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Quick And Dirty: Realistic Skies with Sky Spheres

by Mike Kost

Introduction

Although it is possible to create amazing sunsets and sunrises with media, it's dreadfully painful. Povray takes a long time to render it, and it may not be worth it. Fortunately, sky spheres come to the rescue. This latest Quick & Dirty tutorial illustrates how to create a noteworthy sky using Gimp 2.2 and Povray 3.6.

Quick Reading

Can't fly into the wild blue yonder without some book learning first. A little background material will help nicely:

Sky Sphere Structure

The sky_sphere directive provides a great way to add background to a scene. The sky sphere color gets used when Povray rays never collide with an object. A simple sky sphere structure is shown below:
sky_sphere {
  pigment {
    <function>
    color_map {
        [0.0 <color>]
        ...
        [1.0 <color>]
    }
  }
}
Each sky_sphere contains a pigment. The pigment is almost always a color_map of some kind. The real magic in creating a great sky is generating the color map.

Also important is the function used with the color map. Sky spheres are typically shown using a function of 'gradient y'. This works but has problems. If you get a peak of the sky below y=0, the color_map will wrap as shown below.
gradient y
Two preferrable alternatives are shown below, each with "better" behavior
abs(y)
  
max(min(y,1),0)
function { abs(y) }
function { max(min(y,1),0) }
We prefer using the second alternative, and all the sky spheres in this article do so.

Preparing The Image

To generate that amazing color map, it helps to start with a picture from the real world. I took this picture with my digital camera on an otherwise uneventful flight from Chicago to Austin.
Sky Photograph
To get the image down to something workable, open it up in the Gimp and crop it [1] to a workable section of sky.
Cropping Section In GIMP
Having cropped the image, flip it 180 degrees by using Image->Transform->Flip Vertically. This gives the image below.
Cropped Sky
Althought it seems goofy, it's easier to work with the sky upside-down. Just believe for the moment - it'll be explained in a minute. Having gotten this far, I suggest saving off the small section of sky so that it's easy to refer to.

First Sampling Pass

From here, engage the Color Picker tool [2] and zoom in real close (say 800%). We're going to be looking at individual pixels and it helps to be able to find them.
Zoom In On Image
This is where the tedious work begins. To get a good sky_sphere, we'll slowly iterate through the section of sky and pick colors to place in the color_map. Every pixel has an X and Y value. Y increases as you move down the image and X increases as you move to the right of the image. This is why the image is flipped - Povray thinks increasing Y is up while Gimp thinks increasing Y is down. By inverting the image, we keep the ground level (Y = 0) low and the sky (Y = a lot) high.

The sky segment is 270 pixels tall, numbered 0 - 269. When creating sky_spheres, start by picking the first pixel (Y = 0), the last pixel (Y = 269 in this case), and then filling in about every 20 or so.
Eye Dropper View

Now, stepping through approximately every 20 pixels, we get the color map entries below.
[  0/269 color rgb <120/255, 79/255, 51/255>]
[ 20/269 color rgb <255/255,246/255,203/255>]
[ 40/269 color rgb <236/255,223/255,214/255>]
[ 60/269 color rgb <166/255,176/255,201/255>]
[ 80/269 color rgb <103/255,127/255,171/255>]
[100/269 color rgb < 66/255, 97/255,143/255>]
[120/269 color rgb < 47/255, 75/255,122/255>]
[140/269 color rgb < 37/255, 60/255,102/255>]
[160/269 color rgb < 32/255, 51/255, 84/255>]
[180/269 color rgb < 27/255, 42/255, 71/255>]
[200/269 color rgb < 25/255, 36/255, 58/255>]
[220/269 color rgb < 22/255, 31/255, 48/255>]
[240/269 color rgb < 18/255, 27/255, 42/255>]
[260/269 color rgb < 15/255, 21/255, 33/255>]
[269/269 color rgb < 15/255, 21/255, 33/255>]
Taking this and rendering it in Povray gives the image below
First Pass

Second Sampling Pass

Looking at the first rendering, there's some things that aren't right. The horizon doesn't capture the yellowish sunset glow and the blue has obvious bands to it. To fix this, look through the first pass color map, and find regions where the color changes a lot between entries. Sample new pixels to fill in the gaps. A second pass is shown below with the new color_map entries marked.
[  0/269 color rgb <120/255, 79/255, 51/255>]
[  1/269 color rgb <141/255, 83/255, 46/255>] // New
[  2/269 color rgb <177/255, 86/255, 41/255>] // New
[  3/269 color rgb <235/255,128/255, 72/255>] // New
[  5/269 color rgb <255/255,159/255, 72/255>] // New
[ 10/269 color rgb <255/255,218/255,112/255>] // New
[ 20/269 color rgb <255/255,246/255,203/255>]
[ 30/269 color rgb <255/255,240/255,219/255>] // New
[ 40/269 color rgb <236/255,223/255,214/255>]
[ 50/269 color rgb <205/255,204/255,212/255>] // New
[ 60/269 color rgb <166/255,176/255,201/255>]
[ 70/269 color rgb <129/255,149/255,182/255>] // New
[ 80/269 color rgb <103/255,127/255,171/255>]
[ 90/269 color rgb < 79/255,110/255,154/255>] // New
[100/269 color rgb < 66/255, 97/255,143/255>]
[110/269 color rgb < 52/255, 84/255,131/255>] // New
[120/269 color rgb < 47/255, 75/255,122/255>]
[140/269 color rgb < 37/255, 60/255,102/255>]
[160/269 color rgb < 32/255, 51/255, 84/255>]
[180/269 color rgb < 27/255, 42/255, 71/255>]
[200/269 color rgb < 25/255, 36/255, 58/255>]
[220/269 color rgb < 22/255, 31/255, 48/255>]
[240/269 color rgb < 18/255, 27/255, 42/255>]
[260/269 color rgb < 15/255, 21/255, 33/255>]
[269/269 color rgb < 15/255, 21/255, 33/255>]
Rendering again, we get this second and better scene. The image on the right shows the difference between the first and second renderings [3].
Pass 2 Difference Pass 1 To Pass 2

Third Sampling Pass

To get a little bit better matching with the original image, we did one more pass through. Most of the focus was placed on filling in the regions with large changes in the blue component. Again, the new color map entries are marked.
[  0/269 color rgb <120/255, 79/255, 51/255>]
[  1/269 color rgb <141/255, 83/255, 46/255>]
[  2/269 color rgb <177/255, 86/255, 41/255>]
[  3/269 color rgb <235/255,128/255, 72/255>]
[  5/269 color rgb <255/255,159/255, 72/255>]
[  8/269 color rgb <255/255,203/255, 94/255>] // New
[ 10/269 color rgb <255/255,218/255,112/255>]
[ 13/269 color rgb <255/255,233/255,148/255>] // New
[ 15/269 color rgb <251/255,241/255,172/255>] // New
[ 20/269 color rgb <255/255,246/255,203/255>]
[ 30/269 color rgb <255/255,240/255,219/255>]
[ 40/269 color rgb <236/255,223/255,214/255>]
[ 50/269 color rgb <205/255,204/255,212/255>]
[ 55/269 color rgb <185/255,190/255,209/255>] // New
[ 60/269 color rgb <166/255,176/255,201/255>]
[ 65/269 color rgb <149/255,163/255,190/255>] // New
[ 70/269 color rgb <129/255,149/255,182/255>]
[ 80/269 color rgb <103/255,127/255,171/255>]
[ 90/269 color rgb < 79/255,110/255,154/255>]
[100/269 color rgb < 66/255, 97/255,143/255>]
[110/269 color rgb < 52/255, 84/255,131/255>]
[120/269 color rgb < 47/255, 75/255,122/255>]
[140/269 color rgb < 37/255, 60/255,102/255>]
[160/269 color rgb < 32/255, 51/255, 84/255>]
[180/269 color rgb < 27/255, 42/255, 71/255>]
[200/269 color rgb < 25/255, 36/255, 58/255>]
[220/269 color rgb < 22/255, 31/255, 48/255>]
[240/269 color rgb < 18/255, 27/255, 42/255>]
[260/269 color rgb < 15/255, 21/255, 33/255>]
[269/269 color rgb < 15/255, 21/255, 33/255>]
The result is rendered one more time and differenced against the second pass color map. Much better.
Sky Sphere Pass 3 Difference Pass 2 To Pass 3

Final Sky Sphere

For a final rendering, add some water to reflect the sky. The final image is below. The water is a plane texture is from Christoph Horrman's "Realistic Water with POV-Ray" tutorial.
Final sky_sphere rendering

Want To Know More?

As with many other topics, there's very little on the 'net on sky spheres. If you know of a sky sphere tutorial, please e-mail it in to contact@povray.tashcorp.net.

Notes And Disclaimers

[1] - I've assumed you can get around in the Gimp. If you're wondering what cropping is, try cgo:GIMP Lite Quickies, Crop.
[2] - As before, if you're unfamiliar with the color picker, try The GIMP Documentation: Color Picker Tool.
[3] - To difference two images in Linux, run 'composite -compose difference image1.png image2.png out.png' from the command line. This uses ImageMagick.

Published: 01/03/06
Last Edited: 01/03/06

Copyright (C) 2006 Mike Kost