XYZ Conversion

[Joseph Owens]

[Vladimir Kucherov] “Basically I’d assume that Resolve converts sRGB/Rec709 to XYZ assuming a D65 whitepoint”

Maybe that’s the problem. Is DCI D65? I’d assume its quite a bit lower, more like cinema projector carbon-arc.

Actually closer to Xenon, somewhere in the vicinity of 5500K.

Maybe someone on the avs forum might be able to help in more detail.
https://www.avsforum.com/avs-vb/showthread.php?t=1126589 starts dealing with some of these issues.

jPo

You mean “Old Ben”? Ben Kenobi?

[Vladimir Kucherov]

That does seem to make sense – the difference could be the white point. Unfortunately I just can’t seem to get which reference white it’s converted to… hmm.

[Oyvind Stiauren]

According to the DCI standard (https://www.dcimovies.com/specification/index.html), the XYZ coordinates for the white point is: X’=3794, Y’=3960, Z’=3890. Calculated to a white point that will give you a white point of 5798 K. So your assumption was not to far off on that Joseph.

But the problem here is more complicated that that.

The matrix you are using is first of all made for converting XYZ coordinates to sRGB/Rec.709(D65). To make it do it the other way you’ll have to invert the matrix. And secondly it’s just converting the sRGB/Rec.709 RGB values into the XYZ color space and it’s not converting the primaries from Rec.709 to DCI.

Anyway you can not just use a matrix to convert from Rec.709 RGB to DCI XYZ. The transfer curve of Rec. 709 and DCI is different and that can not be adjusted in a linear matrix.

It’s a fairly complicated matter understanding well the whole XYZ, RGB color space thing. A good place for more information is Bruce Lindblooms website (https://www.brucelindbloom.com)

I think it’s more common to use a LUT (made with Cinespace or similar), to do the conversion from Rec.709 to DCI.

—
Oyvind Stiauren
Post production supervisor, Colorist
Terminal

Mexico City

[Vladimir Kucherov]

Thanks Oyvind,

I guess I was shorthanding some stuff I’m doing. I have been reading Bruce’s amazing site for a while now. My biggest question was to understand exactly what DaVinci’s XYZ lut.

In essence, my other alternative is to use ImageMagick, linearize sRGB color space, matrix transform, change white-point with another transform, and take it back to 2.6 gamma (that’s the spec as far as I could tell). But IM is painfully slow.

I don’t have access to software to build luts yet, unfortunately, so hacking away in the meantime!

[Oyvind Stiauren]

I’ve made a LUT for you with Cinespace. It’s assuming a input of Rec.709 with gamma 2.2 and it’s outputting DCI XYZ.

https://www.terminalmx.com/clients/creative_cow/ITU-R_709_g2.2_DCI_XYZ.dat.zip

I know it looks brighter than the “Export 16Bit XYZ Tiff” option in Resolve. But on the other hand my LUT is almost a perfect match to what Fraunhofer’s easyDCP Creator makes doing the same type of conversion. So pick your poison.

Cheers,

—
Oyvind Stiauren
Post production supervisor, Colorist
Terminal

Mexico City

[Vladimir Kucherov]

Wow, I think that about does the trick. I owe you many a beer if you’re ever in NYC, Oyvind.

[Oyvind Stiauren]

Wow, I think that about does the trick. I owe you many a beer if you’re ever in NYC, Oyvind.

Deal. 🙂

—
Oyvind Stiauren
Post production supervisor, Colorist
Terminal

Mexico City

[Erik Anderson]

You wouldn’t be able to make the inverse of this LUT also too, would you? 🙂

It would be most helpful for using DCPs as a replacement for HDCAM SRs (while the tapes are insanely expensive)…

Thanks in advance!

Erik

[Sandeep Jawalkar]

hi Oyvind Stiauren can you please attach the DCI_XYZ.xml file i couldnt find it… it would be very helpful if you can mail me to sandeepvfx2006@gmail.com please….

[Dimitrios Papagiannis]

Hi Oyvind

if you still have that LUT for xyz conversion as an xml laying around i would greatly appreciate it if you could email me it when you get a chance. No rush.

dir1@mac.com

Thanks

Dimitrios