I'm sure most of you who map were faced with the problem of not having that "right" texture for a particular wall, floor, etc...
Well, you could always try to make your own!
Every city has industrial districts, "ugly" concrete walls, pavements, grates, old buildings, the sort of things that make really interesting maps and very good textures.
This won't be an in-depth tutorial about pixel tweaking your ultimate seamless texture, but it will help you get started and explain a few things.
First of all, you will need a camera:
It can be a film camera if you already have one, but you will get far superior results and much less hassle if you use a digital camera. Since the biggest textures currently in use today are 512x512 (or 1024x1024, if you really want to go overboard) you won't be needing a gazillion megapixel camera.
A 3 megapixel camera will be fine in most situations, and a 4 megapixel camera will do perfectly (buying a camera over 4 megapixels for just mapping/texturing work is overkill).
The camera should be able to take RAW format pictures (only needed if you really need a lot of detail), but at least it should be able to take pictures on a low-compression setting (Jpeg-fine on most cameras). Other features you should look out for are: proper white-balance setting (to get the colours right) and good light metering. A zoom lens is just an added bonus, giving you flexibility and allowing you to frame and capture small details further away.
Finding your dream texture:
Now that you have a camera and you are familiar with the controls, it's time to snap some textures! If you start a map, you will have a general idea of how you want your map to look like. Will it be set in an industrial district? Will it be an office map? Will it feature city center parts? You need a basic and vague concept for a map to be able to know what kind of textures you will be needing.
If you have a list or ideas, you can go out and look for areas in your city that are similar to your map; of course, you might not want to wander into the rougher areas of the city with a shiny, new, and (probably) expensive camera.
Try to look for big flat surfaces that are evenly lit. Warehouse and building walls are your best bet, but anything large and square will do. Try to pick walls that are fairly uniform, as any outstanding details will look horrible when tiled on a large surface. Also, try to pick rainy days for taking photographs as these are the days when the highlights on the walls are the most natural (but if you are doing the next de_dust, the sharp highlights and shadows provided by sharp sunlight are better).
Taking the photo(s):
First, I like to take a general photo of the area, showing the context of the texture. This helps you to find matching textures and gives you a general idea about the texture (and these shots also help you remember where you took the photo).
After this is done, you can go and take the picture that will be the texture itself. Try to maximise the area you are taking a picture of, as this will give you a bit of freedom when editing the texture at home. Try to align yourself so that you are facing the wall (or other surface) at a perpendicular angle. This will help combat perspective distortion, and will spare you from having to correct and align the texture in Photoshop later.
After this is done, I like to take a third photo of the surface from a very steep angle and from close-up, so that it gives me a sense of elevation to use for an eventual bump/normal map.
Of course, you can take as many pictures as you want, but these three are usually enough for a start. You can always go back to where you took the first shots and do some more if the need arises later.
Editing the photos:
For this, you will need some kind of photo manipulation program. Photoshop, Paint Shop Pro and The Gimp are all suited for this. I prefer Photoshop, but it is an expensive program and takes a while to get into (but it is sort of the industry standard, and it is very powerful). PSP is a nice program, it is cheaper but not as powerful as the Ps. The Gimp is free, but I find the interface awkward to use and it does lack some features I like in photoshop.
Once you have a program which you're familiar enough with you can start editing you raw photos to turn them into pretty tileable textures.
The basic workflow of editing a texture for me is this:
Find a part of the photo that has a square area that is fairly blemish free
Crop said area into a square, or ratio that you final texture is going to be in
DO NOT resize the texture just yet! It is much easier to work on a large detailed texture, then on a small pixelated mess.
Try to look for things that will look bad when repeated because of tiling (high contrast things like holes, scratches, cracks and graffiti will usually look strange), and try to clone them over with a blemish-free part of the texture. Try to think how your texture will tile and for what pupose you will use it. Don't worry if it isn't very pretty, as most pixel errors will get scaled down and disappear as you resize the texture to its final size.
Use the Offset tool often. The offset tool in Photoshop allows you to offset the photo so that the edges appear in the middle of the frame, allowing you to see how the edges will line up if the texture is tiled in-game. Try to blend the edges into each other using the smudge and clone tools.
If you are happy with the result, then save you work in a lossless/uncompressed format like TGA, BMP, or PNG. Saving as a JPG will result in a degradation of quality over time if you resave often. JPG was invented as a format for web use, or for things that you don't touch often, so don't use it if you want your textures to stay crisp and sharp.
Tips on editing textures:
Make sure that the textures you are using together will go together. This may sound like common sense, but you have to get some things right in order for your textures to look like they are part of the scenery, not just something thrown on the walls.
To achieve this, make sure that your textures have the same saturation and colour as the rest of the textures you are using, and make sure they are the same brightness. Try to avoid sudden shifts in tone and brightness in the environments that you apply the textures to.
So, I hope this is enough to get you started. I didn't go into too much detail, as the web is full of tutorials showing you who to make seamless textures and the like. Try Google!
But... If a lot of people say that they would like to have an in-depth tutorial from start to finish, then I will probably write one.