did before with the red layer and alpha layer.
Your image won’t look too much different right now, it will just look like the green alpha channel rather than the visible red alpha channel when you opened it. Go back to the top bar and choose Colors > Components > Recompose. This will alter your original image, so the one you have open in layers will stay open. Go back up to the top and select the original image to go back to it, or close your layered image.
Now, your image should have changed from mostly blue to a transparent, mid-range gray with only a few elements visible. This is how it is supposed to look. If you don’t have transparency or it looks very different, then you probably messed up somewhere. Generally, I find it easier to go back to the original cleaned up bitmap (that you saved for future use) and start from scratch rather than trying to figure out where I messed up. That is usually faster.
Next, export your single-layer image as a PNG or DDS file (your preference). You will need a DDS plugin to save DDS files.
Open your item in Sims4Studio and import your new image file in the normal texture category. You will see a small preview in the box, which will probably show more details than you were able to see in GIMP. If it looks correct, save it and go to check it in game or in CASTools (which has a feature for previewing bumpmaps that can help you check for alignment problems). CASTools can be particularly useful if your computer doesn’t open the Sims quickly and you want to preview multiple bumpmaps. The only issue with CASTools is that it doesn’t really look much like it will look in game.
In CASTools. You have to select to see the bumpmap in the Previewer tab.
In game.
As I have only been making normal channels for a few months now, it’s possible I have missed some things, so if you know an easier or better way let me know and I can update the tutorial. I hope this is useful to you!
