HDR output.

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pbacot
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HDR output.

Post by pbacot » Sun Aug 11, 2019 6:10 pm

I have seen that working with different exposures of the render (as adjusted in the render window) and imported as layers in GIMP helps me make a better interior image. I adjusted the brightness, gamma, and temperature and output 4-5 different images, a few of which were useful in blending.

I was finding that before, trying to fix the .png output after the fact was killing the image.

Then I decided to try the .hdr output. I expected it to provide bracketed exposures. I guess I am wrong about that. the single image looked like a very bright exposure in Preview, and it looks like mostly black with faint white lines in Gimp. 2.10 (which supports .hdr).

I found this comment by Fletch:
Simple answer:
Firstly, if you have Pro version you can save your rendered image as .hdr format. This is the High Dynamic Range image you are looking for. Open the .hdr in a photo editor specifically designed for adjusting HDR images, then save the image from there in the desired format, such as .png (lossless but compressed format)

Secondly, you don't have to save as HDR you can use (Pro or hobby) Twilight's directly internal HDR tonal adjustment capability to adjust the exposure during rendering or after it is completed using the Tone Mapping feature of the Render Dialog.
I gather from this that an .hdr image does not have the bracketed exposures. It's a flat image made up from the exposures. I also gather from this comment, that there is no particular advantage in the .hdr output over what can be done with the postpro window in Twilight.

1. Is that true? The point of exporting .hdr is the same as tonemapping within Twilight?
2. I gather from the comments that the .hdr output in a proper editor should give enhanced control of tonemapping over adjusting a .png for example. I wonder why my output was black, whereas other (downloaded) .hdr i have opened look fine.
3. Is there any way to automatically obtain bracketed exposure images from a rendering for use as layers in PS or GIMP?

Does anyone have any recommendations? Thanks.

Chris
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Re: HDR output.

Post by Chris » Mon Aug 12, 2019 4:31 pm

The HDR output is not bracketed exposure, as you have pointed out. What it is is essentially raw output. HDR is "High Dynamic Range"; this doesn't mean it has multiple levels of exposure, it means the image data is not constrained to the standard RGB [255, 255, 255] range. It is what is called "floating point" data which means it can be values like 1.23, 15.9, 9856.45.

Where this becomes useful is that you can actually create your own bracketed exposures. There is software that can be used to exactly that. I have no idea if Gimp has that capability (I didn't Gimp supported HDR now). You could actually use TWR to do that too. Render with exposure set to 1. Save the image. Adjust exposure to 0.8 (no need to re-render) and save. Adjust exposure to 1.25 and save. I've actually used that exact process myself.

pbacot
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Re: HDR output.

Post by pbacot » Mon Aug 12, 2019 7:51 pm

Thanks. Yes. It seems in GIMP you can create the different exposures, given an HDR image (I don't find a plugin or process to automate it). There is 32 bit floating point support. I haven't got a usable HDRI into GIMP from my rendering yet, but will experiment more. As I noted above. It just looks black. A simple daylight rendering (a cube) is just pure black when brought into GIMP, whereas a an HDR pano from the web looks fine.

Chris
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Re: HDR output.

Post by Chris » Mon Aug 12, 2019 11:31 pm

That's weird. I just tried it. I did see a few artifacts, but overall it imported into Gimp (2.10.8) just fine.

pbacot
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Re: HDR output.

Post by pbacot » Tue Aug 13, 2019 1:44 am

I updated my profile. Maybe there's a clue.

Using Twilight 2.13.7 Pro. Gimp 2.10.8

I did find this plugin which may have been developed for pre-floating-point GIMP but maybe will try it on an HDR. https://sourceforge.net/projects/gimpsimulatehdr/

Fletch
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Re: HDR output.

Post by Fletch » Tue Aug 13, 2019 9:31 am

I gather from this that an .hdr image does not have the bracketed exposures. It's a flat image made up from the exposures. I also gather from this comment, that there is no particular advantage in the .hdr output over what can be done with the postpro window in Twilight.
The bracketed exposure method is a "cheat" to achieve and HDR image "look". The HDR is the next step after taking the bracketed exposures. So the HDR you save from Twilight should, in theory, be better than the bracketed exposure because all of the exposures are contained in the one file. The challenge comes in finding a good software to manipulate the image and save it in a printable format. The idea of the image editing software is that you should be able to tone down the overly bright parts and brighten the dark parts all at once, as opposed to using a bracketed exposure method.

That said - you should NOT use any post processing on your RAW render if you are saving as HDR - save it as HDR and edit it elsewhere.
I just tested this, and the HDR output seems good.
1. Is that true? The point of exporting .hdr is the same as tonemapping within Twilight?
Not exactly, it depends on the software. If you open the HDR in Adobe CC or inside of Topaz Studio or some other manipulation software such as GIMP then each will have their own ways of interacting with the info in the HDR. The tonemapping in Twilight is sufficient, but is not as powerful as software specifically dedicated to the tonemapping of HDR images.
2. I gather from the comments that the .hdr output in a proper editor should give enhanced control of tonemapping over adjusting a .png for example. I wonder why my output was black, whereas other (downloaded) .hdr i have opened look fine.
It's hard to guess, (assuming you did not adjust exposure inside of Twilight before saving the HDR!) you could share the hdr output with us here (or PM a link to it to me) and we can take a look. If I had to guess, Gimp's tools are not as developed as others for manipulating HDR. However, I found this tutorial about working with HDR in GIMP from 2016
Topaz Studio is an example of another independent program that can manipulate HDR images for you brilliantly.
3. Is there any way to automatically obtain bracketed exposure images from a rendering for use as layers in PS or GIMP?
follow Chris' advice above

pbacot
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Re: HDR output.

Post by pbacot » Tue Aug 13, 2019 5:11 pm

Thanks, Fletch. That's a good article. Maybe that will show me how HDR editors ought to work. I thought you had to have control of each exposure to make them come out clearly, or create the more vivid look (often exaggerated) in HDR photography.

I saw that before. but my HDR did not come out as usable. I've done the output without adjusting postpro in Twilight (However--what does that mean? there are always settings there--should I check that the values are set to "1.0"?).

Fletch
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Re: HDR output.

Post by Fletch » Wed Aug 14, 2019 2:53 pm

for Post Processing, instead of "Simple" choose "None"

Fletch
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Re: HDR output.

Post by Fletch » Wed Aug 14, 2019 2:57 pm

Debevec website is great for learning about HDR, and there you can download HDR Shop software for HDR manipulation.

pbacot
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Re: HDR output.

Post by pbacot » Wed Aug 14, 2019 11:09 pm

With postpro set to none, I still get a solid black image in GIMP. Must be something with GIMP and Twilight's brand of HDR.

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