ReframeRT changes the number of frames in a sequence of video directly from the timeline. The faster method for getting the same results as this plugin is to use the speed curve which was a later addition. But if you need very precise results, ReframeRT is most useful. There are two ways to do this, which can be selected from the checkboxes in the configuration GUI. The first Stretch mode changes the number of frames in the sequence, and therefore its length, but not the framerate. The Downsample mode instead keeps the length of the movie by varying the framerate. It is important to understand that the plugin works by varying the frames, the possible change of fps is only a side effect of the creation of new frames due to interpolation. The interpolation algorithm is simply the slope of a linear curve. This plugin is keyframable and the Interpolate option works between keyframes. A simpler and more handy version is the Speed PerCent plugin.
Stretch mode multiplies the current frame number of its output by the scale factor to arrive at the frame to read from its input. The scaling factor is not entered directly but using a number of input frames to be divided by the number of output frames.
Scale factor (SF) = Input frames / Output frames
For slow motion we leave 1 for the frames of the input and we increase the number of frames of the output (for example putting 8 for the output we have slow motion 8×, with SF = = 0.125). For fast motion we leave 1 for the output and we increase the number for the input (for example 8 to have 8×, with SF = = 8). Another possibility is to put the frame rate of the media (e.g 120 fps) in the input and the project frame rate in the output (e.g 30 fps) or the opposite.
The stretch mode has the effect of changing the length of output video by the inverse of the scale factor. If the scale factor is greater than 1, the output will end before the end of the sequence on the timeline. If it is less than 1, the output will end after the end of the sequence on the timeline. The ReframeRT effect must be lengthened to the necessary length to accommodate the scale factor. Change the length of the effect by clicking on the endpoint of the effect and dragging.
Although stretch mode changes the number of the frames read from its input, it does not change the framerate of the input. Effects before ReframeRT assume the same frame rate as ReframeRT. In stretch mode to create a fast play effect enter a value greater than 1 to get accelerated playback. For a slow motion effect, use ReframeRT in stretch mode with a value less than 1.
Example: you have a clip that you want to put in slow motion. The clip starts at 33.792 seconds and ends at 39.765. The clip is 5.973 seconds long. You want to play it at ths normal speed. You divide the clip length by the playback speed ( 5.973÷0.4) to get a final clip length of 14.9325 seconds. You create an in point at the start of your clip: 33.792 seconds. You put an out point 14.9325 seconds later, at 48.7245 seconds ( 33.792 + 14.9325). You attach a ReframeRT effect, set it to 0.4 and stretch. You change the out point at 48.7245 to an in point. You start your next clip after the slow motion effect at the 48.7245 out point. You can do this without making any calculations by first applying the effect and then lengthening or shortening the bar to where the stretched movie ends.
Now in the timeline we have the affected part of the plugin where we see the slow/fast effect, and the continuation of the timeline from where the plugin ends. We then have to select the interval on which the plugin acts and render it or transform it into a nested clip or nested asset. In this way we can replace or overlap it with the part of the timeline that originally included all of the part we wanted to slow down/speed up. See also the Reframe render effect for direct rendering.
Downsample mode does not change the length of the output sequence. It multiplies the frame rate of the output by the scale factor to arrive at a frame rate to read the input. This has the effect of replicating the input frames so that they only change at the scaled frame rate when sent to the output. It does not change the length of the sequence. If the scale factor is 0.5 and the output frame rate is 30 fps, only 15 frames will be shown per second and the input will be read at 15 fps. Downsample is only useful for scalefactors below 1, hence the name downsample.
Downsample mode changes the frame rate of the input as well as the number of the frame to read, so effects before ReframeRT see the framerate×scalefactor as their frame rate. If the scale factor is 2 and the output frame rate is 30, the input frame rate will be 60 and the input frame number will by doubled. This will not normally do anything, but some input effects may behave differently at the higher frame rate.
The CINELERRA-GG Community, 2021