When taking photos, the wonderful range of the eye becomes obvious compared to the limitations of even the best digital cameras. The human eye works in a wide range of environments from midday sun to the faint glow of moonlight, but our cameras do not see this same range. This means a scene containing very dark and light areas that all appear to eye normally can in a photo loose part of the scene with areas either washed out or lost in shadow.
HDR attempts to compensate for the lesser dynamic range of a camera by taking multiple images that together cover the entire dynamic range of the scene and combining them together to produce a photo that better presents the full dynamic range in the original scene. Many high end graphic processing packages such as Photoshop contain the ability to create HDR images. Other specialty programs designed only to create these images also exist. Creating these images does not require expensive specialized software. Here we’ll look at using the free open source Luminance to produce HDR images.
What is HDR?
In photography, the doubling or halving of light is referred to as a stop. The human eye can handle somewhere around 14 stops of dynamic range, the difference between the darkest and brightest portions of a scene. By comparison, a modern digital sensor can deal with about 8 stops of difference. In a scene with a greater difference between its darkest and lightest portions than those eight stops, either the brightest areas are blown out (made so bright all detail is lost) or lost in shadow (becoming so dark that detail is lost). The human eye viewing this same scene would see both areas fine.
We can compensate by taking multiple exposures that together cover the entire range of the scene. HDR software then takes the information on the shutter speed, aperture, and ISO stored with each image to determine the dynamic range of the full scene. It then creates a normal photo that shows the full dynamic range of the original photographed scene.
Step One: Get the Correct Photos
Luminance divides the process of creating an HDR image into two parts. First you create an HDR image from the separate LDR images. Then you covert this HDR image to a normal image in JPG or other formats. We’ll start with a scene demonstrating the use of HDR.
Here you can see two images taken of rocks with woods behind them. You can easily see that when exposing for the rocks, the woods fade into shadows and almost all detail is lost. When exposing for the woods, the rocks appear as indistinct white blobs. When looking at this scene in real life, I could clearly see both areas. The images combine to give us full and correct exposure through the range of the scene that was photographed. In spite of the fact that one is slightly overexposed, these can still be used to create a final image.
Step Two: Start and Create HDR
First download and install the Luminance software from the [web site link]web site. I’m using version 2.30-beta1 for these examples. When you begin the program you will be presented with a blank screen. Click the New HDR Image button which will start a wizard to guide you through the process.
First you will be asked to select the images to work from. You will need at least two, but can use more if the range of your camera or the range of the scene require this. Luminance will then calculate the exposure value of each image using the EXIf information stored in the image. If this information is missing, you will need to supply either the EV for each image or the exposure difference in stops between the images.
You will also usually want to select auto align images. For most circumstances, the align_image_stack method works best and should be left selected. Then click Next. Luminance will then process the images and align them. This can take several minutes depending on the number of images, the size of the images, and the complexity of the images.
Once the alignment is complete, you will be taken to the Editing Tools. Here you can tweak the alignment and crop the HDR image in case areas of the source photos do not overlap. When you are satisfied with the alignment and cropping, click Next.
You can then choose the profile to use for HDR creation. When you are more familiar with generating HDR images, you can also specify a custom profile. For most images, the default Profile 1 will produce a good result and is a safe choose if you’re unsure which to use. Now click Finish and Luminance will begin generating the HDR image.
If you are not happy with the result then go back and try a different profile and see if that gives you a better resulting image.
Step Three: Edit the HDR Image
You now have an HDR image created from your original LDR images. The options you have to the HDR image are:
You also may want to adjust the pre-gamma value. This value represents how the contrast of the image colors is processed. Luminance will preview any change you make allowing you to choose the result that looks best.
Step Four: Tone map the HDR Image to an LDR Image
You will now need to select an operator used to map the image. They are listed in order of most useful for photographic images. To being with leave the defaults for the operator. You can see a preview of each operator at the the right and can hover over a preview image to see the associated operator. You can also drag a selection over the image to tone map only a section to produce a faster preview if you would prefer to test different options quickly.
When you have found the operator that gives you results that you like, select the size you would like for the resulting final image. Now click the Tonemap button to start the process. When complete a new window will appear with your final tonemapped image at the resolution you specified.
If the resulting image appears too bright or too dark, you can adjust the levels of the image using the Adjust Levels button. This works much as the adjust levels within Photoshop, GIMP, or any other image processing software does.
If you are not happy with the results, you may need to tweak the settings a bit. The documentation for Luminance provides information on the settings and the results produced. Creating an HDR image is not an exact science. Often the results will be what you’re looking for, but sometimes you’ll need to either bring the resulting image into another program to tweak it or adjust settings to get the results you want.
When you’re happy with your resulting image, click the Save As… button. This will let you choose the location and filename to save the image to and for formats such as JPG choose the quality of the saved file. You can also save the HDR image in the Luminance EXR format if you would like to work with it again later.
Using Luminance we’ve taken a scene we could normally not capture accurately with our camera and produced a final result that reflects the original scene. From the two LDR photographs taken on site, Luminance produced an HDR image that accruately represented the try range of lighting present. We then used Luminance to produce a standard LDR image that reflected this greater dynamic range.
Luminance is not the easiest HDR program, but it comes at a perfect price (free) and provides a lot of power. This how to has shown the basic process of creating an HDR from Luminance, but there are many other features such as batch processing to explore. Once comfortable with the basics try these other options and see what types of photos you can create.