You can only check between the upper and lower points if in the virtual world, from where you are standing, they are one above another. But imagine you are standing at the bottom corner of a wall looking at the other corner of a wall. Now the points probably won't even be on the same X value. Although in reality, that formula is not about finding how tall the wall is, it's about figuring out what the scale factor you need to multiply your texture by.

And textures, I think this is where the misunderstanding is. If you simply draw a bounding box on the screen and fill it with a texture, yes, the wall will have a texture. But the texture won't change when the wall does. You could stand 2 feet from the wall, or 200 feet from the wall, and the texture would always be the same size. If the texture was a brick wall, at 200 feet away, the entire wall might just be a single brick. You have to scale the texture down so that as the wall gets smaller, the texture does too.

This same thing applies if you are looking at the wall from a weird angle, if you are doing what I said earlier, looking from the bottom corner to the top corner, obviously, the bricks closer to you will be bigger than the bricks farther away from you, and for this you need texture scaling.

Here is a simple game I made a while back using this concept. You can see that the wall gets closer, you have to scale the sprite up, otherwise, it wouldn't appear to get any closer.

He gave pictures in his previous post.

Here is the website I was talking about earlier, it works completely of the screen coordinates of the triangle, and uses a method called linear interpolation.  It also works in scanlines, which is beneficial to us calculator users since scanlines are special for our screen and can be drawn rather quickly. 

http://www-users.mat.uni.torun.pl/~wrona/3d_tutor/tri_fillers.html

Are you sure that's the right URL? I got the 'server not found' error

Yes, now it works.

Thanks, this is really useful. I'll save that link, and this will be the first thing I'm going to code tomorrow. Today, I haven't got enough time left.

Congrats on that. Could you post screenies when you finish the tex draw routine?

Also, that raycasting tutorial is great.

bad news: I think I'm going to stop with textures:I just can't get it right on my computer, and i noticed this:

Note! I find it hard to get good results using fixed point math because of inadequate precision

But that's not the main problem. The main problem is that I think that it would be way to slow. The normal triangle is fast enough, but on my computer, with 2.8GHz CPU, It takes over 5 secounds to draw the textured triangle. I know that game maker is very slow, but i don't think it's speed can be beaten by a calculator. A game just won't be fun when you have to wait for half an hour for each frame.

I think I'm going to make a 3D engine with solid faces, no textures. Unless somebody here knows how to beat the speed of a computer with a calc. (or is GM just way to slow to compare it to?)

this can be caused by 2 different things: the algorithm is very inefissient, or Game Maker is very bad at drawing. There should be some way to efficiently draw triangles

btw: I just tested GM's drawing speed, and it turns out that drawing points with gm is really slow. Could there still be a chance that this is possible? I mean: OpenGL can render over a milion textured triangles with over 2000 fps. But openGL uses the graphics card, and a calculator hasn't got a good graphics card. Is there a chance that i could get reasonable speeds with this, or should i just give up?