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The Skies of DOOM2, Ultimate DOOM and Final DOOM

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There are 3 skies in DOOM2.

SKY1	SKY2	SKY3

Each one is tied to a specific group of maps. For example

We know from playing DOOM2 that the sky for MAP14 is the city landscape, but in the editor the sky is F_SKY1,

therefore we surmise that F_SKY1 is just a universal place holder for a sky image.

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Let's investigate of how the skies were added for the game.

In DOOM2 textures are added as socalled patches, which are those textures found between P_ markers. Open the IWAD DOOM2.WAD with Slade3 for a closer look. Type rsky in the filter:

Here we find the patches RSKY1, RSKY2 and RSKY3, which are the real sky images, in the group P1_START to P1_END, which in turn is between P_START and P_END.

Selecting each patch we can deduce these facts:

Now that we know that the sky image for MAP01 is called RSKY1 let's clear the filter and scroll down the list to RSKY1

The sky images are registered in the lumps TEXTURE1 and PNAMES.
To find out what that looks like set the filter to tex and find the lump TEXTURE1. Doubleclick it to open the Texture Editor. Scroll down to SKY1 or set the filter to sky and see the three skies. For example:

To recap our findings:

1. For MAP01 the sky image PATCH is called RSKY1.
2. The sky image is registered in TEXTURE1 and PNAMES.
3. The actual sky is called SKY1.
4. The patch for the sky image is 256x128 pixels in size.
5. the texture repeats 4 times to span the horizon.
6. The patch must be a tiled image.
7. Each sky is bound to a group of maps.

This, of course, also applies to DOOM and Ultimate DOOM, with the exception that the patches are called SKY1, etc instead of RSKY1, etc..

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The skies of Final DOOM

However, along comes Final DOOM to show us that the sky textures are not limited to the 256x128 size. As can be seen from TNT.WAD and PLUTONIA.WAD, sky texrures can also be 512x128 or 1024x128 in size. In the case of 512x128 the texture is made up from 2 256x128 tiled patches and will repeat 2 times, once for each hemisphere. And in the case of 1024x128 the texture is made up from 4 256x128 tiled patches and will not repeat.

But, as it turns out, patches for sky textures can also be single tiled patches of 512x128 or 1024x128.

Now, that we know that the sky image can have different sizes, 256x128, 512x128 and 1024x128, you may wonder how that image is projected onto the DOOM horizon. Notice that the width doubles in each case, and that is no coincident.

Consider the DOOM landscape as a 360° view:

1. A sky image of 1024x128 will project around a full 360° sky
2. A sky image of 512x128 will project around a 180° view, therefore will repeat twice
3. A sky image of 256x128 will project around a 90° view, therefore will repeat four times

In each case it is vital that the sky image tiles horizontally, so that no breaks are detectable.

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Happy fragging.
Kappes Buur, Jan 18, 2019  updated Feb 2021

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