Tuesday, October 26, 2010

My HDR Technique

Since I keep referring to it over on NSN, I thought I'd write a short post on what my HDR processing technique really looks like...

Shooting: AEB +/- 2 stops. (Occasionally, I'll do five exposures at +/- 2 stops, but neither of my cameras allows for this easily, so I only do when I really think I need the extra stops, like when the sun is in the frame.) Manual exposure mode to make sure I really get +/- 2 stops--if you're in an auto mode, if the light changes, so could the exposure. ISO 100 for the lowest noise (on most Canon cameras anyway--50 might give lower noise on the cameras that have it, but at the expense of dynamic range). Tripod, mirror lock-up, cable release. Level the camera, and shoot away. Check the histograms to see that in the darkest shot, the highlights aren't blown and in the lightest shots, there is some space on the left side (lots of space, preferably, since that's where noise lives).

But this post is really about the Processing:

I'll do this by example. This is a shot I recently posted on NSN. First, the three individual exposures:

This scene probably didn't need an HDR treatment, but I think it can still be improved by it.

These images were first converted to TIF in ACR. The only adjustment I made in ACR was to set the white balance, and I set it the same on all three shots. This is important--if your camera is set to AWB, you could get different white balances for the different exposures. It's also important not to make any other changes in the RAW conversion step. (Well, there are probably some changes that would be okay, but I'm not going to go into that.)

I take these into Photomatix and generate an .HDR file (or .EXR file--for most practical purposes it doesn't really matter which format). I set Photomatix to "Align source images by matching features", to "Reduce chromatic aberrations", and to "Attempt to reduce ghosting artifacts--background movements (eg water or foliage)" with "High" detection. I actually do this in batch mode, since I'll come back from a trip with perhaps dozens of sets of images to make into HDR files.

I can then open the HDR file and do tone-mapping. This is where it gets complicated in Photomatix, as there are so many options and sliders to play with. My main warning here is that the image you see on the screen doesn't have to look like the final result. In fact, it probably won't, as I'll show. I actually do two tone-maps for my image, one using the Details Enhancer (DE) method, and one using the Tone Compressor (TC) method. The DE operates locally, trying to maximize local contrast across the image. The TC method acts globally, to map the HDR space into a LDR space in a uniform way (so that two pixels with the same HDR values in different parts of the images will have the same LDR values). The DE method can result in halos, while the TC method will not. But don't worry about that now...

Here are my two tone maps for the above image. I have no idea what settings I used for each of these--I just play around until I get something I'm happy with.

Here's the DE version:

And here's the TC version:

Neither one is something I'd be happy with. The DE is very flat, and the TC is very contrasty--in particular the gradation in the sky has been emphasized. (Note that in the DE version it actually reduced this gradation from the original.) If we zoomed right in, though, we'd see better detail in the DE version.

Next, I take these two images and put them into Photoshop. My technique is to put the DE layer on the bottom and the TC layer on top, with an Overlay blending mode and an opacity somewhere between 20-40% (I forget what actual value I used here). When we do this, we get the following:

Now we're getting somewhere. I think this is better than either of the individual tone maps, and certainly better than the "0" exposure by itself. But every image I do any processing on (whether it's HDR or not) gets an optimization treatment of levels, curves, and saturation, often with masks to selectively apply the effects to different parts of the image. The treatment is always done manually to get what I feel at the time is the best presentation I can get from that image, while staying true to what I remember the scene looking like in my mind.

Below, I'll show what I got when I applied these optimizations to both the above blended HDR image and the original "0" exposure alone.

First, the HDR:

And what I'd have gotten if I didn't shoot HDR and just optimized the "0" exposure:

To me, there's really no comparison. The HDR really is more vibrant than the other. And at the same time, it has less gradation in the blue sky, which I know is objectionable to some people.

But to be fair, I'd have done a couple of things differently if I hadn't shot for HDR. First, I'd have shot a bit more to the right (as I had more room on the right side of the histogram in the "0" exposure than I did on the left). This likely would have helped with noise in the shadows (which you can't really see at this web-size anyway). Secondly, I would likely have done some adjustments in ACR (like "fill light"--or other tools in that part of ACR). But for this image, I don't think I would have done much.

So there you go. My basic HDR technique in a nutshell. I shoot every landscape image with bracketing, on the assumption that I will apply this technique to it.


At October 31, 2010 at 10:18 AM , Anonymous Anonymous said...

Good summary, Paul. Spot on for how good, "photorealistic" results are achievable with Photomatix. It's pretty straight forward, once the basic workflow is figured out.

With Photomatix, we have the control to focus strictly on local contrast enhancement (Details Enhancer method) or global contrast enhancement (Tone Compressor method). Or do both and blend under full control with layers in Photoshop to get a balance of local and global adjustments.

Sometimes, though, I wish I could streamline all of this into a single pass and get on with things just that bit quicker. :) The fun thing about some of the new generation of HDR tools coming out now is that they create the ability to blend the local & global contrast enhancements in a single operation during tone mapping.

For example, I've been having fun working with Oloneo PhotoEngine the past couple of months. Like other new gen tools, OPE doesn't offer "tone mapping algorithms". Instead it offers a series of adjustment tools including tonal compression strength, detail enhancement strength, and global contrast strength. It goes beyond and offers a unique style of curves tool, as well as some interesting saturation curves tools. OPE is in beta right now but shows some strong promise... I'm looking forward to seeing where they take the product.

Of course Nik HDR Efex Pro is now shipping and should provide an interesting new take on HDR. As well, there's Unified Color's HDR Expose package. Photomatix is still a major workhorse and I expect it will remain so; but HDR tools have taken a big step forward with the new generation of products hitting the market...

At December 6, 2010 at 9:43 AM , Blogger fizzog said...

Thank you so much - I found this incredibly helpful and insightful.

I've been looking at Nik HDR and Photomatix for the last few days, trying to find a way of creating full but also realistic HDRs. (Many of the offerings out there are just too quirky, surreal and 'painted' for my taste!)

Your blend of DE and TC works a treat in PSE9, and has helped me plump for Photomatix.

A great post - thanks!


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