[Bug 787582] videotestsrc: -I and +Q regions are wrong

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=787582

--- Comment #3 from David Schleef <ds at schleef.org> ---
Julien woke me up from retirement to comment on this bug.

The original RGB numbers (which are still used) were probably made up from some
quick calculations by me when originally writing it, and I certainly did not
care about accuracy at that time.  Years later, it kind of solidified into part
of the ABI, as people (like me!) started using the output of videotestsrc as a
repeatable pattern for tests.

Note that pattern=smpte was never meant to *be* the SMPTE pattern, but just be
*similar to* the NTSC SD SMPTE pattern.  Note that the color bars are 100% and
that there's a snow field.  I think the super-black -7.5 and sub-black +7.5
areas are slightly incorrect as well, since -7.5 corresponds to Y=-4 (I think?)
and cannot be described in YCbCr space.  Also something about -I and +Q should
be at 50% chroma.

I also occasionally used the difference between RGB and YUV patterns to notice
when a pipeline had been negotiated differently.

Minor trivia: One of the -I or +Q is so far out of range that naive colorspace
converters will calculate and clamp it incorrectly, making it bright orange.

I actually rewrote the smpte pattern in the pre-1.0 timeframe to conform to the
SMPTE HD pattern specification, which happens to be well-specified for digital.
 I managed to lose the patch, so it never got in.

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