Linear response TIFFs into PTGui for TrueHDR

[Fabien]

Hi,

After fixing chromatic abberations, I am exporting my RAW images from CaptureOne to TIFF 16bits, using the Linear Camera response curve.

I remember it was necessary to convert TIFFs to 32bits in the past, to make sure it was applying a linear camera response curve to the files. Is it still the case today?
Isn't it the same to set PTGui camera response curve to be linear (Shape = 0, Shift = 0)?

I find the results when using the Linear Response TIFFs much more vibrant, contrasted and realistic. It is definitely different from the results I'm getting when inputting RAWs directly in PTGui, but these RAW should be using a linear response curve anyway. Which one is the correct way to do it, for the best lighting accuracy.

Thanks,
Fabien

[PTGui Support]

Hi Fabien,

PTGui 12 can use linear files in 8 or 16 bit mode as well. Keep in mind
there are two things in play:

The ICC profile may (or may not) contain a gamma curve. PTGui 12
recognizes the ICC profile, and I assume CaptureOne will add the correct
profile, so this should work out of the box.

And your raw converter may apply an additional curve in order to create
a good looking image.

If you load RAWs in PTGui, it will apply a curve by default, see the
Post Processing side bar. Indeed if you set shape,shift=0 then
processing is completely linear. You can compare this to your CaptureOne
workflow, the image should look identical in terms of contrast.

If you load TIFFs or JPEGs in PTGui, PTGui doesn't change the curve. But
for certain operations (vignetting, HDR) PTGui will still need to know
the curve because these operations are done in linear luminance space.
PTGui applies the curve (in Exposure/HDR) in reverse, performs the
correction and then applies the curve again.

Hope this helps!

Kind regards,

Joost Nieuwenhuijse
www.ptgui.com

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[Fabien]

Thank you for you quick reply.

" If you load RAWs in PTGui, it will apply a curve by default, see the Post Processing side bar."

From what I can see, the Tone Curve in the Post Process is not applied to the 32bits HDR output, but only to the Panorama Editor display (and most likely Tonemapped HDR, which I don't use so I'll ignore it).

The White Balance, on the other hand, is applied to the 32bits HDR output, which is great, but a bit confusing as I was expecting the whole Post Process panel to not have any impact.

The Camera Response curve, on the other hand, is applied to the 32bits HDR output.

Loading RAW files, the Camera response curve inside the Exposure / HDR panel is empty because PTGui treats the RAW as linear, I assume. The 32bits output is slightly washed out and desaturated, not similar to one of the images used in the bracket.

I am kind of used to get that kind of results when merging HDR, but I just always wondered if this was correct.

Loading TIFF 16bits (linear response from CaptureOne), PTGui adds a default Camera Response curve than can be optimized and is rather flat. It gives me more contrasted, vibrant image that is close to a single image of that bracket. Which is surprising in some way, as I would expect it to be flatter, as the linear response curve does flatten the photo.

Loading TIFFs 32bits  (linear response from CaptureOne), PTGui does not add a default Camera Response curve by default (expects linear) and the output is identical to the TIFF 16bits one, if I set the Camera response curve of this one to be completely flat.

I don't think CaptureOne applies automatically the correct ICC on export, it's up to me to pick it, and I don't have a Linear sRGB ICC by default (but I can make one: http://fnordware.blogspot.com/2008/05/making-linear-icc-profile.html). However, I don't think this would make any difference, since we saw that the TIFF 16 and TIFF 32 give the same result if removing the Camera Response Curve.

CaptureOne outputs the image with a flat linear response curve, but my understanding is that it is still using a sRGB gamma? Anyway, reading this thread, it doesn't seem to be the silver bullet I was waiting for as the linear curve does seem to preserve extreme highlights so it's not 100% linear: https://support.captureone.com/hc/en-us/community/posts/360012422518-What-is-Linear-Response-

I'll give dcraw linear TIFF a try, but I would have liked to keep the process as simple as possible.

 

[PTGui Support]

On 26/10/2021 16:30, Fabien wrote:
> Thank you for you quick reply.
>

> /" If you load RAWs in PTGui, it will apply a curve by default, see the
> Post Processing side bar."/

> From what I can see, the Tone Curve in the Post Process is not applied
> to the 32bits HDR output, but only to the Panorama Editor display (and
> most likely Tonemapped HDR, which I don't use so I'll ignore it).

Correct, it's applied to the LDR output only.

> The White Balance, on the other hand, is applied to the 32bits HDR
> output, which is great, but a bit confusing as I was expecting the whole
> Post Process panel to not have any impact.

Yes that's not entirely straightforward. Actually wite balance
correction is applied before blending because the images might have
different white balance. Thus it appears in the HDR output as well.

> The Camera Response curve, on the other hand, is applied to the 32bits
> HDR output.

PTGui needs to convert your processed TIFF files back to linear
luminance space before merging them. So the camera curve is applied (in
reverse).

> Loading RAW files, the Camera response curve inside the Exposure / HDR
> panel is empty because PTGui treats the RAW as linear, I assume. The

Right.

> 32bits output is slightly washed out and desaturated, not similar to one
> of the images used in the bracket.
> I am kind of used to get that kind of results when merging HDR, but I
> just always wondered if this was correct.

That's the thing: truly linear images look kind of washed out. If your
CaptureOne processed images don't look bland or washed out, I think they
are not truly linear.

> Loading TIFF 16bits (linear response from CaptureOne), PTGui adds a
> default Camera Response curve than can be optimized and is rather flat.

That camera response curve doesn't affect the image. It's only used when
you apply vignetting correction, for temporarily mapping to linear space
and back. You don't notice the curve itself.

> It gives me more contrasted, vibrant image that is close to a single
> image of that bracket. Which is surprising in some way, as I would
> expect it to be flatter, as the linear response curve does flatten the
> photo.
>
> Loading TIFFs 32bits (linear response from CaptureOne), PTGui does not
> add a default Camera Response curve by default (expects linear) and the
> output is identical to the TIFF 16bits one, if I set the Camera response
> curve of this one to be completely flat.
>
> I don't think CaptureOne applies automatically the correct ICC on
> export, it's up to me to pick it, and I don't have a Linear sRGB ICC by
> default (but I can make
> one: http://fnordware.blogspot.com/2008/05/making-linear-icc-profile.html).
> However, I don't think this would make any difference, since we saw that
> the TIFF 16 and TIFF 32 give the same result if removing the Camera
> Response Curve. > CaptureOne outputs the image with a flat linear response curve, but my
> understanding is that it is still using a sRGB gamma?

Possibly, but it doesn't matter. The gamma will be embedded in the ICC
profile and PTGui will convert to linear gamma when loading. There's no
difference between Linear sRGB and sRGB with gamma, assuming the ICC
profile is correct.

> I'll give dcraw linear TIFF a try, but I would have liked to keep the
> process as simple as possible.

Why not use your raws directly in PTGui? It's the same as using dcraw.

Joost

[Fabien]

The camera response curve does greatly affect the colors and contrasts of the 32 bits HDR output in my tests:

" Why not use your raws directly in PTGui? It's the same as using dcraw."

Maybe this is what got fixed in PTGui 12 with the color management, but the Unity team did recommend to use dcraw as the only trusted software to extract RAW information in the most linear way (https://blog.selfshadow.com/publications/s2016-shading-course/unity/s2016_pbs_unity_hdri.pdf)
The reason why I also don't use RAW directly if because I need to fix chromatic aberration and CaptureOne does wonders at preserving details.

Thanks a lot for your help.

[PTGui Support]

On 26/10/2021 18:14, Fabien wrote:
> The camera response curve does greatly affect the colors and contrasts
> of the 32 bits HDR output in my tests:

If you feed PTGui TIFF files, it will have to reverse engineer the curve
you used when developing the raw files, in order to go back to linear
lumincances. It will never be able to do this perfectly.

> The reason why I also don't use RAW directly if because I need to fix
> chromatic aberration and CaptureOne does wonders at preserving details.

> Photo_LtjkkU3Rne.jpg

Those can't be the same raw files! I see details in the right image
which are missing in the left hand image.

Joost

[Fabien]

This was probably not a fair comparison, I agree. They are the same files used, but the stitching prioritized a bad image corner when using the RAW files, and used another cleaner image when using the TIFF files. 
But overall, the sharpness and chromatic aberration fix make a huge difference.

[Fabien]

After further testing, I concur that using RAWs directly in PTGui does maintain better color accuracy (comparing color chart from HDRI to a color chart rendering with that HDRI environment) and that using TIFF with the default camera profiles in Capture One isn't as accurate, even worse when using linear response curve.
I created ICC profiles for my camera using the color chart(https://www.youtube.com/watch?v=wcOn1SyP7WA), I then use the ICC with the linear response curve which will do proper color correction and neutralize any wrong color cast, and exporting TIFFs making sure to select the sRGB ICC profile.
This provides great results, close to the RAWs but with even better color accuracy, and fixed chromatic aberration and purple fringing.