[Tig] LUTs for Video File Codecs

Dan C. Tatut dtatut
Tue May 2 22:53:40 BST 2006


--
Thanks to oktobor for supporting the TIG
--
Hi Jack,

you do not have to wait for 4D LUTs. They have been around for a long time but in another area than post-production (go to the university next door, they may be using lookup-tables with a number of dimensions that is higher than 4 :) )

However a LUT is just a dictionary. A dictionary just translates one word into another. In the case of LUTs in color correction systems, the LUTs approximate a color space transformation.

Color is just one property of a pixel. Codecs use pixel's color information but also position in space (x,y coordinate in the frame) and position in time (which frame in the movie to be compressed/decompressed).

There is no way to preview compression through a LUT since a LUT will reproduce the same results again and again based on the same input. This is not the case for a codec, since the codec compresses a pixel based on its color, spatial and temporal position. Moreover compressors like MPEG will produce content-dependent results (that's what they have been designed for). 

Photoshop has a tool that allows you to preview compressing for web-like image formats (e.g. JPEG, GIF). What actually Photoshop does, is compressing the image in memory and decompressing it again and display the result. This is the preview of the compressed file. 

Another issue is that all the codecs tend to have their own tricks to compress images. There is no rule in image compression... that's why you have so many available.

Ideally one should output the media and send it to a compressor, decompress it again and compare differences. This can be achieved by plugging your color grading system's SDI output into a real-time MPEG encoder for example, then plug the output into a real-time MPEG decoder and feed-back the color grading system with the result. Not so easy but feasible.

Dan Tatut
CEO
CHROME Imaging
105 Rue de Lyon
CH-1203 Geneva
Switzerland

Phone: +41 22 807 23 60
Fax: +41 22 807 23 70
Mobile: +41 78 659 11 04
WWW: http://www.chrome-imaging.com


-----Original Message-----
From: Jack James [mailto:jack at surrealroad.com]
Sent: mardi, 2. mai 2006 23:40
To: Dan C. Tatut
Cc: 'TIG'
Subject: RE: [Tig] LUTs for Video File Codecs


Perhaps a 4D LUT would do it?
Now we just need to wait for someone to invent one.

-----Original Message-----
From: tig-bounces at tig.colorist.org [mailto:tig-bounces at tig.colorist.org] On
Behalf Of Dan C. Tatut
Sent: 02 May 2006 21:51
To: Tim Montgomery; tig at colorist.org; tig-bounces at tig.colorist.org
Subject: RE: [Tig] LUTs for Video File Codecs

--
Thanks to oktobor for supporting the TIG
--
Hi Tim,

Codec degradation cannot be simulated by a LUT. a LUT just translates a set
of colors into another (e.g. for simulating film output, etc). Codecs not
only work with pixel intensities (in RGB, YUV or any other colorspace) but
they also work with several frames (temporal compression). Moreover a codec
is not supposed to change colors. Color changes are just artifacts due to
the missing information (usually discarded by the coded at compression time
to limit necessary bandwith).

Best, Dan

Dan Tatut
CEO
CHROME Imaging
105 Rue de Lyon
CH-1203 Geneva
Switzerland

Phone: +41 22 807 23 60
Fax: +41 22 807 23 70
Mobile: +41 78 659 11 04
WWW: http://www.chrome-imaging.com


-----Original Message-----
From: Tim Montgomery [mailto:tim.montgomery at finishedit.com]
Sent: mardi, 2. mai 2006 22:40
To: tig at colorist.org; tig-bounces at tig.colorist.org
Subject: [Tig] LUTs for Video File Codecs


--
Thanks to oktobor for supporting the TIG
--
Is there such a thing as a LUT that simulates Codecs like Quicktime and
Windows Media?  Is there a general rule when grading for codec degredation?



_______________________________________________
Tig list - http://tig.oktobor.com/mailman/listinfo/tig
TIG wiki: http://tig.colorist.org/wiki

_______________________________________________
Tig list - http://tig.oktobor.com/mailman/listinfo/tig
TIG wiki: http://tig.colorist.org/wiki



_______________________________________________
Tig list - http://tig.oktobor.com/mailman/listinfo/tig
TIG wiki: http://tig.colorist.org/wiki




More information about the Tig mailing list