Skies and Floors
Author: Brian Lozier
If you don't have the correct flag set for the sky or for floors, you will get some strange effects. If the floor flag isn't set correctly, the player will sink into the ground and kind of ice skate everywhere. If the sky flag isn't set correctly, your skies won't look like skies, even if a sky texture is there, and also, players projectiles won't appear to go forever, they will just hit like a normal wall.
To create a surface that looks like a sky or floor, all you have to do is select the surface, and give it a sky or floor texture. (the easiest way for me is to double click it in 3D preview) Now that the texture is correct, you have to set the flags correctly. To do this, go back to the editing window, and making sure the correct surface is selected, press [enter]. This will bring up the item editor. In the item editor, set the "Surface Flag" to 204 for sky, or 5 for floor. The flag for sky is shown below.
There is also another way, if you double click the place where you just entered 204, it will invoke a screen that lets you select all kinds of flags. Ever wanted those cool sound effects like the player is walking on metal or dirt? That's here. Just use the checkboxes to select which ones you want, and the rest is history.
That's it, all you have to do now is save and gob the level to test it out.