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Author Topic: RawTherapee and Blown Channels  (Read 9687 times)

ppmax2

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RawTherapee and Blown Channels
« on: August 18, 2014, 09:51:38 pm »

Hi--

I've been using RT virtually non-stop for the past month and am totally blown away with the results I can achieve with this application.

However one scenario has me stumped and I can't tell if it's my technique (or lack thereof in this case), or if there is something wonky about the way that RT (the AMAZE demosaicer) handles blown RGB channels in RAW files.

I have a few images that have me baffled when it comes to handling really blown/overexposed regions of the scene. Imagine taking a picture directly into the sun where the sunlight reflects off clouds. In other RAW programs, these regions appear to be rendered without any attempt to reconstruct or clamp these blown areas. However, in RT with zero highlight compression or reconstruction applied, I see strange halos as the light transitions from totally blown to within the dynamic range of the camera.

Here is a shot of a sunset as rendered in Aperture. No adjustments other than fiddling with the black point.


Here is the same shot as rendered in RT. Note the hot yellow areas surrounding the white hot area in the center:


Granted, the RGB channels are totally blown in these areas, but the result in Aperture looks much more natural. How do I make RT handle these situations more gracefully? I've tried highlight reconstruction, highlight compression, playing with RGB curves, playing with Lab curves, changing exposure, minimizing saturation ...I just can't make the yellow halos go away. The problem may exist between computer and keyboard (PEBCAK)...or is there some channel clamping bug in RT?

thanks again,
PP
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Fine_Art

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Re: RawTherapee and Blown Channels
« Reply #1 on: August 18, 2014, 11:00:02 pm »

When you click highlight reconstruction, it has several options. The one I get the best results with is color propagation.
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ppmax2

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Re: RawTherapee and Blown Channels
« Reply #2 on: August 18, 2014, 11:57:48 pm »

Thanks Fine_Art...been there done that...the highlight reconstruction options all yield some type of halo...

I've been playing with the CC and CL curves and am getting closer. Bending the CL curve way over desaturates the halos and gets the result I want. I'll probably need to do some hue shifting since I'm getting lots of orange...it's amazing to think that all these types of manipulations have been built into other RAW renderers...or at least they handle these edge cases better.







Thanks
PP
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Bart_van_der_Wolf

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Re: RawTherapee and Blown Channels
« Reply #3 on: August 19, 2014, 03:08:29 am »

Granted, the RGB channels are totally blown in these areas, but the result in Aperture looks much more natural. How do I make RT handle these situations more gracefully?

Hi,

Are the RGB channels really blown? Check the histogram of the Raw data (the RT histogram has a toggle to show that) or use a tool like Fast Raw Viewer.

If there is some data, which may get pushed further outside of the RGB range after White balancing, use the Raw panel's White Point correction and Highlight Preservation controls to pull them back in a bit further before demosaicing, and only then attempt to tame the highlight transitions with a roll-off.

Cheers,
Bart
« Last Edit: August 19, 2014, 03:35:51 am by BartvanderWolf »
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== If you do what you did, you'll get what you got. ==

ppmax2

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Re: RawTherapee and Blown Channels
« Reply #4 on: August 19, 2014, 10:37:26 am »

Thanks for the suggestions. Unfortunately I'll be traveling for a few days and won't be able to access my RAWs but I am interested to try handling this scenario by changing the white point. Bending the CL curve like shown above feels heavy handed....and this seems like an issue that can be solved "upstream."

Thx PP
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ppmax2

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Re: RawTherapee and Blown Channels
« Reply #5 on: August 23, 2014, 10:03:40 pm »

I'm finally able to revisit this thread...

To reiterate, it appears there is some mojo that other tools have that RT lacks when it comes to dealing with extreme highlights. While each tool has a different default render profile, it appears RT posterizes the output in extreme highlight areas, and this is virtually impossible to get rid of. Here are some samples, each 1:1 zoom to the same area in the image

1. RT: note the posterized yellow bands:



2. Aperture


3. Capture One


4. LR 4



As you can see, the RT render has a distinct yellow/green cast and the transitions between these deep yellow areas and the white blown regions in the clouds looks  posterized (for lack of a better term). As you can see below in the image from RT showing only the green channel, the yellow/green areas in the clouds appear to be directly correlated to the blown areas in the green channel:


Here is the same RT image with highlight alert turned on:


For reference, here is a screenshot of the RAW histogram:


I have also posted the CR2:
http://ppmax.duckdns.org/public.php?service=files&t=2580a5573701cb3ea9b53ea234f1b7d6


To combat this effect I have tried everything:
Highlight reconstruction (all modes); luminance recovery *kinda* works but it's virtually impossible to get rid of the yellow/green cast and the banding
Highlight compression (several different values)
Lab adjustments (reduce a and/or b)
White point
RGB curves (lowering green channel output, and/or bumping blue and red channels)
CL curves (see above)
Different demosaic algorithms
...etc

It appears RT has a strange behavior (bug?) when it comes to rendering images that have blown or nearly blown highlights.

Any clues about how to fix, or what may be causing this? Clearly, from the images posted above, RT's render is the strange one of the 4 tools...

thx much--
PP
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ppmax2

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Re: RawTherapee and Blown Channels
« Reply #6 on: August 23, 2014, 11:21:11 pm »

FYI: I cross-posted this in the RT forums here
http://rawtherapee.com/forum/viewtopic.php?f=1&t=5603&p=38536#p38536

MichaelEzra may be onto something...


PP
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Fine_Art

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Re: RawTherapee and Blown Channels
« Reply #7 on: August 24, 2014, 06:46:38 pm »

The biggest difference I see is that the other screenshots white out the sky, while the RT screenshot moves it to lower tones with high saturation.

If the data is blown there is no magic to retake the shot filling in the missing parts. If you move the image in RT to be bland like the other programs the blown bits will be similarly hidden. Vice versa. The key to such HDR scenes is multiple exposures or automated brackets.

I remember one pano I did with the sun rising over mountains. I kept dropping the exposure compensation 1ev at a time until only the disk of the sun was blown. It took me to the limits of the D600 at -6ev if memory serves.
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ppmax2

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Re: RawTherapee and Blown Channels
« Reply #8 on: August 26, 2014, 07:38:44 pm »

The biggest difference I see is that the other screenshots white out the sky, while the RT screenshot moves it to lower tones with high saturation.

If the data is blown there is no magic to retake the shot filling in the missing parts. If you move the image in RT to be bland like the other programs the blown bits will be similarly hidden. Vice versa. The key to such HDR scenes is multiple exposures or automated brackets.

I remember one pano I did with the sun rising over mountains. I kept dropping the exposure compensation 1ev at a time until only the disk of the sun was blown. It took me to the limits of the D600 at -6ev if memory serves.

Thanks for the reply...conversation over at the RT forum seems to indicated there is a bug in how RT handles color scaling in these highlight areas. Here's a link to the thread:
http://rawtherapee.com/forum/viewtopic.php?f=1&t=5603&p=38564#p38564

I also linked to a CR2 in that thread. If you open it and try to correct it I think you'll find that there is something amiss...I also noticed that trying to set the WB value by using the eyedropper to pick a color in one of those regions essentially fails...there is no way to change the color cast. I believe that this behavior indicates there is a bug in the image processing pipeline.

I understand your comment about not blowing data--the problem with RT seems to be that there is some transform being applied that is not user controllable. Per your comment above, there should be no scaling of tones unless explicitly added.

As I mentioned above, the results shown are from default rendering from each app--and no highlight compression or channel clipping has been applied. I also performed other tests and was able to show that the two dcraw based apps that I have all have this problem with 5D Mark III CR2 files with similar blown highlights. If the channels are blown they should be rendered as white--not the posterized yellow/green mush as seen below. To me it looks like some RGB channel info is being clipped or ignored...there shouldn't be "bands" of super-saturated color in those regions.

thx for your comments and interest--
PP
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