A simple approach to HDR-blending
or: Getting rid of ugly blending halos
One of the biggest problems in panoramic imaging is the typical high dynamic range of the scenery. Many commercial applications address this problem but the higher the brightness differences are, the more images you need to blend to get a sophisticated result.
This tutorial addresses the generation of a blended image from 2 shots in general - with a special focus on reducing the resulting blending halos.
Preparation: How to shoot the images
If your camera can do spot-metering, enable it. This will help you to measure the exact spot which you want to use for your different images. If your camera has no spot-metering, shoot the image and check it thoroughly if it fits your needs.
The "light" image: This image will hold the information about the darker areas of your blended image. Capture it without caring about the bright areas in your image. It should look like a "normal" photo.
The "dark" image:
The enemy of digital photography is over-exposure. You need to avoid an overexposed image under any circumstances. You can restore pretty much out from darker areas in your digital image, but you can't restore any information out of a bright white area.
To avoid over-exposure, expose towards the brightest areas in your image.
Standard blending process
This is how a standard blending process will look like. Please notice the strange halo around the lamp. We're going to avoid this.
Processing the images
Step 1: Blending
Several ways exist to blend the images together for a nice visual appearance: GUI-based with a software like Photomatix - Contrast Blending (Exposure Blending) - or some manual work with Photoshop as described below...
Load both images and copy the dark image as a separate layer over the light image. Add a layer mask with the dark image completely masked out.
- As you can see there are "burnt out" white areas in the light image. All the following steps have to be done separately for each coherent white area in the image.
- Prepare a layer brush (=the normal brush) with soft edges and a radius which is a little larger than the area you want to get from the dark image. You can see an example of the brush size for the large area here:
- Click to "paint through" and don't care if the brightness doesn't fit perfect. This will be addressed in the next step.
- Now adjust the color saturation of the blended areas. Reduce your layered image to one layer. Use the Sponge Tool (Photoshop shortcut: O) in saturate mode with a strength of about 10% to carefully adapt the color saturation:
Step 2: Removing color halos
Now we have to eliminate the resulting color halos for a correct image appearance. This step may sound complicated but is very straightforward as soon as you are used to the process.
- Use the eyedropper tool to retrieve a color sample of the surrounding area where the colors are still correct. Use a multi-point matrix to have a good color sample.
- Now select the brush tool. Switch the painting mode from "Normal" to "Saturation". Paint over the color halo. Don't be irritated if the color becomes worse than before.
- Switch the painting mode from "Saturation" to "Color". Paint over the color halo.
- If you feel that colors are too intense, you may use the Sponge Tool (Photoshop shortcut o) in desaturate mode to carefully reduce the colors. A tool-strength of about 10% is strongly suggested.
--Bvogl 16:50, 18 Jul 2005 (EDT)