Advanced POV-Ray Effects: Animation

by D.W. Hyatt

Using the Clock Variable

It is possible to send to the POV-Ray program an external value called a clock variable that can be used animate objects within a POV-Ray scene. The value is passed via an external script using an additional K parameter in the form +Kn where n is a number representing the clock value.

This clock value can be utilized in many ways within the source file in order to do animations or modify characteristics in the ray traced image. Consider the following examples:

POV-Ray Command	Explanation
sphere { <0,0,0>, *3.0 + 0.5 clock**}	This would cause the radius of a sphere to get bigger by 0.5 times the clock value.
sphere { <0,0,0>, 3.0 pigment { color rgbf < *0.0 + 0.1clock**, 0, 0, 0> } }	This would cause the color of the sphere to get redder by 0.1 times the clock value.
sphere { <0,0,0>, 3.0 translate < 0, *3 (sin(radians(30clock))*, 0> }	This will cause the sphere to move up and down in the y direction depending upon sine of the angle defined as the clock value multiplied by 30 degrees. Note the use of a function to convert the angle into radians.
camera { location <0, 5, -10> look_at <0, 0, 0> rotate <0, *30clock**, 0> }	This will cause the camera to rotate 30 degrees times the clock value

Using Scripts and the Clock Variable to Help Make an Animation

In order to make an animation, the programmer must make a series of separate frames with the objects in different locations. Each of those frames must be saved as a separate image and then put together to make some kind of movie. This will be the real advantage of the clock variable because we can define the animation in terms of that value, and then pass different numbers to the script.

The simple povray script we have used before can be easily modified to include a clock variable that can be added to the outfile name. Look carefully at the following script to see how it is done:

Example Script: clockpov


   x-povray -I$1.pov +V +D0 +P +X100 +FP16 +O$1$2.ppm +K$2

If the above script is called with the following arguments:

Example Call:

clockpov sphere 05

The script will use the filename sphere ( argument $1 ) and the clock value 05 ( argument $2 ) for the parameters in the script. Povray will look for the source file sphere.pov, and it will create a PPM image outfile called sphere05.ppm that is a combination of both arguments. This will be extremely handy when we use another program to animate the images and create an animated GIF for a web page.

Animating the Output Files

The output files can be animated using a module from the UNIX program called ImageMagick. To just animate the various frames, just type:

animate sphere*.ppm

sphere00.ppm	sphere01.ppm	sphere02.ppm	sphere03.ppm
sphere04.ppm	sphere05.ppm	sphere06.ppm	sphere07.ppm
sphere08.ppm	sphere09.ppm	sphere10.ppm	sphere12.ppm

This will take the files that start with sphere and end with .ppm and will animate them in sequence depending upon their file names. That is why it is convenient to have them named with a numerical value attached. With ImageMagick, there are controls to speed up or slow down the animation as well as make it go forward or reverse.

Making an Animated GIF Image

To create an animated GIF image, ImageMagick has another routine that is quite helpful called convert. Although it is possible to convert directly from the PPM images, it is better to use xv to make some smaller GIF images of each frame, and then use the command:

convert -delay 20 -loop 0 sphere*.gif animatespheres.gif

This will take all of the source frames and will make them into one animated GIF image called animatespheres.gif. The -delay 20 argument will cause a 20 hundredths of a second delay between each frame, and the -loop 0 will cause the gif to loop over and over again.

Click on the image to the right to see it animate. This sequence has used many of the above concepts including a rotating camera, the moving red sphere that goes up and down, and the changing color of the translucent sphere as it goes from blue to clear. Twelve frames were used to make the animation. The source code is listed below.

Source Code

Conditional Clock Values

Sometimes one might want the clock to do one action for a while and then do something else later. This can be accomplished by a using conditional statement.

For instance, the following code shows how to use conditional actions for the povray clock variable using the #if and #else statements so that the sphere is drawn one way when the clock is less than 5, and another way the rest of the time. The code must also have the #end statement to show the parser where the statements belonging to the conditional stop and other normal statements continue. This code also shows how to define a new variable with #declare. In this case, we have created something called XShift that we use to translate or shift along the x-axis that only happens when the clock exceeds the value of 5 in the example below.


#if (clock < 5)
  sphere { < 0, clock * 0.1, 0 >, 1 }

#else
  sphere { < 0, clock * 0.5, 0 >, 1 }
  #declare XShift = (clock - 5) * 0.1
  translate < XShift, 0, 0 >

#end

Using the Cluster to Create an Animation

Animations can be generated more quickly by running POV-Ray on the Linux Cluster.