Hey,

So I've been fooling around with GIMP filters to make textures, and I'm not really sure if I'm 'doing it right'. Let's start with some screenshots of the textures painted onto a terrain in Unity (they might be really low quality, but I'm sure hoping they aren't...):

[attachment=6831:MyTextureExample.jpg]

On the left is the same texture, but before I manipulated its Levels. I fooled around with the Input Levels and didn't really touch the Output Levels for the right-side one. I'm bad at judging these kinds of things - does the right side one look better? Should I have not manipulated the levels as much, but still manipulated them?

Here's another example of a different texture I made and also did some level manipulation to it (left side is the original, right side is after level manipulation):

[attachment=6832:MyTextureExample2.jpg]

The environment that I was planning on using these textures for was inside a large cave system. Dark and brown-red was the color scheme.

So, along with the previous questions about levels, I'm pretty much asking this:

How do these look? I'm guessing they make you sick to your stomach...
What should I do to them to make them look better? What shouldn't I do when manipulating filters?

I've been kind of randomly using layer modes in GIMP and fooling around with them until I get a look I can agree with. I'll usually use Screen, Overlay, Soft light, Hard light, or Grain Merge. Sometimes I'll use Grain Extract after inverting the color of the layer. Should I know anything specific about what these do, or is it OK to just look at them and see if I think it looks good?

Hopefully these questions aren't too stupid.

Thanks!

Those textures to me seem noisy, but with no sense of depth or texture. They're just randomly grainy swatches of color.

Do you have reference photos? I highly recommend you either take a look at some reference photos when planning your textures, to get a feel for the kinds of patterns and shapes that actually occur in rock, or go ahead and use the reference photos as direct sources for your textures. Merely thrashing around with a bunch of filters and such, without a reference or any idea of what you really want to achieve, is only going to net you very random and inconsistent results.

Using purely Gimp filters as well to me is very limited. Filters and effects typically work best when they have good source material to work upon, rather than as an end unto themselves. Gimp procedurals are limited mostly to simple cloud noise or variations of simple cloud noise, making it difficult to do anything more elaborate than the same kind of "noisy color swatch" as you display here. Real stone/dirt/etc... is going to have patterns and variations that are difficult to achieve with simple cloud noise alone. You can go a long way toward adding a feeling of depth and texture by using a bump-map source, as it provides patterns of light and shade that are appropriate for a feeling of surface roughness.

Also, you might want to do a little research into color tones and temperatures, and how colors should be chosen for game environments. The red colors you have chosen are extremely "warm"; and extremely warm colors, I have found, tend to not work so well for environment background elements such as walls and floors. You might want to tone the temperature down so that the important elements of the scene, the characters and objects, are not eclipsed by the background. Try desaturating the colors a bit, adding a bit of grey. You can still have the brown and red shades, but not so in-your-face bright and hot.

Google image search for red rock and you will find some very good looking red stone, none of which is anywhere near as bright and hot and saturated as those red textures in actuality.

Those textures to me seem noisy, but with no sense of depth or texture. They're just randomly grainy swatches of color.

Do you have reference photos? I highly recommend you either take a look at some reference photos when planning your textures, to get a feel for the kinds of patterns and shapes that actually occur in rock, or go ahead and use the reference photos as direct sources for your textures. Merely thrashing around with a bunch of filters and such, without a reference or any idea of what you really want to achieve, is only going to net you very random and inconsistent results.

Using purely Gimp filters as well to me is very limited. Filters and effects typically work best when they have good source material to work upon, rather than as an end unto themselves. Gimp procedurals are limited mostly to simple cloud noise or variations of simple cloud noise, making it difficult to do anything more elaborate than the same kind of "noisy color swatch" as you display here. Real stone/dirt/etc... is going to have patterns and variations that are difficult to achieve with simple cloud noise alone. You can go a long way toward adding a feeling of depth and texture by using a bump-map source, as it provides patterns of light and shade that are appropriate for a feeling of surface roughness.

Also, you might want to do a little research into color tones and temperatures, and how colors should be chosen for game environments. The red colors you have chosen are extremely "warm"; and extremely warm colors, I have found, tend to not work so well for environment background elements such as walls and floors. You might want to tone the temperature down so that the important elements of the scene, the characters and objects, are not eclipsed by the background. Try desaturating the colors a bit, adding a bit of grey. You can still have the brown and red shades, but not so in-your-face bright and hot.

Google image search for red rock and you will find some very good looking red stone, none of which is anywhere near as bright and hot and saturated as those red textures in actuality.

That makes a lot of sense. I've been wondering how I could add some sense of depth to a texture, as these look so flat and terrible. Can you apply a bump map to a texture with GIMP, or do you have to save a bump and the texture itself and apply the bump to the texture in your engine? I ask because I'm not sure if you can apply a bump to textures when painting them onto terrains in Unity.
Or did you mean the bump should be a photo of dirt/stone/whatever you're making?

Regarding a source image, should I open it in GIMP, make it's layer mode Normal, and then add extra effects as layers?

You can apply a bump map in Gimp. Ideally, though, you would create a separate bump or normal map and apply it via shader, enabling you to perform correct shading. However, if that is not possible, you can use the Filter->Map->Bump Map filter in Gimp to create a bump-map from a noise layer, and apply the bump map to a diffuse color texture using a multiplicative layer.