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YUV/RGB Color Space Issues




On 16 Dec 97 13:56:53, telecine wrote:
> Known for some time now the color spacing problem in Discreet logic
> machines
> has been a difficult problem to solve.
> The flame takes in a digital 4:2:2 signal and spits out a signal chopped
at
> 100 units.
> making all highlights turn a Different color or pure white
> All we have been able to do is color correct in RGB to make the problem
go
> away.
> Is the another solution ?
> When Will Discreet logic fix this Problem ?
>
> Micah Kirz
> Nice Shoes
> New York
>

     First, let me try to clarify what the issues are regarding YUV/RGB
conversion. YUV and RGB are just two different models to represent the same
reality: colors. There are many other models to represent colors (CIE,
YCbCr, YIQ, ...), each of which may be related to different markets and
technologies. But let's come back to the ones that are of interest to us:
YUV and RGB. Both are as valid to represent colors and both have their
advantages.

     The reality is that these different representations may be required to
interact, and  to do that you have to take decisions. Let me use a very
simple analogy to try to illustrate what needs to be achieved. Let's
pretend that the RGB color space has the shape of a balloon and that YUV
has the shape of a box (actually the RGB space is cube-shaped while YUV is
diamond-shaped and they are skewed/rotated compared to each other). You
have to decide if you want to fit the balloon into the box (RGB
representing a subset of YUV) or if you want the balloon to include the box
(YUV as a subset of RGB). Both approaches are technically possible. The
second has the disadvantage of wasting a lot of dynamic range in colors and
thus producing more color banding artifacts. The first approach (RGB as a
subset of YUV) has the advantage of making best use of the dynamic range
and also of the fact that most YUV colors that are not enclosed in the RGB
space are considered to be illegal by most television broadcasters.

     Even though the ITU-R 601 standard states that Black is set at 16,
White at 235 and that the extra range (0-15, 236-255) is there for headroom
to minimize clipping of noise and overshoots, most analog devices and A/D
converters can still impunitively use that portion of the signal to store
video information (BTW that's only done in NTSC..). However I would like to
point out that with the advent of DTV in the US, storing and using
information in the reserved portion of a signal may become something that
you will want to avoid as much as possible since the US DTV broadcast
standard is now digital and based on the same constraints of
footroom/headroom as D1.

     That being said, we have recently investigated the possibility of
taking advantage of new architectures provided by the DIVO (Onyx2) and
Octane video I/O boards.  We are investigating the possibility of
digitizing all the information carried by a D1 signal but we want to be
careful before introducing this new technology: we are working very closely
with SGI in order to anticipate and avoid any and of all the potentially
problematic issues that may arise as a result of this change.

     All Discreet Logic systems are built on open computer-platforms
manufactured by Silicon Graphics (SGI) which are based on very powerful RGB
Image Processing Engines. We believe that there are a lot of advantages to
this approach, let me cite a few:

* Resolution-independence: The SGI graphics engines are not built as fixed
resolution hardware. Our effects system can deal with any spatial
resolution. FIRE is format independent and supports NTSC, PAL, PAL 16:9
resolutions and will eventually handle HDTV resolutions too, which we've
demonstrated on several occasions recently (SIGGRAPH, IBC, Crawford Digital
opening, SMPTE NY Fall conference). And by the way, to answer someone's
question earlier this week, yes we can sustain a full HDTV noncompressed
stream playback in real-time (1080x1920  at  30p) on a disk based system.

* Image Processing Power: Almost all the research done in the Image
Processing field is based on RGB models. This has allowed Discreet Logic to
develop the highest quality tools available: tracker/stabilizer, true
perspective 3D DVE with light sources and camera, keyer, color corrector,
warper, ... literally taking post-production to new levels. We have also
developed expertise in exploiting the SGI graphics engine, unlike systems
from other manufacturers (some of them now defunct) that only process using
CPUs.

* Huge R&D: We can leverage all the research done by SGI and bring it to
the post-production market. SGI is probably  has the largest image
processing research and development labs in the world. Their markets
include scientific and medical imagery, industrial design, military and
aeronautics, meteorology, entertainment, ... We believe that there is much
to be leveraged from all of this research, and that the whole entertainment
industry can widely benefit from it.

* Versatility: Many of our systems run on the same platform; this is a big
advantage to our clients, helping them to be flexible in the types of jobs
that they attract.  In addition, the use of a open system enables our
clients to use our systems for secondary purposes when they have down-time
(if they do!!!): 3D computer-graphics rendering, running other software,
...

     We believe that for most people, the advantages of our RGB-based
systems outweigh the disadvantages. We are aware of the issues related to
RGB/YUV conversion and are looking to address the ones we can in the
future. We are always working closely with our customers and with SGI to
make this issue more seamless in the future.

     As was discussed a lot over the last few days in this newsgroup, it
may now be time to decouple the signal formats, resolutions and frame rates
we use in the post and broadcast industry from the ones that are going to
be broadcast or received at the other end. With the advent of scanning
devices that internally natively operate in RGB (e.g. Philips/BTS Spirit)
and full chrominance (e.g. the Cintel C-Reality), and with DTV becoming a
reality in the months to come with the many different formats it brings, we
believe it's time to take a fresh new look at how we work and take full
advantage of the flexibility that computer-based solutions bring.

     Hopefully this explanation gives you a better understanding of our
situation and that we'll see you join the DL family of users soon!

Stephane Blondin
Editing Product Specialist
Discreet Logic



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