Gamma question at DVinfo.net

Val Jeannot · October 11th, 2011, 08:44 AM

Hi,

Could it be possible to obtain the specs of the different gamma curves in FirstLight ? Sorry if this has already been posted, but I couldn't find the answer.

More specifically, what is the spec of the standard 2.2 gamma setting ? Is it what it says, i.e exp(2.2), or is it sRGB (linear then exp(2.4), making an overall curve of near 2.2, but not the same)

If the 2.2 gamma is exp(2.2), couldn't this be an issue because without being an expert, I think basic windows editing programs expect a sRGB gamma curve. I checked VirtualDub source code and it's the case.

Again, in this case, could it be possible to add a sRGB gamma in FirstLight so we could get perfect Linear->sRGB mappings ?

Thanks

David Newman · October 12th, 2011, 12:48 PM

It is a true 2.2 gamma. sRGB is not used commonly for video, REC709 is and that is a slight variation on 2.2 gamma that we do intend to support at some point. All very subtle variations. However we are not changing the curves from the source if input and output (encode and decode) curve match. So if you have encoded from an sRGB source, that is what you get.

Val Jeannot · October 13th, 2011, 03:24 AM

Yes sRGB is not very used in video, I guess rec709 would be a better idea.

I just checked again Poynton HDTV book, and is states that all video, including SD, should be encoded using rec709 transfer function (linear RGB to gammified R'G'B') (pages 268 and 297).

Suppose I get some Super8 footage scanned on a SI-2K with Cineform codec in RAW format. I would like to take the footage, send it in VirtualDub for some deshaking , and then put it on a Blu Ray.

The place where the SI-2K sensor info is gammaified and becomes R'G'B' is in FirstLight, but it can't do Rec709 which is the norm.
So the only way I can have a Rec709 encoding is use FirstLight to do SI-2K->Linear, then use some tool like maybe AfterEffects to do the Linear->Rec709 only to do the gamma encoding, and then I would have the correct gammaified file.
It's a complicated process that could be done in FirstLight that already does on the fly gamma mappings.

Of course I could also use some filter to do the 2.2 to rec709 mapping in VirtualDub, but with only 8 bits color depth, there would be a loss of information, that would't happen with a native rec709 file.

Last thing, I think that adding sRGB and Rec709 gamma profiles would allow to use FirstLight to transcode sRGB footage (MJPEG for instance) to Rec709 on the fly, and would add a nice feature to this cool image-correcting tool.

Thanks !

October 11th, 2011, 08:44 AM	#1
Val Jeannot Tourist Join Date: Sep 2011 Location: France Posts: 2	Gamma question Hi, Could it be possible to obtain the specs of the different gamma curves in FirstLight ? Sorry if this has already been posted, but I couldn't find the answer. More specifically, what is the spec of the standard 2.2 gamma setting ? Is it what it says, i.e exp(2.2), or is it sRGB (linear then exp(2.4), making an overall curve of near 2.2, but not the same) If the 2.2 gamma is exp(2.2), couldn't this be an issue because without being an expert, I think basic windows editing programs expect a sRGB gamma curve. I checked VirtualDub source code and it's the case. Again, in this case, could it be possible to add a sRGB gamma in FirstLight so we could get perfect Linear->sRGB mappings ? Thanks

October 12th, 2011, 12:48 PM	#2
David Newman CTO, CineForm Inc. Join Date: Jul 2003 Location: Cardiff-by-the-Sea, California Posts: 8,095	Re: Gamma question It is a true 2.2 gamma. sRGB is not used commonly for video, REC709 is and that is a slight variation on 2.2 gamma that we do intend to support at some point. All very subtle variations. However we are not changing the curves from the source if input and output (encode and decode) curve match. So if you have encoded from an sRGB source, that is what you get. __________________ David Newman -- web: www.gopro.com blog: cineform.blogspot.com -- twitter: twitter.com/David_Newman

October 13th, 2011, 03:24 AM	#3
Val Jeannot Tourist Join Date: Sep 2011 Location: France Posts: 2	Re: Gamma question Yes sRGB is not very used in video, I guess rec709 would be a better idea. I just checked again Poynton HDTV book, and is states that all video, including SD, should be encoded using rec709 transfer function (linear RGB to gammified R'G'B') (pages 268 and 297). Suppose I get some Super8 footage scanned on a SI-2K with Cineform codec in RAW format. I would like to take the footage, send it in VirtualDub for some deshaking , and then put it on a Blu Ray. The place where the SI-2K sensor info is gammaified and becomes R'G'B' is in FirstLight, but it can't do Rec709 which is the norm. So the only way I can have a Rec709 encoding is use FirstLight to do SI-2K->Linear, then use some tool like maybe AfterEffects to do the Linear->Rec709 only to do the gamma encoding, and then I would have the correct gammaified file. It's a complicated process that could be done in FirstLight that already does on the fly gamma mappings. Of course I could also use some filter to do the 2.2 to rec709 mapping in VirtualDub, but with only 8 bits color depth, there would be a loss of information, that would't happen with a native rec709 file. Last thing, I think that adding sRGB and Rec709 gamma profiles would allow to use FirstLight to transcode sRGB footage (MJPEG for instance) to Rec709 on the fly, and would add a nice feature to this cool image-correcting tool. Thanks !