ZDoom

Hello, and welcome to my tutorial on adding decals. This is a basic tutorial for adding decals to your map.
Anyways, lets get started.

First off, you need an image you want to use. For example, maybe you wanna use a movie poster for a city map with a nice little theater.
Here's an image I would use for example:



Use this image as an example here. If it's too big, resize it. But it should be fine like this.
Anyways, download the image, and add it to a WAD file containing a map. Don't forget the P_START and P_END markers!
Oh and add it to TEXTURE1 before continuing.

Now that you've done that, you'll have to create a new lump called DECALDEF. This is the lump that defines all decals.
Here is some sample code to use for a start.
The number at the top line equals the decal ID. Note that decals do NOT share the same ID as actors.

Here is the DECALDEF page on the ZDoom Wiki for more info.

Now that you've done all that, go into GZDB and place a "Decals" actor on any linedef. Make a square as an example level.
Go to the "actions" tab for the actor and set the ID to the decal's ID. Go into your WAD, and look at that! You have a decal in your Doom level!

If you have any questions, ask away!

I'll be giving this a go in a day or two. I'll be sure to get back to you on how it goes.

I was following this tutorial, and ended up with some problems - decal was not portraying texture as needed.

Texture in question was 128x128 (with scaling settings both in textures lamp and decaldef). However, decal based on 64x64 looked fine.