Jun 162012

Here is the next installment of HDR tips by Unified Color VP of Marketing John Omvik. Here’s John:

In my last post I discussed the importance of firmly securing your camera when shooting exposure brackets for HDR scenes. The key was to eliminate movement between frames that can cause merge and alignment issues. Once the camera is secure, our attention needs to shift to the objects themselves.

This image has moving water and clouds, the smooth de-ghosting option is best for this type of scene. It smoothly blends the images producing sharp results in stationary objects like the rocks in the foreground and blends the movement in the water.

Since we are taking multiple exposures that will be merged together it is also just as important to avoid moving objects in your scene that can change their location or position from one frame to the next. Objects moving between frames are called ghosts because they can show a combination of both the moving object as well as the background scene and appear ghost like. Since we are dealing with HDR and multiple exposures that render different brightness levels from the individual frames, a ghost may not appear as a solid image, but only part of a tonal range of the object. For example if an object moves between frames, the shadow portion of the object may render in one position while the highlights render in another.

This image has stationary objects and a person walking through the frame in the upper right corner.

There are 2 types of ghost effects, and many different ways to address them, I call them rooted and mobile ghosts. A rooted ghost is slight movement in what is really a fixed object. Example of these are leaves of a tree, water in a stream or even clouds in the sky. In most cases these have some anchor point in the image that isn’t moving (for clouds, the blue sky counts as an anchor). In these cases we are not so concerned with detail in the moving parts and prefer to smoothly blend the motion preserving the sharpness of the other elements in the image. In HDR Expose 2, we refer to this as “Natural” Ghost reduction. Mobile ghosts are objects that actually move from one position in the image to another between frames, such as a car driving or a person walking through a scene. Rather than blend the object which would look strange we prefer to isolate the object with a sharp edge that better defines the object and allows us to more easily clean it up in post. In HDR Expose 2 we call that the “Sharp Edges” ghost reduction. For images that have both rooted and mobile ghosts there is the smooth edges option that attempts to best address both types of motion. If you are using 32 Float v2 to process your HDR images and Photoshop to do the merge step, you have the option of selecting a “key frame” that the others are compared against in order to isolate movement.

In this case the natural option produces undesirable results the moving object (person) is blended with the background and each other.

A close up look at the sharp edges method produces much better results. There is still a slight outline of the ghost from the other frame, but these are minor and can easily be retouched out. Another option would be to wait until the gentleman had made his way through the frame, but that is not always possible in crowded areas.

De-ghosting of any type is just additional image processing that you are better avoiding if at all possible. So until we have cameras that can capture the full dynamic range of a scene in a single shot, or can bracket so fast that they can freeze motion between frames, it is best avoid shooting in-door shots in busy locations with lots of people moving through a frame or shooting outdoor scenes on windy days.