HDR for repro work - avoid highlight clipping

[torger]

I use HDR for improving noise performance in shadows in repro work. The only thing you want for this type of work is a file (typically 16 bit tiff) with as little noise as possible and no clipped highlights, but it should have the exact same look as a single-shot file -- just less noise. You don't want fancy grunge-looking tone-mapped HDR. I've tested a few HDR softwares an surprisingly this seemingly easy task is hard to achieve in most software.

I found that neutral preset in SNS-HDR works decently well for this, but there is one problem -- highlights can get clipped (even if there is an input file without clipped highlights) so one need to manually adjust brightness to avoid clipping, and since there's no detailed clip indication one may need trial and error to find the optimal exposure.

Some highlight clip indicator or auto-set to maximally-exposed-without-clipping (i e file automatically ETTR) would be much helpful in this case.

[Sebastian Nibisz]

It is difficult to implement. SNS-HDR operating on curves, so the values ​​are never greater than the limit. I can not determine when it is overexposure.

[torger]

Would it instead be possible to when saving to TIFF have a checkbox "ETTR optimize" so then at save time the exposure is automatically scaled (possibly by trial-error inside the program if it cannot be calculated in advance) so the maximum brightness hits or is close to 255/65535 value and no clipping? If the file is to be further processed in say photoshop or lightroom it is very valuable to have an optimally exposed 16 bit TIFF as input.

It also would be logical for the "neutral" setting that the maximum value in the saved TIFF file would represent the maximum value in the darkest file (shortest shutter speed) in the bracketed input, so if there is no clipped highlights in the darkest input, the output has not either.

For scenes with highlights that can be captured, picky photographers don't like clipping, so a feature specifically aimed at highlight preservation and optimal exposure I think would be really valuable and strengthen SNS-HDR's already strong position as HDR software for photographers aiming at highest possible image quality.

[Old Baldy]

I noticed that some of my merged/tone-mapped output images were very, very close to the highlight clipping border, and that the full spectrum (high to low) was being used as shown on the histogram, so I could not simply decrease (or increase) the brightness/EV when pulled back into Lightroom, where I do my minor PP after the merge......but on the other hand, I'd EXPECT the output to fully utilize the available range after tone-mapping, so this in itself is not a problem for me.

I haven't noticed any significant clipping of the highlights, but will take another look. If that has occured, then I'd agree that this seems to be something that ideally should not occur (duh!) but I'm curious by the response that the clipping border cannot be known within the calculations/program??? Perhaps I'm misunderstanding the response (I am definitely not an expert)

[den]

What kind of highlight clipping is the concern? One or more of the R, G, B channels or the Luminoisty?

SNS-HDR Lite/Home/Pro in its present version 1.4.8.1 does not seem to clip Luminoisty when merging/tone-mapping virtually created exposures... even when merging the clipped highlight image versions...

See illustrations for v1.4.5.2 in this thread: http://www.sns-hdr.pl/forum/viewtopic.php?f=8&t=213. In the virtually created image versions, the white point is changed but the black point remains the same... what is really being changed by SNS-HDR is the base image's original 0 to 25%; 25% to 50%; 50 to 75%; and 75 to 100% tone range luminoisty histogram distributions within a 0% [0] and 100% [255] tone range with no shadow or highlight luminoisty clipping...

For R, G, or B channel clipping, one could perhaps use Home/Pro's "Highlights Protection" slider adjustment while monitoring the Histogram changes before saving/opening the SNS-HDR created image to an external editor...

...granted the SNS-HDR Histogram dialog is perhaps more of a guide in its present form than a diagnostic tool.

[torger]

Only looking for luminosity clipping may cause one of the individual R G B channels are clipped, which can cause color distortion. The goal here is that if the darkest input file has no clipped R G B channels, then the output file should not have either, but it should be tuned as close to saturation as possible to make full use of the file format's dynamic range.

I guess color space stuff may make this a bit complicated inside the program, for example if you export with a smaller color space than the input files have, but I work with ProPhotoRGB input and output.

I've only done limited testing so far, I've noted that in some cases the neutral preset does not clip, in others it does. When it does clip, then by manually adjusting the brightness slider with trial-and-error (using histogram as a coarse guide) I can restore highlights while keeping a well-exposed file. It is this step I'd like to have automatic, like a checkbox when saving tiff or something. In some cases you may want to clip (when there's just too much dynamic range in the scene) so you cannot just have it always enabled. One could have that the neutral preset does not clip highlights but instead have them ideally exposed as close to saturation as possible (that would be logical I think), but it seems like the program's HDR engine does not really work that way and then I think it is unnecessary to change that, but instead have a checkbox "ETTR highlights" when saving tiff.