(High Dynamic Range Imagery) HDRI (High Dynamic Range Imagery)

History of HDRI Not just a digital process! Using multiple exposures to fix extreme luminance range was used as early as 1850 by Gustave Le Grey In the following image Gustave used one exposure for the sky and a longer exposure for the sea HDRI is an old technique and predates digital photography.

Such rendering was impossible at the time using standard methods, the luminosity range being too extreme. Le Gray used one negative for the sky, and another one with a longer exposure for the sea, and combined the two into one picture in positive.

Here we see a modern HDRI Here we see a modern HDRI. This is the result of taking 3 or more exposures and combining them. The amount of exposures is up to the photographer, using up to 10 exposures to create 1 image is not uncommon. The more exposures used, the more dynamic the range.

HDRI or HDR High Dynamic Range Imagery Allows greater range between the light and dark areas of an image This range is “Dynamic” because the light values can be modified Represents the range of intensity levels seen by the human eye more accurately HDRI is a set of techniques used in imaging and photography to reproduce a greater dynamic range of luminosity (light) than standard digital imaging or photographic techniques can do. The aim is to present the human eye with a similar range of luminance as that which, through the visual system, is familiar in everyday life. The human eye, through adaptation of the iris (and other methods) adjusts constantly to the huge dynamic light changes seen in our environment. The brain continuously interprets this information so that most of us can see in a wide range of light conditions. Most cameras, on the other hand, cannot. HDRI is a way to get close to the power of the human eye.

Part of the problem with HDRI imagery is it’s overuse or heavy handedness. HDRI can be used to enhance the light and detail in an image, or overused to create a painterly look. Because it breaks the rules of lighting by showing more range of light all at once it can create almost surreal or overpowering results and should be used with purpose.

HDR File Format vs HDR Imagery (HDRI) One is a file format and one is a photographic technique The HDR file format contains extra infrared data for light simulations. These are used in 3D animation sometimes to provide real world lighting conditions for 3D animation, such as these light probes to the right. Don’t confuse the HDR file format for the HDRI process. One if a format, the other is a technique. HDR file format contains extra information for light mapping and is sometimes used to create realistic lighting in 3D animations.

HDR Photography Multiple Low- Dynamic Range images are combined This process is called “Tone Mapping” Shooting multiple exposures for tone mapping is called “Bracketing” Due to the limitations of printing and display contrast, acquiring an HDR image is only half the story; one must also develop methods of observing the results. Consequently we have firstly, the methods of obtaining the information in the first place and secondly, the methods of rendering that information to be available to a standard monitor or printing device, this is tone mapping.

Multiple exposures can create interesting results in many types of scenes. The flexibility if the process lets you show the details you want and really have full control over the final image.

HDR Photography Any camera that allows over or under exposure can be used to create HDRI Some cameras have automatic exposure bracketing (AEB) These separate exposures are combined with Photoshop HDR images can represent a greater range of luminance levels than can be achieved using more 'traditional' methods. Images such as those containing many real-world scenes, from very bright direct sunlight to extreme shade or very faint nebulae. It is often achieved by capturing and then combining several different exposures of the same subject matter.[1][2][3][4] Non-HDR cameras take photographs with a limited exposure range, resulting in the loss of detail in bright or dark areas. The two primary types of HDR images are computer renderings and images resulting from merging multiple low- dynamic-range (LDR)[5] or standard-dynamic-range (SDR)[6] photographs. HDR images can also be acquired using special image sensors, like oversampled binary image sensor. Tone mapping methods, which reduce overall contrast to facilitate display of HDR images on devices with lower dynamic range, can be applied to produce images with preserved or exaggerated local contrast for artistic effect.

HDRI Tips Shoot multiple exposures with a tripod, each exposure must be in the exact same position Portraits are difficult without AEB (Auto Exposure Bracketing), but the look of HDRI can be “faked” with a single exposure In photography, dynamic range is measured in EV differences (known as stops). An increase of one EV, or 'one stop', represents a doubling of the amount of light. Conversely, a decrease of one EV represents a halving of the amount of light. To reveal detail in the darkest shadow requires high exposures. Inversely, to prevent 'bleaching out' of detail in very bright areas, one must choose very low exposures. Most cameras cannot provide this range of exposure values within a single exposure, due to their low dynamic resolution.

Here we see an underexposed photo, roughly 8 stops below ideal exposure settings.

Next we advance to 2 stops below ideal exposure settings.

Now we advance 2 stops over ideal exposure…

And now 4 stops above our base exposure setting, which will be different for every shot. Adjust and experiment based on the amount of light in your scene. +4 Stops

Combined with Tone Mapping Here are all the exposures combined with a process called “Tone Mapping”. Photoshop provides access to this process through the “Merge to HDR Pro” function. This gives you the ability to dial in your exposure for your desired results. Combined with Tone Mapping