Yellow color issues in mode "exposure fusion"

[Sven Flock]

Hi all,

first of all the specs: MacBook Pro 16" (2019), macOS Monterey 12.2 with PTGui Pro 12.9

I am currently stitching an indoor panorama for a customer. I shot a -2EV/0/2EV raw bracket series on location. After loading the TIFF16bit converted source files into PTGui I noticed a yellow halo/color issue on the floor (at the chair), the wall and the darker areas of the ceiling. Switching to tone mapping algorithm solved the issue in the preview and final rendering.

As I want to adjust the tones myself I usually do not use tonemapping/exposure fusion output at all but export the raw 32bit HDRI. The thing is, that the color issue is also present in the HDRI. Looking at a single exposure layer everything looks normal.

I thought that tone mapping/exposure fusion is only a technique to get HDR to LDR footage and "compress" the tones. So why do I have this color issue in the final HDRI?

How do I get rid of the yellow color issues?

Switching from hardware rendering to software only does not change the result

The problem is also present on a Windows 11 machine with software/hardware rendering in place.

Thanks,

Sven

[Erik Krause]

Am 07.03.22 um 16:48 schrieb Sven Flock:

> I shot a -2EV/0/2EV raw bracket series on location. After loading the
> TIFF16bit converted source files into PTGui I noticed a yellow
> halo/color issue on the floor

Did you shoot at a fixed white balance? And did you convert the images
all the same, without auto settings?

If both answers are yes, please make a set of images available on your
favorite file sharing site and post the link here. If the problem also
occurs with jpegs, half size jpeg copies would be preferred.

It would also be interesting which color space you used for the TIFFs.

--
Erik Krause

[Sven Flock]

Hi Erik,

thanks for the quick reply.

Here are your answers:

• White balance has been fixed (5500K)
• Only lens profile has been applied
• Adobe color profile has been chosen (default in Lightroom Classic)
• Adobe RGB color space for uncompressed 16bit TIFFs

I have to prepare the JPEG test. As a single TIF file is over 300MByte in size, I hope that the issue is still present in the JPEG use case.

[PTGui Support]

Hi Sven,

If the issue is present in the HDRI then it might related to the way you
are developing the raw files. Raw converters apply toning curves and
other magic to make the image look more pleasing. But for assembling
HDRs PTGui needs to undo all of this in order to get to the actual
colors of the scene. Use settings as neutral as possible in your raw
converter, or load the raws directly in PTGui.

But if it's present it the HDRI I would expect the issue to be present
in the tone mapped output as well. Are you sure this is the case?

Also could check your source images to see if the color difference is
visible? Take the -2 image and increase brightness by +1 EV, and take
the 0 image and decrease brightness with -1 EV and compare the colors.
If they look different, you can expect the exposure fused result to have
these differences too. Again any difference here would be caused by raw
conversion.

Kind regards,

Joost Nieuwenhuijse
www.ptgui.com

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[Sven Flock]

Hi Joost,

I have completed your tests, nothing to see there when adjusting the exposure as described. I am using the Adobe Camera Raw Converter.

I think I narrowed it down:

There seems to be an issue with exposure fusion enabled and writing the HDRI out (I have only tested it with TIFF 32Bit version). If exposure fusion is activated, the 32bit TIFF seems to get some "information" from the expsoure fusion setting. It's baked in. When turning exposure fusion off, the HDRI looks great (no yellow color issues). On the other side, I have tested it with Tone mapping and playing arround with the parameters. In every single case, the resulting HDRI looked identical.

I made a video to make it more clear:

https://www.youtube.com/watch?v=bKkG0jLB3PA

In summary, HDRI result should be independent of the tone mapping/exposure fusion parameters. In case of activating "Tone mapping" it behaves that way, in case of activing "exposure fusion" the HDRI gets "biased" by the exposure fusion settings.

Is this by design or is this a bug (or just the case with TIFF32bit but not with other HDRI formats)?

Thanks,

Sven

[PTGui Support]

Hi Sven,

Your observation is correct. You will only get a true HDRI if 'True HDR'
mode is enabled in PTGui. This is not possible in combination with
exposure fusion.

With Exposure Fusion enabled, the HDR output is just a 32 bit floating
point version of the exposure fused LDR image. No intermediate HDR image
is created in this case, PTGui is just fusing the LDR images together.

True HDR mode is enabled if Exposure Fusion is disabled. In this case an
intermediate high dynamic range image is created, which is subsequently
tone mapped for LDR output.

Joost

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[Sven Flock]

Hi Joost,

thanks for confirmation. I got things mixed up. Sorry about that. I forgot about the 32bit output for LDR images which is new in v12, just thought about different techniques to calculate the HDRI in the first place. 

So the problem is with the LDR. I played around with the sigma parameter. The default value of 0,1 is fine -> no orange color issues but the image looks unnatural. Starting at 0,3 the color issue rises and increases until its maximum value of 1.

Is there something I can counteract to provoke that color issue? Do you still need examples to reproduce this? Is this an issue with the sources?

Thanks,

Sven

[PTGui Support]

Hi Sven,

I don't think there's much that can be done about it. At higher sigma,
more of the non-optimal exposures will get mixed in. So more of the
overexposed images is used, and those overexposed images have
desaturated highlights. I think that's the explanation.

Basically you can play with the sliders and see if you get a good
result, but the algorithm is a kind of black box. Or black magic. Or you
could say that I don't fully understand why it (usually) works so well :-)

True HDR and tone mapping is the better approach IMO: the HDR image
first reconstructs the true luminance of the scene, which is then
compressed into an LDR image.

Kind regards,

Joost Nieuwenhuijse
www.ptgui.com

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[Sven Flock]

Hi Joost,

sorry for the late reply, I was ill. 

Lessons learnt -> Only use TRUE HDR.

Thanks for all the help.

Bye,

Sven