Environment Mapping Tutorial
Environment Mapping Tutorial
by Frenzy / TeSkO
Hiya reader. This is a small tutorial to help people understand how simple environment mapping really is :) I'm assuming you have experience with 3d graphics and have done Gouraud shading and texture mapping. Also, I'm assuming you have knowledge of or have used fixed point maths namely 8.8 fixed point. If your answer is yes to all the above then this doc should tell you what you need to know, otherwise I'd suggest you leech some other tutors from ftp.cdrom.com or other such site..
THE ENVIRONMENT MAP
First things first, the environment map. This is just a texture map. It can be any size but for simplicity I'm going to assume a 256x256 texture map. This environment map can be anything you like. However, depending on how your environment map looks depends on how your env mapping will look. What I'm trying to say is this. If you have a good map your objects will look a lot kewler. You can also simulate phong using env mapping just by making an appropriate environment map. A sample environment map is given called 'PHONG.PCX'. Its just a simple map that will make your objects phong shaded. Once you have seen this you should get the basic idea on what an environment map should look like. There is also one called 'METAL.PCX' which will make your objects look metallic/chrome. Looks very nice. I got this from somewhere, dunno where. But it looks good and it demonstrates what the environment maps should look like. BTW, this METAL.PCX looks rather like a face.. How bizarre, I really would like to know where it came from :-)
HOW TO ENVIROMENT MAP
Well, you've read the above and seen the .PCX files and you are still with me. Good, here comes actual env mapping technique.
You have vertex normals for each vertex in your object right? Just like in gouraud. Well, the way we use them is a bit different. In gouraud you took the dot product of the light vector with each vertex normal. Well, in env mapping you have to calculate the U,V coordinates so you can texture map with them. Remember, env mapping is a straight texture map. The only difference is the way you get your U,V coordinates. What you do is this:
You must have your vertex normals in 8.8 fixed point!!!
For every vertex normal in your object compute its U,V coordinate by taking the X and Y components and dividing by 2. This gets them in the range of -128..128, then add 128 to that. That then gives you them in the range of 0..256 which just so happens to be the dimensions of your texture map :) So, you have this:
Now, once you have done that you can simply texture map as normal using these U,V coordinates... Simple eh? Why, did you expect it to be hard??
OPTIMIZATIONS
Of course, make your texture mapper fast. Also, you don't need to rotate the vertex normals. This saves a lot of time. I won't go into this as you might already know. If not then just ask. There is also another way to find your U,V coordinates using spherical coordinates. However, this is not necessary.
FINAL WORDS
Well, thats that done. A good 5mins read? if it didn't help then sorry but it works for me.. If ya have any other questions then just ask. Your surely have some.
Oh yes, I threw together a small example called 'DEMO.EXE' to show you what it should look like. Its crude but its only an example. Don't know how fast it will be on your machine but on a dx4 120 it runs at 51 fps.
My env mapping is not fully optimised at the moment :(
Byeeee
Signed,
Paul aka Frenzy
EMAIL: p.adams@wlv.ac.uk
