meaning of ffmpeg filter option values for modifying contrast/brightness/gamma

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Context: I am using ffmpeg for processing a raw video file in raw <em>y4m</em> format to <em>mp4</em> at various brightness values for an experiment looking at OLED display performance. After looking at the ffmpeg documentation I was able to find the following filter (
Code:
eq
) for modifying the brightness of the video.
<a href="https://ffmpeg.org/ffmpeg-filters.html#eq" rel="nofollow">https://ffmpeg.org/ffmpeg-filters.html#eq</a>

I used the information from the ffmpeg documentation and the following article (<a href="https://wjwoodrow.wordpress.com/2015/07/12/more-ffmpeg-hell/" rel="nofollow">https://wjwoodrow.wordpress.com/2015/07/12/more-ffmpeg-hell/</a>) to get videos with different qualities. But neither the ffmpeg documentation nor the article explains these values in detail.

Question: Now my query is that, what does these
Code:
eq
values range correspond to? Is it a percentage? For example,

Code:
ffmpeg -i input_1080p24.y4m -vf eq=1:-0.5:1:1:1:1:1:1 -vcodec libx264 -b:v 8000k -s 1920x1080 -g 15 -n -r 30 output_1080p.mp4

gives me a output video with reduced brightness. But how do I quantify this brightness reduction? Does this
Code:
eq = -0.5
correspond to 50% reduction in original brightness? In other words, is this a linear scale?

This is my first post on stackoverflow. Thanks!