4. Processing a Single Frame HDR Image
Okay, enough theory! In this section, two HDR workflows will be shown that produce a single frame (i.e. non-stitched) final image from several input images. One workflow is based on Photoshop CS2, then other on Photomatix Pro. In both cases, the example will show how to initially create the HDR image file, and then how to tone map it into a final image.
Key points covered in this section:
- Tools used
- Workflow 1 – PS CS2
- Workflow 2 – PS CS2 + Photomatix
- Comparison of workflow results
The examples will produce a tone mapped image from this sequence:
 |
 |
 |
0
EV (base image) |
-2
EV |
+2
EV |
The base image was shot at 1/15s, f/22, ISO 100. The camera used was a Canon EOS 10D with Sigma 12-24mm f/4.5-5.6 lens zoomed to 13mm. The camera was tripod mounted, and the shutter was tripped with a remote release cable. The exposure sequence was taken using auto-bracketing at an interval of +/- 2 EV. The images were shot RAW and converted to 16-bit TIFF files using Rawshooter Premium at default settings except for +20 detail, +10 saturation and +3 vibrance.
The example screenshots in this section are taken from a desktop workstation (AMD Athlon 64 X2 4800+ dual core system with 3 GB of RAM visible) running Windows XP SP2.
Application software versions include:
- Rawshooter Premium 1.0.3 build 77
- Adobe Photoshop CS2 9.0.1
- Photomatix Pro 2.2.3
The first workflow shows how to produce a tone mapped HDR image using only the tools found within Photoshop CS2. At the moment, CS2 probably can be considered the preeminent tool for image processing and it is “the” workhorse application for many photographers. When CS2 was introduced, it contained new support for HDR image processing and so it is likely to be the first point of contact with HDR.
There are two key functions to use. The first is Merge to HDR, accessed via the File>Automate>Merge to HDR menu in CS2, or the Tools>Photoshop>Merge to HDR menu in Bridge. This function is how you initially combine several images taken at different exposures into a single HDR 32-bit image file.
The second function is HDR Conversion, accessed via the Image>Mode>16 Bits/Channel menu in CS2 while a previously created HDR 32-bit image is open in the editor. Dropping the image mode below 32-bit triggers CS2 to get you to specify how to tone map the HDR image, in this case to get it to fit within a 16-bit DR. (HDR Conversion can also be accessed by dropping the image mode to 8-bit.)
CS2 Merge to HDR
To get the first example rolling, the first thing to do is process the above three example images into a single HDR file. With CS2 open, select the Merge to HDR function via the File>Automate>Merge to HDR menu. This brings up the following panel:
With the drop-down list showing “Files,” I have browsed to my source images and included the three of them. The drop-down can be used instead to select “Folder” which permits adding an entire folder of images, or “Open Files” which will add the files currently open in the editor.
The only other control is a check-box to enable alignment of the input images. I recommend selecting the check-box unless there is a specific reason not to. While it will add processing time, it will help ensure that any minor image alignment errors present in the input images will be eliminated, or at least minimized. If you are sure your images will match 100%, or are not worried about ghosting and blurriness from registration problems, you can uncheck the box to speed up processing.
Clicking the OK button kicks off the first stage of processing and produces a single HDR image. This process takes 15 – 20 seconds on my workstation for this example. Most of that time is spent aligning the input images. Once alignment and initial processing are done, a second large window is displayed:
This window shows a preview of the HDR image, incorporating the full DR of the original sequence. Since your computer monitor is essentially an 8-bit device, and no tone mapping has yet been done, you are able to see only a limited subset of the full DR at any one time.
The histogram on the right hand side of the window gives a snapshot of the total DR of the combined image. Each red tick mark on the horizontal axis of the histogram represents about 1 stop of DR. The white point slider beneath the histogram controls the subset of DR displayed on the monitor. Adjust the slider left and right to examine detail in different tonal areas of the image. Using the histogram, slider and image preview, it is quickly possible to tell where the interesting image detail lies and roughly how well the merge worked (including the image alignment).
Down the left hand side, thumbnails of the input image are shown with an estimate of the relative exposure of each. Each image also has a check-box controlling whether that image’s information is included in the merged image. You can toggle an individual image on and off to roughly determine the contribution its data makes to the overall look.
Be sure to leave the bit depth drop-down showing “32 Bit/Channel” in order to generate an actual HDR image. Selecting 16 or 8 bits will immediately drop the DR of the merged image by clipping shadows and highlights to fit the selected bit depth, based on the position of the white point slider. When 32-bit is selected, the position of the white point slider does not affect the saved data in any way, but the slider’s position will be remembered the next time the HDR file is opened.
The size of the preview can be adjusted using the zoom controls at the bottom if you need to examine some details more closely. When the input images have been selected and the preview examined to your satisfaction, click OK and the HDR image will be generated. This process takes only a few seconds. At the end, the generated HDR image is open in CS2, ready for further work:
Before doing anything else, you can save the HDR image in case you want to come back to the pre-tone mapped version at a later date. Several formats are available to choose from, including Radiance RGBE (which uses the extension “.hdr”) and Open-EXR (which uses “.exr”). This example saved as an .hdr file runs about 18.5 MB in size compared to about 36 MB for just one of the input images in 16-bit TIFF format. In “.exr” format, the HDR image comes in just over 13 MB in size. For a technical comparison of the HDR formats, see Greg Ward’s paper “High Dynamic Range Image Encodings” linked at the end of this article.
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