Export settings and file size

I know there are already discussions about it, but I’ve read a lot of them without being able to solve the problem (if there is a solution).
I have a project that uses three original files (1080, 60fps) for a total size of about 1 GB.
Exporting with:
Preset: H.264 High Profile
1080p and 59,940… fps (automatic value)
Codec quality: 55%
GOP: 299 frames (it is so by default)

I get an mp4 file (about 3 minutes) of over 2 GB.
I’ve tried other presets (YouTube etc…) but the results are similar.

The strange thing is that with a slightly different project (1080p, 25fps, about 2 minutes), using the same H.264 High Profile preset I get an mp4 of 39 MB.
How is this huge difference possible?
Am I doing something wrong?

Thanks for any suggestions.

Because video encoding is complex and different content compresses differently when you use constant quality.

Does the video look OK to you? Then there is nothing wrong.

If you are satisfied with the video quality, then I suggest to accept the file sizes.

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The file size difference is quite extreme, i really wonder why.
There could be several reasons:

  1. fps settings: 60 fps means double the size of 30 fps, try if 30 fps will fit your needs
  2. GOP settings of 299 is weird, i normally use 30 or 60 - but it normally doesn’t change much in the file size. In general large values tend to reduce file size just a bit
  3. Content!!! Its a big difference if you take a photo of a blue sky or bushes, grass … whatsoever with lots of details. A blue sky can be compressed with jpg to near nothing whereas a photo with lots of details (bushes, trees,…) cannot be compressed that easily withoul loosing lots of detail! When your movie contains lots of detail and fast movement it cannot easily be compressed.
  4. Quality setting: the most obvious parameter if you aim at lower file size but with cost of quality. Try what a quality setting of 50% leads to and if you are satisfied or reduce it even more.
  5. Check if the source video uses variable frame rate - Shotcut doesn’t like it - so convert to user-friendly format. This probably leads to bigger file size too.

But the difference is still amazing. I would try changing some parameters - please report :slight_smile:

Thank you for your answer.

Quality is very good.
What seems strange to me is the difference in size, but having very little experience with video, maybe it’s normal.
In your experience, for a 3 minutes video (it’s a trekking video, with lots of camera movement, 3-4 transitions, etc…) exported to mp4 with the above settings is it normal a size of 2GB?

Thank you for your answer.

I’ve tried changing several settings, without important differences in size.
Of course, I can decrease the size by bringing the fps to 30 or 25, the resolution to 720p, and lowering the quality to 45% (in this way I arrive at about 350 MB), but clearly the difference is very noticeable.

Shotcut, for this project, suggest me 299 for GOP (while in the other project the value is 125).
What exactly does GOP mean?

Videos are split into Groups of Pictures (GOP). See my description in this post:

That explains it. All of the motion and probably a lot of details from nature make it very difficult to compress. When you choose Quality-based VBR size goes up to maintain quality. And when the codec detects a scene change (a lot of details moved), then it resets GOP unless you enable the Fixed option next to GOP. Otherwise, GOP is normally a maximum number of frames between key (I) frames. If you change the Rate control to Average Bitrate and Bitrate = 12M you should get a file size around 300 - 400 MB.

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Thanks everyone for the suggestions.
I will carefully read the indicated articles, and do the suggested tests, then update the discussion.

It is exactly as you say, the same project with Average Bitrate/12Mb/s results in a 400 MB file.

Going back to the original H.264 High Profile preset.
I understand the concept of the GOP pretty well.
What it’s still not clear to me, however, is the value I find as default after choosing a preset.
Basically, the GOP value changes for the same preset with different projects, and in general Shotcut shows me high values - in the case of this project, 299 (with less fps, like 25 or 30, I see that the value tends to go down to 125, and in fact if I understood the concept of GOP it seems a logical thing to me).
Is this value estimated by Shotcut as the optimal value (optimal compromise) based on the characteristics of the project?
Can changing it improve or degrade quality?
(I would tend to think that since it is a threshold value, it doesn’t change much in terms of quality).

For a good explanation with examples see here:

Back to basics: GOPs explained | AWS Media Blog.

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It is 5 seconds multiplied by the frame rate without rounding.

Thank you everyone.
I think I have a clearer understanding of the issue now.

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wow, it is really helpful! Thank you.

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