### 5.2.1 Animation Options

Internal animation loop, automatic output file name numbering and the ability to shell out to the operating system to external utilities which can assemble individual frames into an animation, greatly improved the animation capability. The internal animation loop is simple yet flexible. You may still use external programs or batch files to create animations without the internal loop.

#### 5.2.1.1 External Animation Loop

`Clock=`n.n | Sets `clock` float identifier to n.n
`+K`n.n | Same as `Clock=`n.n

The `Clock=`n.n option or the `+K` n.n switch may be used to pass a single float value to the program for basic animation. The value is stored in the float identifier `clock`. If an object had a `rotate <0,clock,0>` attached then you could rotate the object by different amounts over different frames by setting `+K10.0`,`+K20.0`... etc. on successive renderings. It is up to the user to repeatedly invoke POV-Ray with a different `Clock` value and a different `Output_File_Name` for each frame.

#### 5.2.1.2 Internal Animation Loop

`Initial_Frame=`n | Sets initial frame number to n
`Final_Frame=`n | Sets final frame number to n
`Initial_Clock=`n.n | Sets initial clock value to n.n
`Final_Clock=`n.n | Sets final clock value to n.n
`+KFI`n | Same as `Initial_Frame=`n
`+KFF`n | Same as `Final_Frame=`n
`+KI`n.n | Same as `Initial_Clock=`n.n
`+KF`n.n | Same as `Final_Clock=`n.n

The internal animation loop relieves the user of the task of generating complicated sets of batch files to invoke POV-Ray multiple times with different settings. While the multitude of options may look intimidating, the clever set of default values means that you will probably only need to specify the `Final_Frame`=n or the `+KFF` n option to specify the number of frames. All other values may remain at their defaults.

Any `Final_Frame` setting other than -1 will trigger POV-Ray's internal animation loop. For example `Final_Frame=10` or `+KFF10` causes POV-Ray to render your scene 10 times. If you specified `Output_File_Name=file.tga` then each frame would be output as `file01.tga`, `file02.tga`, `file03.tga` etc. The number of zero-padded digits in the file name depends upon the final frame number. For example `+KFF100` would generate `file001.tga` through `file100.tga`. The frame number may encroach upon the file name. On MS-DOS with an eight character limit, `myscene.pov` would render to `mysce001.tga` through `mysce100.tga`.

The default `Initial_Frame=1` will probably never have to be changed. You would only change it if you were assembling a long animation sequence in pieces. One scene might run from frame 1 to 50 and the next from 51 to 100. The `Initial_Frame`=n or `+KFI` n option is for this purpose.

Note: if you wish to render a subset of frames such as 30 through 40 out of a 1 to 100 animation, you should not change `Initial_Frame` or `Final_Frame`. Instead you should use the subset commands described in section "Subsets of Animation Frames".

Unlike some animation packages, the action in POV-Ray animated scenes does not depend upon the integer frame numbers. Rather you should design your scenes based upon the float identifier `clock`. By default, the clock value is 0.0 for the initial frame and 1.0 for the final frame. All other frames are interpolated between these values. For example if your object is supposed to rotate one full turn over the course of the animation, you could specify `rotate 360*clock*y`. Then as clock runs from 0.0 to 1.0, the object rotates about the y-axis from 0 to 360 degrees.

The major advantage of this system is that you can render a 10 frame animation or a 100 frame or 500 frame or 329 frame animation yet you still get one full 360 degree rotation. Test renders of a few frames work exactly like final renders of many frames.

In effect you define the motion over a continuous float valued parameter (the clock) and you take discrete samples at some fixed intervals (the frames). If you take a movie or video tape of a real scene it works the same way. An object's actual motion depends only on time. It does not depend on the frame rate of your camera.

Many users have already created scenes for POV-Ray 2 that expect clock values over a range other than the default 0.0 to 1.0. For this reason we provide the `Initial_Clock`=n.n or `+KI` n.n and `Final_Clock`=n.n or `+KF` n.n options. For example to run the clock from 25.0 to 75.0 you would specify `Initial_Clock=25.0` and `Final_Clock=75.0`. Then the clock would be set to 25.0 for the initial frame and 75.0 for the final frame. In-between frames would have clock values interpolated from 25.0 through 75.0 proportionally.

Users who are accustomed to using frame numbers rather than clock values could specify `Initial_Clock=1.0` and `Final_Clock=10.0` and `Frame_Final=10` for a 10 frame animation.

For new scenes, we recommend you do not change the `Initial_Clock` or `Final_Clock` from their default 0.0 to 1.0 values. If you want the clock to vary over a different range than the default 0.0 to 1.0, we recommend you handle this inside your scene file as follows...

      #declare Start    = 25.0;
      #declare End      = 75.0;
      #declare My_Clock = Start+(End-Start)*clock;

Then use `My_Clock` in the scene description. This keeps the critical values 25.0 and 75.0 in your .pov file.

Note: more details concerning the inner workings of the animation loop are in the section on shell-out operating system commands in section "Shell-out to Operating System".

#### 5.2.1.3 Subsets of Animation Frames

`Subset_Start_Frame=`n | Set subset starting frame to n
`Subset_Start_Frame=`0.n | Set subset starting frame to n percent
`Subset_End_Frame=`n | Set subset ending frame to n
`Subset_End_Frame=`0.n | Set subset ending frame to n percent
`+SF`n or `+SF`0.n | Same as `Subset_Start_Frame`
`+EF`n or `+EF`0.n | Same as `Subset_End_Frame`

When creating a long animation, it may be handy to render only a portion of the animation to see what it looks like. Suppose you have 100 frames but only want to render frames 30 through 40. If you set `Initial_Frame=30` and `Final_Frame=40` then the clock would vary from 0.0 to 1.0 from frames 30 through 40 rather than 0.30 through 0.40 as it should. Therefore you should leave `Initial_Frame=1` and `Final_Frame=100` and use `Subset_Start_Frame=30` and `Subset_End_Frame=40` to selectively render part of the scene. POV-Ray will then properly compute the clock values.

Usually you will specify the subset using the actual integer frame numbers however an alternate form of the subset commands takes a float value in the range 0.0 <=n.nnn <=1.0 which is interpreted as a fraction of the whole animation. For example, `Subset_Start_Frame=0.333` and `Subset_End_Frame=0.667` would render the middle 1/3rd of a sequence regardless of the number of frames.

#### 5.2.1.4 Cyclic Animation

`Cyclic_Animation=`bool | Turn cyclic animation on/off
`+KC` | Turn cyclic animation on
`-KC` | Turn cyclic animation off

Many computer animation sequences are designed to be run in a continuous loop. Suppose you have an object that rotates exactly 360 degrees over the course of your animation and you did `rotate 360*clock*y` to do so. Both the first and last frames would be identical. Upon playback there would be a brief one frame jerkiness. To eliminate this problem you need to adjust the clock so that the last frame does not match the first. For example a ten frame cyclic animation should not use clock 0.0 to 1.0. It should run from 0.0 to 0.9 in 0.1 increments. However if you change to 20 frames it should run from 0.0 to 0.95 in 0.05 increments. This complicates things because you would have to change the final clock value every time you changed `Final_Frame`. Setting `Cyclic_Animation=on` or using `+KC` will cause POV-Ray to automatically adjust the final clock value for cyclic animation regardless of how many total frames. The default value for this setting is off.

#### 5.2.1.5 Field Rendering

`Field_Render=`bool | Turn field rendering on/off
`Odd_Field=`bool | Set odd field flag
`+UF` | Turn field rendering on
`-UF` | Turn field rendering off
`+UO` | Set odd field flag on
`-UO` | Set odd field flag off

Field rendering is sometimes used for animations when the animation is being output for television. TVs only display alternate scan lines on each vertical refresh. When each frame is being displayed the fields are interlaced to give the impression of a higher resolution image. The even scan lines make up the even field, and are drawn first (i.e. scan lines 0, 2, 4, etc.), followed by the odd field, made up of the odd numbered scan lines are drawn afterwards. If objects in an animation are moving quickly, their position can change noticeably from one field to the next. As a result, it may be desirable in these cases to have POV-Ray render alternate fields at the actual field rate (which is twice the frame rate), rather than rendering full frames at the normal frame rate. This would save a great deal of time compared to rendering the entire animation at twice the frame rate, and then only using half of each frame.

By default, field rendering is not used. Setting `Field_Render=on` or using `+UF` will cause alternate frames in an animation to be only the even or odd fields of an animation. By default, the first frame is the even field, followed by the odd field. You can have POV-Ray render the odd field first by specifying `Odd_Field=on`, or by using the `+UO` switch.