I reproduced as well and started debugging it, but I have not fixed it yet. It affects all frei0r-based generators in Open Other.
I fixed this for the next release v19.06.
Be that as it may, what you call “SMPTE color bars” in Shotcut do not comply with SMPTE RP-219 when delivered to the user regardless of Shotcut’s internal scaling, so it’s misleading to call them “SMPTE color bars”.
What does that mean?
Shotcut’s internal scaling
It is not internal; it is the default libswscale behavior to use sRGB.
I think you should inspect the Y’CbCr output. Perhaps, there is some misunderstanding about the RGB colorspace. Shotcut uses sRGB. AFAIK, sRGB is full range and not limited to MPEG levels as you have depicted: that appears to be Studio RGB. Maybe you think Shotcut should use Studio RGB instead of sRGB, but I disagree as we incorporate more sRGB sources from PNG, WebVfx, and generators such as these color bars. Here are similar discussions:
Allow me to go over the role of color bars in professional video.
There is one standard and one standard only for what Shotcut calls “SMPTE color bars”. There is no room for debate or disagreement. I posted a link to the standard previously along with an explanation. Here is the link again:
http://car.france3.mars.free.fr/HD/INA- 26 jan 06/SMPTE normes et confs/rp219 mirehd.pdf
If you’ve ever worked with professional video you know that color bars are not mere decorations. They are a calibrated test signal which is expected to meet certain specifications. They will be monitored by test equipment which is also calibrated to RP-219, to determine whether the video complies with FCC and ATSC specs and is thus suitable for broadcast.
Regardless of the calculations involved, take your eyedropper program and read the colors off the screen. As I posted above, yellow, for example, would be 180-180-16.
The FCC doesn’t care about full range, sRGB or discussions on doom9 when they issue a citation and make a broadcast licensee pay a fine.
How you can possibly compare this approach to professional and broadcast video? The window of an app on a computer is not a broadcast medium. Most computer monitors are calibrated toward sRGB, and that is what this preview targets. If this is how you are testing it, then it is wrong. The FCC and broadcasters are certainly not doing it that way. Maybe you should test the SDI output instead, or, I as suggested before, the Rec. 709 Y’CbCr output of an export. I am done arguing with you about and not changing the RGB handling.