Interior Mapping meets Escher

One cube, six interiors

My Inspiration

My program was inspired by a piece of M.C. Escher's artwork, “Other World” (seen below). In this artwork, the viewer looks from the inside of a cube and sees five walls, and each pair of walls shows a different view angle of the moon's surface. In my program, I have reproduced the effect of that painting, inverted: using interior mapping, I have given each face of the cube a different interior, occupying the same virtual space.

"Other World", by M.C. Escher

Screenshots

Program Logic

C++

The first piece of logic is in my C++ program: it loads all six cube maps (one “interior” cube map for each exterior face of the cube). It then draws the 6 faces of the cube, binding the cube maps in place. The C++ program is also responsible for feeding the world position into the shader as a texture coordinate.

Shader

My shader implements the following techniques:

Alpha Blending

The outer window texture is transparent in the parts where the interior should appear.
The interior textures allow transparency as well -- this will allow the world background to show through.

Interior Mapping

Interior Mapping uses the fragment shader to cast the incoming view ray (“direction”) into the interior of a cube, and decide which wall it hits.

Cube Mapping

The outer world background sphere uses normals to directly use up the cubemap texture.
The cube interiors use the position of the ray-cube intersection to look up the interior wall texture.

Diffuse Lighting

Initially, I computed both the exterior color and the interior color using diffuse shading from the single light source
Later, I decided that the interior looks better full-bright, instead of lit from outside. outside world through them.

Resources

Interior Mapping

http://interiormapping.oogst3d.net/

I had previously seen an interior mapping demo created by “Joost 'Oogst' van Dongen”; this demo provided the base shader code. I fed the provided shader code through the NVIDIA CG compiler to obtain GLSL code, and then revised from there.

Textures

http://humus.name/index.php?page=Textures – Cube Map Textures

http://www.humus.name/index.php?page=3D&ID=80 – Interior Mapping Demo

I obtained my cubemap textures from Emil Persson (“Humus”) -- more specifically, I used the following named cubemaps (note that the camera starts out looking at the "left" side):
Right: "MarriottMadisonWest" Left: "Interior" (from InteriorMapping demo) Front: "DallasW" Back: "Skansen2" Top: Walls are the window texture; top and bottom are from the open-source game Nexuiz. Bottom: "Earth"; exterior bottom is from Nexuiz.

Challenges

Some of the most challenging aspects of this program were as follows:

Obtaining correct “view direction” vectors in the shader -- I ended up having to feed the world coordinates into the shader through one set of texture coordinates.
Dealing with offsets and a cube from (0,0,0) – (1,1,1) – a cube centered around zero resulted in 4 walls and broken cubemap coordinates.
Ensuring that a sideways incoming ray did not immediately collide with an interior wall -- I eventually had to use a factor of "0.999" in the wall distances, to ensure that the virtual inside walls were "behind" the physical walls.
Using alpha-blending to enable viewing “through” the cube from outside, as well as viewing the world from the inside.

Demo and Code Assumptions

I have archived the demo (compiled for Linux) here: dgoyette_interiormapping_demo.tar.bz2

My demo assumes the following:

All textures are in TGA format -- RGB or RGBA
- Higher alpha means more opaque; the reference Interior Mapping demo did the opposite.
All textures reside in a file structure as follows:

<program dir>

Other Notes:

If I really wanted to confuse the viewer, I could have made each of the rooms have a different definition of "up"... but I decided that that would be a bit too evil.

In addition to the Escher painint, I was also inspired by a level from the old game Duke Nukem 3D, "Tier Drops" [youtube.com], that used the game engine design to awesome effect: Picture a spiral staircase with four central landings (also connected via vertical tubes)... and then flatten it so that all four center areas occupy the same physical space. This was the effect I was trying to create in my program.