H264 is Changing my Grade - Yellow Skin Tone Issue

[Ben Roper]

Hey folks,

Just wanted to put this out there in the hope someone might know what's going on and be able to offer some assistance...

I was watching a draft of my latest project which was exported from a 4K 16bit TIFF sequence in After Effects as H264, when I thought, "Why is everybody looking so YELLOW?!" I assumed I'd accidentally added something onto the final sequence and so went looking for the problem, but found nothing.

After a little testing I can confirm that the below result is occurring only when exporting H264... I've never noticed this before - Has anyone got any idea what's going on? AE 'Working Colour Space' is set to 'off', the project can be in 16b or 8b and give exactly the same result.

In particular notice the yellow skin tone and the greens being pushed towards blue, especially in the ferns on the far left.

HD Res:
http://www.rptr.uk/GradeIssue1.jpg

Low Res Attached:

 

[Ben Roper]

Here's another frame exhibiting very yellow skin tones;

HD Res:
http://www.rptr.uk/GradeIssue2.jpg

Low Res Attached:

 

[Stacey Spears]

The encoder is most likely making an assumption about the range. e.g. video vs. pc range. In 8-bit that would be 16-235 vs. 0-255. Often an encoder will assume that RGB is 0-255 and then compress it to 16-235. If your content is already in the proper range, that messes it up. It also has to color convert from RGB to YCbCr. It may not know if it should use 601, 709 or 2020 3x3 matrix.

Then you have similar options on the playback side.

Professional compressionists often use their own software to convert a 16-bit TIFF into an 8 or 10-bit YCbCr 4:2:0 so they have complete control. Then they send the encoder that directly so the encoder does not perform range compression or color conversion. This is the normal workflow for professional Blu-ray Disc encoding.

Not sure if the above helps you or not.

 

[Ben Roper]

UPDATE:

This issue does NOT occur when exporting H264 from an 8bit TIFF sequence through PREMIERE. So I guess that's a work around, but a bit of a pain as Premiere doesn't accept 16bit TIFFs. Any ideas?

 

[Ben Roper]

Thanks for your thoughts Stacey!

 

[Marc Wielage]

I would also suggest starting with a few test frames of SMPTE color bars that you can check on a scope. This will tell you very quickly where and how things are changing. Having a reliably-calibrated monitor with a color-managed output helps a great deal. Computer displays frequently will lie to you under certain conditions.

 

[Stacey Spears]

Just to add to what Marc said. Some encoders will also output an uncompressed file for you to verify if you like. In the case where you feed 8-bit 4:2:0 into the encoder, it can also write out an 8-bit 4:2:0 file and then you can compare the input to the output. x264 and x265 both offer this feature. Then you just need a viewing app you trust.

In my case, I wrote an image viewer / analyzer. It has a very basic WFM and Vectorscope built-in. It also has a pixel viewer. It shows what I call a Macroblock view at a time. This simply means a 16x16 grid of pixels. It shows in the native format. So 8, 10 or 16-bit RGB or YCbCr. I had to create this as a way to validate the test patterns I was building and then encoding. Its a lot of work to verify everything at the pixel level to make sure no funny business is happening.

You might look at FFMPEG or some of the simple front ends for x264 (AVC) and 265 (HEVC). Adobe EE uses Main Concept I believe. Its not bad these days, but x264 is still the king of h.264 encoding. The Doom9 forum is a good place to read about encoding. Note that some of them can be a bit rude to new people. I suppose its no different than any forum. Thought, RU is nicer to new people. RU is a group of working professionals while Doom9 is a group of people who are using encoders to "make backups" of their movie collection. It really is the best resource for encoding.

 

[Ben Roper]

Marc - Thanks!

Stacey - Creating your own software to solve a problem is super awesome. Thanks again for the insight! I'm still not sure why Premiere would create an undisturbed output and After Effects would adjust the colour though?

 

[Will Steer]

Unsure why AE is doing that precisely, but I also wouldn't go with AE as a transcoding tool as it can be finicky if you're not using the colour management settings (i.e. if you haven't defined Rec709 etc as the working and/or output profile).

H264 encodes can sometimes be odd. I see someone has already mentioned the x264 encoder. If you're on Mac, installing x264 as a component of Quicktime is super easy and gets you very reliable high quality results and it's available as a codec option in any encoder with access to Quicktime. I use it as standard for H264 deliverables.

Perhaps try going via media encoder rather than Premiere unless there's audio you need to sync? Alternatively try running the encode through Resolve. Resolve's own H264 encoder is also excellent.

 

[Ben Roper]

Thanks Will!