Andrew Somers
Forum Replies Created
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“What’s the difference between Rec. 709 Gamma 2.4 and HDTV (Rec. 709) in AE? Which one should I use for advanced VFX when working in linear color space? “
You can use any of those or even sRGB in a linearized workspace, as the gamma in the profile is not used, only the color coordinates of the primaries and white point.
When you are working in LINEARIZED space, your gamma is 1.0
sRGB and Rec709 use the same primary coordinates, so there is no difference between them when linearizing your workspace.
None of those profiles should be used for output though. When you are converting from linearized space to render out to a Rec709 video, you want to use the correct gamma for video files, and the gamma of such a profile is closer to 1.9-2.0, and not 2.4.
The “HDTV (Rec709).icc” profile uses a 2.2 gamma curve (and appears no different than sRGB) with no offset as is specified in the actual ITU-R. It does not have the proper Rec709 gamma.
The profile I am recommending is: Rec709-elle-V4-rec709.icc
Downloadable here:
https://github.com/ellelstone/elles_icc_profiles/blob/master/profiles/Rec709-elle-V4-rec709.icc
It is the only one I’ve found that appears to have the correct linear “knee” in the near black area. It seems many other profiles, including those that Adobe distributes, are using an incorrect gamma curve.
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Rec709 has always specified a gamma, it is:
It details the scene referred encoding gamma for cameras and Rec709 video signals.
It is scene referred, not output referred.
In the initial Rec709 spec (1993) output referred gamma assumed to be the “known quantity” of CRT type displays that were in use at the time. Output gamma was defined as Rec1886 in 2011for the display itself, and Rec2035 in 2013 to define the associated viewing environment.
A principle reason a scene referred profile was defined for the SIGNAL (i.e. how you OUTPUT from AE for example) was due to the unknown variations in viewing environment and how the monitor was adjusted for a particular environment.
The correct output gamma for a given display will vary based on the viewing environment.
Thus “scene referred” is the best practice for the signal.When you *export* from AE using the “Rec709-elle-V4-rec709.icc” profile, it uses an approximate 1.9 Gamma Curve as defined in the standard (it is approximate because the standard defines a linearized portion near black that offsets the gamma curve slightly).
There are however other profiles floating around that are “named” Rec709 that are using other gamma curves, for instance the profile named “HDTV (Rec709)” seems to be using a 2.2 gamma curve. While this may be the appropriate one to use for WEB video, it is technically incorrect as far as the Rec709 standard is concerned.
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“We’ve recently started compositing some final assemblies in rec709 as opposed to the default no-color space in AE, and with the no color space, the exports were the same (with expected player differences) between all apps. Once we switched to 709, it seems to be quite a bit brighter in gamma and offset- but the strange thing is that when that file is pulled back into AE it reads it as composited in the full 709 space”
Something I discovered is that not all Rec709 profiles are created the same – some have a technically wrong gamma. This includes Rec709 profiles that ship with Adobe products.
The profile named “HDTV (Rec709)” does not have the correct gamma curve – in fact “HDTV (Rec709)” seems indistinguishable from the sRGB profile!!!
Unfortunately there are many Rec709 profiles out there, with different gamma curves.
The profile named Rec709-elle-V4-Rec709.icc seems to be one of the few correct ones out there. It can be downloaded here:
https://github.com/ellelstone/elles_icc_profiles/blob/master/profiles/Rec709-elle-V4-rec709.icc
This profile seems to have both the correct linearization at levels lower than 0.018, and the correct gamma offset that results (the offset results in a perceptual gamma that varies about 1.9 to 2.0 depending on level).
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Duplicate Post
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“I’ve been trying for SOOO long to figure out when to use Rec. 709 and when to use sRGB and I’m close to giving up and pretending like these things don’t exist in this world. However, I’ll give it one last try.
My questions right now:
(1) To achieve more realistic blendings and motion blur I want to work in a linear color space, but this means that I have to choose between Rec. 709 and sRGB – how do I know which one to pick? (I don’t have any delivery specifications – I’m creating films mostly for fun but they’ll probably end up on YouTube).
(2) Why should I even work in sRGB in After Effects when Premiere and FCPX uses Rec. 709 and the final export will be made from Premiere or FCPX?
(3) Is it a bad idea to embed Rec. 709 color profile when exporting PNG-images for the web, and why?”For LINEAR Working Space:
First of all, if you are using 32 bit linearworking space (gamma 1.0) then it does not matter if you choose sRGB or Rec709 – both use the exact same primaries, so the only difference is the gamma curve. If you are working in linear gamma 1.0, there is no gamma curve (it is a straight line). In a linearized working space, the color profile you choose to work in will only affect the position of the color primaries and white point coordinates.For GAMMA ENCODED Working Spaces:
If you are working in a gamma encoded space (i.e. not linearized), as you might in 16 bit mode, then pick the colorspace you are going to output to so there are no color rtransformations on output. This is particularly critical if you are working in 8 bit for some reason.For Video:
The standard for HD video is Rec709. Rec709 is specified as a “scene referred” standard with a scene referred gamma of approx. 1.9 (it is not a pure gamma curve however, as there is a linearization for very dark levels). A scene referred gamma was chosen for the standard because the output gamma of a display is going to vary depending on the viewing environment and how that display is adjusted for same.See: https://prolostx.squarespace.com/blog/2008/2/22/digital-cinema-dynamic-range.html
It’s useful to note that while the broadcast standard specifies 16 for black level and 235 for white, Quicktime and most web video codecs expect to see 0 for black and 255 for white, so when outputting video for web choose Rec709 (0-255) and not (16-235).
For Web Images:
The worldwide standard for web images is sRGB. When you output still images for use on the web, is it best to output them as sRGB. Embedding the colorspace here is optional, so long as you choose sRGB as the output colorspace.For web use you should never use any colorspace other than sRGB, even with an embedded profile – many browesers and systems are not color managed, and so images not output as sRGB will not look correct even with an embedded profile.
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I’ve already provided concrete examples, as I stated: text, fades, and transparent drop shadow. But I don’t seem to be getting the point across to you, as you are bringing up separate (and not directly related) issues.
At some point when I have time I will provide the workflow reasons with examples. Until then I’ll table the discussion as we aren’t getting anywhere.
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Darby, we are still not discussing the same thing.
I am not talking about blends of colors in linear space.
I am talking about TRANSPARENCY.
White titles coming out using the output module’s conversion to a gamma encoded space do not have the same perceptual transparency because the output module’s alpha channel is wrong.
It is trivial to convert the RGB image data from a linear space to a gamma encoded space. What is needed is to also convert the alpha channel to provide the same relative transparency in the target application.
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Darby, we are talking about two different things.
If you take white text, and set opacity to 10%, the perceived transparency in linear is denser than it is in a gamma encoded space. But that is not what I am talking about.
If you are working in linear space, and use an output module to output to a tiff using a gamma encoded space such as rec709 or sRGB, and your alpha is straight, the image will be perceptually the same when viewed in the gamma encoded space as it was while working in linear space.
However, the alpha will not work on systems that use the gamma encoded space to blend.
However if you import that image into a linear space environment, the alpha values will be correct.
If you convert to a gamma encoded space, the alpha values must be changed to accomplish the same perceptual intent. This means that when after effects outputs from a linear working space to a gamma encoded space, it is not converting the alpha values to create the same perceptual intent. It should be clear that this makes the gamma encoded RGBA output unuseable.
The workaround is to NOT use linear space when the output needs to have an alpha and be in a gamma encoded space.
The color values of the text are correct when opacity is 100%. And when alpha is straight, these RGB values do NOT change. But the opacity as interpreted by a compositing system that is blending in gamma encoded space does not match that of the perceptual intent when working in linear space.
I’ve been playing with modifying the alpha channel values – but ideally there would be an option in the output module to adjust the alpha for the same perceptual intent. It is the alpha. The alpha needs to be modified ALONG WITH the RGB data when outputting from linear to a gamma encoded space.
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At Darby: After Effects’ output module does a good job of converting the image data from linear space to a gamma encoded space. What After Effects fails to do is convert the opacity data from linear to gamma encoded.
The numerical value of the alpha channel relative to a linear ColorSpace results in a substantially different perceptual blend them the same numerical value in a gamma encoded space.
For instance 10% opacity in linear space will appear denser than 10% in a gamma encoded space, even though the input image data is the same.
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That doesn’t solve the problem, as delivery specs are RGBA. If it was only our internal workflow it wouldn’t be a problem, but the image sequence and/or QuickTime must be handed off to another facility, and we have to meet their delivery specs.
Our normal internal workflow is a linear (gamma 1.0) colorspace, but due to this issues we must use a gamma encoded colorspace when we need to associate an alpha channel.
The problem remains: linear colorspaces use a different opacity than gamma encoded spaces for the same perceptual intent, and AE does not compensate.
