user12346: This is the best my actioncam can create.

My goal is to take advantage of this low quality and save as much harddrive space as possible so visible quality remains.
Which measurements can I take using which windows programs to exploit the already low quality and save as much space as possible?

user1871: There are a few video quality metrics available for you to use, primarily SSIM and also PSNR. You can use ffmpeg to convert the video and then compare the output.
Step 1 Convert the video
ffmpeg -i actioncamfile -c:v libx264 -crf 23 -c:a copy -map 0 compressed.mp4

The CRF value modulates the quality. Lower values produce better quality but larger file sizes. Typical values range from 18 to 28, but absolute range is 0 to 51, but 0 being lossless, which will produce an exact looking file but will be bloated.
If you want to test with only a small section, say 20 seconds of the source starting from 15 seconds into the file, you can use
ffmpeg -ss 15 -i actioncamfile -t 20 -c:v libx264 -crf 23 -c:a copy -map 0 compressed.mp4

Step 2 Measure the quality of the conversion
Run
ffmpeg -i compressed.mp4 -i actioncamfile -lavfi "ssim;[0:v][1:v]psnr" -f null -

This command output, at its end, will display lines like this
[Parsed_ssim_0 @ 000000000039ad80] SSIM Y:0.984483 U:0.980458 V:0.980921 All:0.983219 (17.751712)
[Parsed_psnr_1 @ 0000000000398320] PSNR y:42.63 u:43.19 v:44.09 average:42.90 min:42.07 max:46.16

You can aim for a SSIM All value to be at least 0.99, or set your own benchmark, based on what's acceptable.

12346: <p>This is the best my actioncam can create.</p>

<p><a href="https://i.stack.imgur.com/0GbFx.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0GbFx.jpg" alt="Rollei Wifi 5S"></a></p>

<p>My goal is to take advantage of this low quality and save as much harddrive space as possible so visible quality remains.</p>

<p>Which measurements can I take using which windows programs to exploit the already low quality and save as much space as possible?</p>
 1871: <p>There are a few video quality metrics available for you to use, primarily <a href="https://en.wikipedia.org/wiki/Structural_similarity" rel="nofollow">SSIM</a> and also <a href="https://en.wikipedia.org/wiki/Peak_signal-to-noise_ratio" rel="nofollow">PSNR</a>. You can use <a href="https://ffmpeg.org/download.html" rel="nofollow">ffmpeg</a> to convert the video and then compare the output.</p>

<p><strong>Step 1</strong> Convert the video</p>

<pre><code>ffmpeg -i actioncamfile -c:v libx264 -crf 23 -c:a copy -map 0 compressed.mp4
</code></pre>

<p>The CRF value modulates the quality. Lower values produce better quality but larger file sizes. Typical values range from 18 to 28, but absolute range is 0 to 51, but 0 being lossless, which will produce an exact looking file but will be bloated.</p>

<p>If you want to test with only a small section, say 20 seconds of the source starting from 15 seconds into the file, you can use</p>

<pre><code>ffmpeg -ss 15 -i actioncamfile -t 20 -c:v libx264 -crf 23 -c:a copy -map 0 compressed.mp4
</code></pre>

<p><strong>Step 2</strong> Measure the quality of the conversion</p>

<p>Run</p>

<pre><code>ffmpeg -i compressed.mp4 -i actioncamfile -lavfi "ssim;[0:v][1:v]psnr" -f null -
</code></pre>

<p>This command output, at its end, will display lines like this</p>

<pre><code>[Parsed_ssim_0 @ 000000000039ad80] SSIM Y:0.984483 U:0.980458 V:0.980921 All:0.983219 (17.751712)
[Parsed_psnr_1 @ 0000000000398320] PSNR y:42.63 u:43.19 v:44.09 average:42.90 min:42.07 max:46.16
</code></pre>

<p>You can aim for a SSIM <code>All</code> value to be at least 0.99, or set your own benchmark, based on what's acceptable.</p>
