cube2dome
Creates fisheye images from cubic maps

Written by Paul Bourke
First version: June 2002
Update: December 2004 to support automatic conversion of animation sequences
Update: January 2006 to fix a subtle bug that added slight errors in the fisheye
Update: Dec 2008, added subdirectory support to assist with grouping cubic faces

Also, an interactive (real time) version of this is possible, see PanoDome - QT VR on Stereoids

Cubic maps courtesy of Peter Morse (Antarctica model from Terragen)
and Peter Murphy (Photographic).


Introduction

This utility attempts to read groups of up to 6 images rendered as a cubic map and creates as output an angular fisheye image. The input images are assumed to be TGA formatted files at either 24 or 32 bits per pixel (rgb with optional alpha), the output fisheye images will also be TGA files with the matching colour depth, in particular, the alpha channel in the input images will be honoured. Supersampling antialiasing is used to reduce aliasing artefacts arising from the discrete sampling processes.

While there are many potential reasons for wanting this functionality, the motivation for writing this conversion utility is to create fisheye images of animation sequences for dome projection in those cases where the rendering software being used doesn't explicitly support fisheye projection.


Input images
 
cube2dome -w 300 -a 3 10 %c_%d.tga

The utility has been compiled and tested for Mac OS-X command line usage. It has specifically been designed for converting animation sequences and thus supports a versatile file specification scheme natural to UNIX/C programmers and fulldome producers.

Usage string
Usage: cube2dome [options] filemask
       filemask    A C style filename mask, must contain %c then %d for the
                   face name [l,f,r,b,t,d] and frame number. For example:
                      frame_%c_%04d.tga
                   will look for cube faces with one of [l,f,r,b,t,d] substituted
                   for %c and the frame number substituted for %04d (zero padded)
                   New from version 1.06: file mask can now contain two %c fields.
                   This allows frames to be split by directories, for example:
                      sample_%c/frame_%c_%5d.tga
Options
       -w n        sets the output image size to n, default: twice image width
       -a n        sets antialiasing level to n, default: 2
       -n n        starting frame number, default: 0
       -m n        frame number stepping count, default: 1
       -s n        last frame (inclusive and optional)
       -j n        save as jpeg with specific quality (default: TGA, quality 100)
      -bc r g b a  sets the background colour, default: 32 32 32 255
      -mc r g b a  sets the colour for missing faces, default: 0 0 255 255
      -ht n        rotate camera about up vector, default: 0
      -vt n        rotate camera about right vector, default: 0
      -fa n        fisheye angle in degrees, default: 90
      -vp x y z    sets the view position (x,y,z) for offaxis fisheye
       -o s        overlay with the tga file "s" (expects alpha channel)
      -bf n        fade towards the back of the fisheye, n = fadepower
       -d          verbose, debug mode

File naming conventions

The file names for the 6 cubic faces are specified in a fairly general way but there are some rules/assumptions as well. The file name is assumed to consist of two items, a single letter that indicates the face of the cube and a number that indicates the frame number (cube2dome has specifically been designed to convert animation sequences). The whole file name is specified by a C style format string. For those not familiar with this, two examples are given below. The face letter is indicated by a %c and the frame number by %d. The cube face letter are as follows "l" - left, "r" - right, "f" - front, "b" - back, "t" - top, "d" - bottom/down. The frame number can be any non negative number, from 0 upwards.

Example 1.
If the frames are called l_ice_2.tga, f_ice_2.tga, r_ice_2.tga, t_ice_2.tga, d__ice_2.tga, b_ice_2.tga, then the file name mask passed to cube2dome would be %c_ice_%d.tga.

Example 2.
If the frames were called room_l0003.tga, room_f0003.tga, room_r0003.tga, room_b0003.tga, room_t0003.tga, room_d0003.tga, then the mask would be room_%c%04d.tga.

The output fisheye TGA files will have exactly the same file name style as the input images but with a "c" replaced in the cube face letter position. So in example 1 above the output file will be c_ice_2.tga, and in example 2 the output fisheye image will be called room_c0003.tga.

Options
  • -a
    Discrete sampling artefacts are reduced with supersampling, that is, pixels in the output image are sampled multiple times, the final pixel value is the average of corresponding pixels in the cubic maps. In general values of 1 or 2 can be used for preview images, 3 and 4 are typical for final images. Note that the conversion time increases roughly as the square of the antialiasing value!

  • -vt
    rotates the fisheye view by the camera right vector, this is particularly useful for dome and planetarium applications where differences in dome and seating arrangements can imply different definitions of "front".


    -vt 0 (the default)
     
    -vt 40

  • -ht
    rotates the fisheye view about the up vector. Note this is performed before -vt described above.

  • -d
    Debug mode can be helpful for diagnosing problems, mostly intended for use by the developer.

  • -fa
    Sets the fisheye angle (new: March 2005), the default is 180 degrees. Used primarily to bring the horizon into shot on a horizontal dome.


    -fa 180 (the default)
     
    -fa 200

Additional notes
  • If one of the faces of the cube is missing or the file corrupt, it will appear as a solid blue patch. This ensures that animations sequences with missing or corrupt files can still be processed and checked for other problems. This also means that if only 5 faces of the cube are required for a particular fisheye alignment then the 6th face need not be rendered. A new option (-mc) now allows an arbitrary colour to be assigned to a missing face.


    Submarine cubic map
     
    Bottom face missing or corrupt

  • All the input images are assumed to be square and all the same dimensions. The expected dimensions are determined from the first uncorrupted cubic image found.

  • When specifying the file name mask the cube face letter (%c) must precede the frame number.

FAQ

What is the appropriate resolution for the cube map images for a 4K fisheye?

Imagine the coverage of the front cube face in the fisheye, it is about 1/2 the width (or height), and about 1/2 the left face and right face make up the rest of the width of the fisheye. So, the cube maps for close to a 1:1 mapping should therefore be 2K wide.
If you are not bound by powers of 2 and if you can afford to render 2500 pixel faces then that should give you enough resolution to be safe, make sure you do your final cube2dome renders with antialiasing of 2, perhaps 3 if you can afford the time. In many cases having high resolution cube faces (4K) would be a bad thing (believe it or not), you have the chance of aliasing effects for high spatial frequency objects of your scene.