HDR Tutorial – Part 3

Assuming you’ve already read parts 1 and 2 and played with some of the information there, you probably have a bunch of bracketed exposures of a scene that you want to turn into a High Dynamic Range image. What’s next?

The first thing you need to do is make sure that you have software capable of merging your bracketed images. You have several choices here including the following:

• HDRSoft Photomatix
• Ariea HDR Max
• Artizen HDR
• FDR Tools
• Adobe Photoshop CS2 or later

You may be surprised to see Photoshop in this list since it’s not normally considered an HDR application. I use it in one very particular case, however, which I will cover later in this section.

All of these tools have an evaluation version you can download for free to try out. The evaluation copy will generally be crippled somehow, either allowing use for a limited time or watermarking any images it saves but they are well worth downloading to see which tool fits your style of photography. I use Photomatix and Photoshop so will concentrate on these two in this tutorial. I’ve played with HDR Max and quite like its user interface. I’ve not tried either Artizen HDR or FDR Tools but have seen some superb results from each and will definitely be trying them out sometime soon. By the way, the kind folks over at HDRSoft are offering readers of this tutorial 15% off the usual price of Photomatix if you enter “DaveWilson” in the coupon code box when you check out (By way of full disclosure, I should also point out that they give me a commission on copies sold via this site but I used the product happily for 4 years before signing up as an affiliate and am a delighted customer regardless of this arrangement.)

Assuming you’ve downloaded and installed one of the tools, you are now ready to generate an HDR image. I’ll assume you are using Photomatix 3.2 for the following but I expect the process is somewhat similar with Artizen HDR and FDR Tools. Note that, if you are an Adobe Lightroom user (as I am), there is a better way to accomplish this but I will describe the method using Photomatix on its own first and add the Lightroom info at the end of the page.

Merging Your Exposures

The following step-by-step list is based on the Windows version of Photomatix Pro 3.2. If you have a different version of Photomatix or are a Mac user, I suspect you will be able to follow along fine but I apologise if you find this is different from the interface you are used to.

1. Open Photomatix Pro 3.2 and press the “Generate HDR image” button at the top of the “Workflow Shortcuts” window that you should see.
2. In the “Generate HDR – Selecting source images” window, either press “Browse…” and navigate to the folder containing your bracketed exposures or drag and drop them into the white area of the window. Make sure you include all 3, 4, 5 or whoever many exposures you took in your sequence for a single image. Once you have all the images listed in the box, press “OK” to continue. You will be rewarded with the following dialog box:
   

   Photomatix Options

3. The options you select in the dialog will vary depending upon how you took the image. The main choices are:
   • If you shot your brackets handheld or reckon that you may have moved the camera at all between taking any of the images, make sure “Align source images” is checked. If I have this checked, I usually also check “By matching features”. This slows down the conversion but does a better job.
   • I tend to keep “Reduce chromatic aberrations” checked but, to be honest, I’ve not checked to see how much of a difference it makes.
   • Again, I tend to keep “Reduce noise” checked.
   • If the images I am merging contain any movement whatsoever – leaves blowing in the wind, people walking, cars moving, etc. – I check “Attempt to reduce ghosting artifacts” and whichever of the two options seem more appropriate given the images I am working on. If you have any movement of objects in your scene, you should prepare yourself for some Photoshop rework afterwards since you will almost certainly see some odd ghosting in the final image.
   • I leave the “Raw conversion settings” in their default states. These are designed for cleverer people than me (the kind who understand colour management).
4. With the options set, press “OK” to start the process of merging your images. Depending upon the speed of your machine, the number of images you are merging, the size of the images and the options you selected, this may take anything from 30 seconds to a few minutes to complete.
5. Once the merge is complete, you will be shown a representation of the final HDR on the display. Don’t freak out! It looks absolutely terrible at this point. If you remember from part 1, your monitor can’t display an HDR image so you see a really bad rendering of it. This does not mean that your image is wrong or somehow messed up, it’s just that the image contains a whole lot more information than the computer can display. If you don’t believe me, here’s what one of my cowboy boot images looks like at this point in the process.
   

   Raw HDR Displayed by Photomatix

You now have another choice to make. You have generated a true HDR image but do you want to save it or move straight on to tone mapping to generate your final output? If you want to experiment with different tone mappings, saving the HDR file is probably a good idea so chose “File/Save As…” from the Photomatix menu and navigate to the folder in which you want the image stored. Notice that you have some strange file types listed. Several of these types are specific HDR image file formats. From the reading I’ve done, OpenEXR is my favourite so I use this. I notice that Photomatix 3.2 now offers 2 different variants and I have no idea which is better so I would pick the “ZIP Compression” one for now (though I reserve the right to change this later once I find out whether other tools support both flavours and determine which writes the smaller files).

You now have a real, live HDR image on your hard disk so it is time to move on to part 4, “HDR Tone Mapping” and process this back into something that looks good on a normal display.

What about Photoshop?

Earlier I mentioned the one special case where I use Photoshop to merge my HDRs. Photomatix does a great job in most cases but if I’ve shot my bracketed images handheld or if I notice significant camera movement between the images, I will use Photoshop to generate my EXR file instead. It’s ability to accurately align misaligned images is amazing and definitely very much better than Photomatix (though, I should note that Photomatix 3.2 is supposed to have improved in this area. At the time of writing, I’ve just upgraded so have not tested this out yet).

To merge in Photoshop:

1. Choose “File/Automate…/Merge to HDR…” from the main menu.
2. Select all the bracketed image files for the scene and press “OK” to start the merging process. This will take a lot longer than it does with Photomatix but you should find that the alignment is superb.
3. Partway through the conversion process, you will be shown a window with a representation of the final HDR image and thumbnails of each of the individual images used to build it. All I do here is make sure that the Bit Depth is set to “32 Bit/Channel” then press “OK” to continue the process.
4. The merge process chugs along for another minute or so then you are shown the final HDR image. As with Photomatix, this will look terrible (usually flat, dull and really nasty) but don’t worry – this is normal.
5. Use “File/Save As…” to save your HDR image. Again, I use OpenEXR (.exr) format but you can choose whichever format you prefer.

With your HDR image saved to a file, you can now open it in Photomatix Pro using File/Open… and it’s ready for tone mapping.

Move on to part 4, “HDR Tone Mapping,” to find out how.

Any hints for Lightroom users?

If you are lucky enough to use Adobe Lightroom 2, you should install the Photomatix Lightroom plugin. This is included in the download for Photomatix Pro 3.2 and lets you export images directly from Lightroom into Photomatix for merging. I highly advise this because Lightroom does a far better job of converting raw files than Photomatix (the folks at HDRSoft even admit this). By using the export plug-in, I see a marked improvement in the sharpness and overall quality of my merged HDR images.

To use the Lightroom plugin, highlight the images you want to merge in the Library module and right click one then chose “Export/Photomatix Pro” from the context menu. Chose the options you want in the normal File Export dialog and press “OK” to export the files. You will them be shown a dialog similar to the one above to allow you to select the Photomatix options. The only different option relates to the file type to export. To maintain maximum quality, I always use “TIFF 16-bit” but you can experiment with the 8 bit formats and see how you get on. Again, I work on the principle of never throwing away information until I absolutely have to so using a 16 bit format allows me to keep all the information in the raw files intact when they are passed to Photomatix.

Once you are finished with Photomatix and tone map the image as described in part 4 of the tutorial, it will be automatically reimported into your Lightroom catalog.