Am learning Blender for video editing and having a great time with it (brain hurts a bit, though).

Have added the gamma cross effect to a project and it is all working beautifully, except ... as the fade in happens using the gamma cross, the frame rate drops to 25fps. The project is in 50fps.

All worked fine with no dropped frames at 50fps prior to adding the effect.

On Xubuntu-core 15.10 and Blender 2.74. GTX 970, 16Gb RAM and i7 6700.

TIA and just ask if any further info required.

  • $\begingroup$ do you actually mean the preview drops to 25fps or the output movie - say an avi format - has a frame drop from 50fps to 25fps causing stuttering in the video? $\endgroup$
    – hawkenfox
    Commented Feb 24, 2016 at 17:39
  • $\begingroup$ Thanks for replying and sorry about the delay. Good point. Haven't checked if it's there after it's rendered to avi. It drops to 25fps when I'm editing the raw footage and audio and add the fade from black using the gamma cross and fade to black at end. $\endgroup$
    – Bucky
    Commented Feb 27, 2016 at 14:22
  • $\begingroup$ Will do test, render a short avi with a fade in and out and report back. Was going to say that in the last comment in a new paragraph but didn't realise hitting enter would post my comment! :) $\endgroup$
    – Bucky
    Commented Feb 27, 2016 at 14:24
  • $\begingroup$ I'll just clarify: I fade from black using the gamma cross at the beginning of the footage. While the gamma cross is happening, the frame rate stutters around 25fps. Once the gamma cross section is finished, all good and back to a steady 50fps. Same in reverse at the end. All good until 25fps when the gamma cross fade to black happens. $\endgroup$
    – Bucky
    Commented Feb 27, 2016 at 14:27
  • $\begingroup$ If that happens only during preview and not the final out put avi file, that could well just mean that your system have a hard time managing the memory required to playback the gamma cross section. If that's the case there is nothing to worry about. $\endgroup$
    – hawkenfox
    Commented Feb 27, 2016 at 14:56

1 Answer 1


The Preview Frame Rate of a project in the Video Sequence Editor depends on many things:

  • Speed of your CPU.

  • Amount/speed of RAM available.

  • Size of the original image vs that of the project.

  • Compression scheme (codec) and bitrate of the file.

  • Speed of the hard drives.

  • Complexity of the effects applied.

  • Number of simultaneous streams.

  • Size of the preview screen.

The first time you play the timeline, blender will decode the original frames form your file, compute whatever transformations you applied to them, and cache the resulting frames into RAM. Some of those operations it can do in real time and most of them it can't, depending on the factors listed above.

Once the timeline has been cached, the timeline should preview in real-time at the project's frame rate (if your machine is indeed capable of that), or at least it should play at a constant speed.

Changes on elements of the timeline strips will invalidate the cache and new frames will need to be recalculated.

Usually you can only cache a few seconds for playback in real-time.

The amount of frames you can cache is limited by the amount of RAM designated in User_Preferences/System/Sequencer-Clip_Editor/Memory_Cache_Limit.

To evaluate complex effects it's recommended to limit the range of frames to preview. Use the P key and drag to create a preview-range. To clear the preview range use AltP or press on the clock icon on the bottom of the VSE or Timeline window.

Playing frames that have not been cached will mean that blender has to calculate things on-the-fly, which might have an impact on performance and result in fluctuating frame rates.

The preview frame rate in the timeline will not affect how the final product will playback. Once you've rendered to a video file you are just playing a single video stream with all of the effects baked into it, meaning that all of the transitions and effects are part of the images, so no more complex computations need to take place, and you should expect a constant playback speed (again, depending on your computer's power and the compression scheme being used).

  • $\begingroup$ Sorry for the delay. Yep, rendered a short sample with a fade from black and playing back the rendered section in Blender runs at a steady 50fps. So, end result is AOK and that's what I'm after. Thanks all. I have this site bookmarked. :) $\endgroup$
    – Bucky
    Commented Mar 1, 2016 at 13:32

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