user17609: ffmpeg does a very good job of getting rid of a green screen, with a command like this:
ffmpeg -f lavfi -i color=c=black:s=606x1080 
       -i input.mp4  
       -filter_complex 
            "[1:v]chromakey=0x70de77:0.1:0.2[ckout];
            [0:v][ckout]overlay[out]" 
       -map "[out]"  output.mp4

but it replaces the green screen with solid black, whereas I would like it replaced with a transparent background, alpha = 0.
I tried setting the color to #00000000 (8 zeroes indicating a zero alpha), but that still resulted in a black background.
Strangely, when I set the color to say red, it gives a red tint to many unfiltered areas.
So I would like a transparent background, but I'm also curious what the color option is doing.

user1871: Output by MP4 defaults to the H.264 codec for the video. H.264 does not support alpha.
You can output to .mov with a video codec -c:v of png or qtrle. There are others but they have restrictions on resolution and framerate.
You can't use the overlay filter as it doesn't output to a pixel format with alpha. So, just the chromakey filter, and no color source.

17609: <p>ffmpeg does a very good job of getting rid of a green screen, with a command like this:</p>

<pre><code>ffmpeg -f lavfi -i color=c=black:s=606x1080 
       -i input.mp4  
       -filter_complex 
            "[1:v]chromakey=0x70de77:0.1:0.2[ckout];
            [0:v][ckout]overlay[out]" 
       -map "[out]"  output.mp4
</code></pre>

<p>but it replaces the green screen with solid black, whereas I would like it replaced with a transparent background, alpha = 0.</p>

<p>I tried setting the color to <code>#00000000</code> (8 zeroes indicating a zero alpha), but that still resulted in a black background.</p>

<p>Strangely, when I set the color to say red, it gives a red tint to many unfiltered areas.</p>

<p>So I would like a transparent background, but I'm also curious what the color option is doing.</p>
 1871: <p>Output by MP4 defaults to the H.264 codec for the video. H.264 does not support alpha.</p>

<p>You can output to <code>.mov</code> with a video codec <code>-c:v</code> of <code>png</code> or <code>qtrle</code>. There are others but they have restrictions on resolution and framerate.</p>

<p>You can't use the overlay filter as it doesn't output to a pixel format with alpha. So, just the chromakey filter, and no color source.</p>
