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Laser Show Systems - Laser Graphics

Tweening Frames for Smoother Animations


Creating smooth moving and realistic animations requires that a large number of key frames be digitized.  This is time and labour intensive.  A well known animators technique called 'tweening' can be used to create frames in between the key frames for smoother movements with less digitizing.  Points per second, Frames per second and their effect on perceived flicker is also discussed.



For the purpose of this discussion, let's say you have digitized 10 key frames of a character animation and you plan to make a 20-frame animation (running on twos) playing back at 15 FPS from the 10 original key frames.  Running on twos is an animation term that means that each key frame has been used twice - in this case, once as a key frame and once as a tweened frame.



Before we continue, we need to take into account the effect of flicker and to optimize our animation in terms of points per frames and frames per second (FPS) rate to minimize flicker.
Flicker is defined in the ILDA glossary as "A perceptual effect in laser-drawn graphics when the laser cannot complete its path before the eye's persistence of vision sees the image fade. The effect, usually undesired, is that the image is flickering or pulsating."  In order to avoid the perception of flicker, it is best to keep the PPS and FPS rates as high as possible. The lower limit of flicker perception is between 16 and 18 FPS.  Old black & white movies ran at 16 FPS and had lots of flicker.  Any images running slower than 16 FPS will have perceptible and annoying flicker.  To reduce flicker to a minimum, 20 FPS or above is recommended - modern movies run at 24 FPS and flicker is almost imperceptible.
Here is a table of suggested operating parameters for various frame rates on ILDA standard 12K and 30K scanners.  Note that these are suggested maximums for conservative ratings and may need to be adjusted to suit your system.  When in doubt, always go for the highest frame rate, at the highest PPS rate, to reduce flicker. For example a, 10 FPS animations can easily be run at 20 FPS by drawing each frame twice.

Rated scanner speed

Suggested operating speed (1)

FPS rate

Maximum points per frame (2)

12K 10K 10 FPS  995
12K 10K 12 FPS 825
12K 10K 15 FPS 660
30K 24K 10 FPS 2,390
30K 24K 12 FPS 1,990
30K 24K 15 FPS 1,590
30K  24K 20 FPS 1,190

Note (1):  It is suggested that you operate the scanners below rated maximum speed to avoid thermal problems and to allow for 'headroom'.  This is especially true if you plan to swap frames to other systems which may not be exactly tuned to the rated speed.
Note(2):  The recommended maximum points per frame do not divide evenly into the total available points per second to allow for inter frame/track blanking points.

You may want to adjust the number of points in each frame to meet the recommendations outlined above so as to have minimal flicker in your finished animation.  Note that the 12K images running at 10 FPS and 12 FPS are likely to exhibit considerable flicker.  Most graphics systems allow for independent setting of the FPS and PPS rates.  Thus, a 15 FPS animation can be played back with a high PPS rate so as to minimize flicker while still keeping the number of frames required reasonable.


Hand tweening

Now that you have the 10 frames edited (and saved), copy each frame to the following frame (original frame 1 to frames 1 and 2, original frame 2 to frames 3 and 4, etc.) then save the 20-frame block. When played back at 15 FPS you will have a animation that is quite jerky as each frame is a key frame and is repeated twice.
You will now be editing the frames for tweening purposes. Unlike typical frame editing, you will not be adding or deleting any points.  You will be using tools such as rotation, move and resize to create minor differences in the frames.
In order to move or rotate only a part of a frame, you will need to make only the points you want to move or rotate active.  Some laser graphics systems provide a way of selecting which points will be active and can be edited. If all points are active, then any rotation, sizing or other edits applied, will affect the entire image. With most systems it is simplest to make all points in the frame inactive, then activate only the points you wish to modify.
Go to frame 2 and compare the position of the character with the position of the character in frame 3 (key frame 2). If, for example, the character is walking across the screen, which foot is up and how far has it moved in the next frame (3)? You can now make all points in frame 2 inactive except for the leg points. Use the rotation and move tools to position the leg in frame 2 halfway between its position in frame 1 and frame 3.
Simply moving one part of the frame (the leg) is not usually enough to convey a smooth movement. Look at the angle of the body between frames 2 and 3. Use the rotation and move tools to adjust it to be halfway between the positions in frame 1 and 3. You can look for other details to modify such as arm position(s), clothing details, etc. The more subtle differences there are between frame 1 (the key frame) and frame 2 (the tween frame), the smoother the resulting animation will appear.


Testing tween frames

Test the editing of the tween frame by playing just frames 1 and 2 on the laser projector to see the differences (use a slower frame rate that shows each frame two or three times). When you are happy with this comparison, add frame 3 to the playback and observe the movement of the character in the three frames.
When you are satisfied with the first three frames, repeat the editing process on frame 4 (2nd tween frame) comparing it to frame 5 (key frame 3), then testing it in playback between frame 3 and 4 then 3, 4 and 5 and so on.
Keep repeating this tweening process on even-numbered frames until you get to frame 20. At this point it is cumbersome to keep switching from frame 1 to frame 20 to compare the movement. Copy frame 1 into frame 21 for tweening and comparison purposes. When you have completed the tween frame editing, save only frames 1 to 20 and discard frame 21 (which is a copy of frame 1).
Your animation should now move much more smoothly than the un-tweened version. Now that you have completed your animation, adjust the output rate of your system so that it is running at the desired FPS. Play the 20 animation frames again and you will see the animation as you intended it with the tween frames smoothing the transitions between the movements in the key frames.
You will want to digitize at your system's default speed (to see the onset of flicker and to accurately place points) but it is not likely that you will want to play the animation at these speeds. The animation speed speed in frames per second is more important for character movement than the speed setting in points per second provided the PPS setting is high enough to eliminate flicker.


Computer tweening (morphing)

Most high-end graphics software provides a tweening or "make frames" function. You need a starting frame and ending frame (usually with the same number of points in each frame) and a minimum of one empty frame in between (but usually more).
The tweening function works by moving the points in a straight line from their positions in the starting frame, to their positions in the ending frame; via as many steps as there are blank frames available. Some systems also allow for rotation of the image as it is being tweened.
Due to the linear nature of the point movement, the effect is uncontrolled and looks more like the "morphing" effect seen in movies and TV.    This effect can be most useful and interesting when used with text, logos or silhouette drawings (with no internal detail).
These effects are not very useful when tweening character animations since it is almost impossible to insure that certain point numbers will form the same features in the image in all key frames, you will have to hand-tween frames to obtain the best results


DISCLAIMER: Some of the information in the Backstage area is provided by the persons or companies named on the relevant page(s). Laser F/X does NOT endorse or recommend any products/services and is NOT responsible for the technical accuracy of the information provided.  We provide this information as a service to laserists using the Backstage area. 

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