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Grading Light Illusion Forums / Grading /

Color Space Conversion in practice

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Author patwintersgill
Mono
#31 | Posted: 27 Aug 2013 08:34 
This is a very interesting discussion.
I suppose working in rec709 and then fitting that into XYZ for cinema delivery, is the same principle as working in P3, and fitting that into XYZ?
Does anyone work natively in XYZ? I've never heard of anyone doing that, if not then I suppose colour space conversion is an inherent part of any cinema delivery workflow. Steve, what do you think?
Best,
Pat Wintersgill
Creativity Media
www.creativitymedia.co.uk

Author Steve
Scene
#32 | Posted: 27 Aug 2013 08:57 
Yep, it's exactly the same
And no, it is not possible to work natively in XYZ - no display can do that!

So yes, there are always conversion for DCI deliverables.

Steve

Author patwintersgill
Mono
#33 | Posted: 28 Aug 2013 15:14 
Thanks Steve, that's very interesting. In that case to me it makes more sense to work in Rec709 - unless you were working with a lot of CG-originated material, or had more money than you knew what to do with
Best,
Pat Wintersgill
Creativity Media
www.creativitymedia.co.uk

Author Vassily
Mono
#34 | Posted: 9 Oct 2013 14:16 
Hi guys,
I am learning the colour space conversion and have couple of questions.
If I need to create the conversion LUT from for example rec709 to DCI P3 I select Source colour space - DCI P3 and Destination - Rec709. Hope this correct. I am using the Fit Chroma option and create new LUT. It indicates 95% within the target gamut. This makes sence, the p3 little bit large than rec709.
Now I create the same LUT but with Out of gamut option and it reports the only 41% within the target. Why do I experience the lower matching of the colour spaces with and without the OOG? Is that correct?

Then I burn-in the both conversion and OOG LUTs to a picture which just was taken by the camera of my phone. Below is the results:
https://docs.google.com/file/d/0B7hOTOW3Q0HJdUhLSHBQMVYzVUU/edit?usp=sharing]or iginal
https://docs.google.com/file/d/0B7hOTOW3Q0HJV3hTa0VITkE4V2c/edit?usp=sharing]re c709 to dci p3 conversion
https://docs.google.com/file/d/0B7hOTOW3Q0HJUld4OUFuc0pRYkk/edit?usp=sharing]re c709 to dci p3 out of gamut

The picture with rec709 to dci burnt in LUT looks like I expected, little bit greenish and darker in shadows (bigger gamma value) and maybe little bit more saturated. But out of gamut warning LUT indicates a rather lot of missed detailes. Is it true?

Author Vassily
Mono
#35 | Posted: 9 Oct 2013 14:18 
sorry, the images have not appeared in the post but they can be accessed by the links above

Author Steve
Scene
#36 | Posted: 9 Oct 2013 14:27 
The LUT you generated is P3 to rec709 - you have Source and destination the wrong way round...

Source is always the Colour Space within which the images SHOULD be seen - so Rec709 in this case.
Destination is the Colour Space they will be seen in - so P3.

Steve

Author Vassily
Mono
#37 | Posted: 9 Oct 2013 14:39 
I am trying to create the conversion LUT which was mentioned in the start of the topic. I mean if we are working and grading in rec709 and then need to convert to deliver into the P3. So need to create the LUT which will convert graded material from rec709 colour space to P3. Should in this case the source be Rec709 and destination P3 still?

Author Steve
Scene
#38 | Posted: 9 Oct 2013 14:42 
Yep - as the images were graded in Rec709 they are expected to be seen in Rec709, so that is their Source Colour Space...

P3 is the new colour space they will be seen in, so is the Destination for the images!

Author Vassily
Mono
#39 | Posted: 9 Oct 2013 14:57 
OK, thank you, Steve. It looks like the understanding is coming to my mind.. I was confused because the images with applied LUT as you discribed looks desaturated, thought it should be more saturated because the p3 gamut is wider. But now I realize it will look the same at the P3 display as the original Rec709 image on the Rec709 display. Correct?

Author Nepomuk
Mono
#40 | Posted: 16 Oct 2013 21:37 
I would like to ask you about the level issue.

A project was graded to video levels. It seems simple, but how do I scale it to full range?
I my DI System I could offset and scale the luma, but it should be more precise with a LUT, I imagine.

Thanks.

Author Steve
Scene
#41 | Posted: 17 Oct 2013 08:42 
All DI systems i have ever used have an option for re-scaling between data and video levels.

No LUT should be required, but is can be done by a LUT as well.

Steve

Author Vassily
Mono
#42 | Posted: 27 Oct 2013 20:36 
Hi Steve,
Sorry to my returning to the question which looks to be absolutely clear..
We have Dolby PRM monitor which can operate in P3 gamut and we have material to grade for digital cinema delivery. But maybe we also will need to have a ProRes copy of this graded material in Rec709.

Which setup would you prefer in this case to grade on P3 monitor and then convert a copy to Rec709 or grade on Rec709 and convert to P3?

I assume from the start of the topic most of peoples here working on Rec709 and this is not a problem but this is mostly due to they don't have P3 display. But if I do have the P3 display and the DCP is the main goal for delivery should I anyway work in Rec709 just because I will need the rec709 copy also. Or it is not so big problem to convert to smaller gamut. I understand that go from smaller gamut to wider is smoother but how to evaluate the risk when going from wider to smaller gamut?

In the first page of the topic there was a question which appeared to be without an answer. Maybe because there is no simple answer.. If 99% of normal scene is within Rec709 gamut why people is developing newer wider standards like P3 and more? Just for damn marketing?

And one more question. I have heard an opinion it is worth to work on monitor which is set to P3 gamut (dolby one) and D65 white point (truth D65 with x=0.3127 y=0.329) and gamma=2.6. Sounds like some hybrid format because digital cinema white point is x=0.314 y=0.351. Does it makes any sense? I consider it is better to operate either in native DCI P3 format (P3 gamut, D-cinema white point, gamma 2.6) or in native Rec709 format (Rec709 gamut, D65 white point, gamma 2.2 - 2.4). Am I right?

Author Vassily
Mono
#43 | Posted: 30 Oct 2013 10:25 
Anybody here?
There is no new posts all over the forum for third days..

I am all confuse again about the conversion between different color spaces and different white points..

Rec. 709 standard has 6503K (x=0.3127, y=0.329) white point, let's name it D65 here. DCI P3 standard has 6302K (x=0.314, y=0.351) white point, let's name it D-cinema.
If we create a conversion LUT from Rec709 to P3 which is converting everything (colour gamut, gamma and white balance) and apply this LUT to an image this image will look absolutly the same at Rec709 display (original image) and at P3 display (image through the conversion LUT). Absolutly means if the image has some white area with balanced R, G and B channels and 6500K temperature then after conversion and viewing at P3 display this area will also have 6500K temperature. This sounds clear. Same image has the same look at different standard displays with correctly applied conversion. OK, good. But.. if we have a look at waveform monitor then white area of the image will be unbalanced (B channel has the biggest level, R little bit smaller and G much smaller). This makes sense also. So, all material converted from Rec709 to P3 will look like unbalanced and shifted to blue and magenta at the vectorscope.

The main instrument of the colorist is not the only display device but the scopes also. Doesn't this mean that the same colorist will create the different look of the image if he works at P3 D-cinema display and if he works at Rec709 D65 display. And in first case white objects will have about 6300K temperature and in the second case they will be about 6500K.

What I am trying to understand, do we need to convert the white point when converting between different formats?
Again, for example, if we are grading on P3 digital cinema projector and then converting to Rec709 copy (including the white point) the image will look absolutly the same but won't it be too grinish to compare to other material which TV people are used to see?

Sorry for my overloading the topic.. )

Author Steve
Scene
#44 | Posted: 30 Oct 2013 10:44 
Please use Peak Chroma, or Fit Chroma to overcome the white point variations.

This is explained in the user guides and manuals: http://www.lightillusion.com/profiling_manual.html#peak
Hers it says:

Peak Chroma can be used to 'pull' all 'peak chroma' values within the resulting LUT cube. This can potentially help when converting from a larger colour space to a smaller one, or when generating a calibration LUT for a display that has a white point that is significantly different to the target.

Steve

Author Vassily
Mono
#45 | Posted: 30 Oct 2013 11:23 
Afraid I was not understood correctly.. sorry.
I am not about the from large to smaller conversion but any.

I will try another way to ask. For example I have material which was graded at Rec.709 display. Now I need to create a copy for digital cinema. I am creating the conversion LUT. Select Rec.709 in Source colour space and select DCI P3 in destination colour space. That's all? Shouldn't I change the white parameters of destination to x=0.3127 and y=0.3290?

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