High Dynamic Range imaging
High Dynamic Range imaging (HDR) has come some way since I first ran across the initial phenomenon in the late 90's. HDR is a file format for images that contains numbers. The numbers represent so much pictorial information, no monitor or printer can display the data in it's entirety.
The HDR image is like a digital negative in as much as making a print involves adjusting the enlarger aperture and light source to an exposure level consistent with the photographer's goals. No one exposure represents the negative entirely. Lighter exposures reveal different details than a lower exposure.
An HDR image is formed by taking a series of images that vary only in shutter speed, and combining them in a defined way. Think of an unsliced loaf of bread as an image. T he several images represent every nth slice in a sliced loaf. The other slices are filled in using highly educated guesses to get a full, sliced loaf. Each slice represents a specific shutter speed.
Better versions of the process use statistical analysis to determine characteristics of the filled in slices.
Image readers offer the user a slider to control which exposure will be printed, much like the enlarger in a physical system.
But this is only one part. Even with the production of the HDR image, the viewer has to use a compressor to determine the nature of the extracted data.
The compressor can be any one of several styles. The extraction policy can be objective, i.e. the depiction of all the detail in an image regardless of level of exposure.
Strictly speaking, the image will not be realistic in as much as the objects normally hidden in a dark image will be visible as will the objects normally hidden in a light exposure. In a real image, one must choose between the two, and cannot have both.
Another compressor may have an aesthetic philosophy in attempting to create 'beautiful' images.
Yet another may attempt to duplicate the effects of the human visual physiology in the imagery. The human eye adapts to varying conditions in a dynamic way.
Finally, one compressor that is quite popular, attempts to duplicate a process invented and developed by Albert Eisenstadt and a co-worker, called the 'Zone Method'. The attempt is very good in it's results.
Each compressor reduces the amount of data in the image to an amount printable or displayable on a monitor.
One of the first was the Retinex method. Later, after the Reinhardt Tonemap (Eisentadt Zone Method) came the Zvi Devir HDRC(sic) which made the most successful, to that date, attempt at objective compression. Most detail in an image is made visible using that compressor.
Having chosen a compressor, one must adjust the variables presented by the originator of the compressor. The Reinhardt compressor has several variables that can be adjusted. Fortunately for the uninitiated, most compressors come with a workable set of default values for that compressors variables. A useful analogy would be the knobs and dials on a machine. The machine comes ready to use, out of the box. But adjustments are possible.
One of the potential uses of the HDR image is as a storage for historical scenes. Detail that was lost in historical photographs can be stored in an HDR image.
More aesthetically pleasing imagery can be created with the HDR image, that with a standard image.
It is easy to see that High Dynamic Range photography is a field in, and of, itself. Much like specializing in B&W or 35mm slides, or wide format (4x4) photography.
The HDR image is like a digital negative in as much as making a print involves adjusting the enlarger aperture and light source to an exposure level consistent with the photographer's goals. No one exposure represents the negative entirely. Lighter exposures reveal different details than a lower exposure.
An HDR image is formed by taking a series of images that vary only in shutter speed, and combining them in a defined way. Think of an unsliced loaf of bread as an image. T he several images represent every nth slice in a sliced loaf. The other slices are filled in using highly educated guesses to get a full, sliced loaf. Each slice represents a specific shutter speed.
Better versions of the process use statistical analysis to determine characteristics of the filled in slices.
Image readers offer the user a slider to control which exposure will be printed, much like the enlarger in a physical system.
But this is only one part. Even with the production of the HDR image, the viewer has to use a compressor to determine the nature of the extracted data.
The compressor can be any one of several styles. The extraction policy can be objective, i.e. the depiction of all the detail in an image regardless of level of exposure.
Strictly speaking, the image will not be realistic in as much as the objects normally hidden in a dark image will be visible as will the objects normally hidden in a light exposure. In a real image, one must choose between the two, and cannot have both.
Another compressor may have an aesthetic philosophy in attempting to create 'beautiful' images.
Yet another may attempt to duplicate the effects of the human visual physiology in the imagery. The human eye adapts to varying conditions in a dynamic way.
Finally, one compressor that is quite popular, attempts to duplicate a process invented and developed by Albert Eisenstadt and a co-worker, called the 'Zone Method'. The attempt is very good in it's results.
Each compressor reduces the amount of data in the image to an amount printable or displayable on a monitor.
One of the first was the Retinex method. Later, after the Reinhardt Tonemap (Eisentadt Zone Method) came the Zvi Devir HDRC(sic) which made the most successful, to that date, attempt at objective compression. Most detail in an image is made visible using that compressor.
Having chosen a compressor, one must adjust the variables presented by the originator of the compressor. The Reinhardt compressor has several variables that can be adjusted. Fortunately for the uninitiated, most compressors come with a workable set of default values for that compressors variables. A useful analogy would be the knobs and dials on a machine. The machine comes ready to use, out of the box. But adjustments are possible.
One of the potential uses of the HDR image is as a storage for historical scenes. Detail that was lost in historical photographs can be stored in an HDR image.
More aesthetically pleasing imagery can be created with the HDR image, that with a standard image.
It is easy to see that High Dynamic Range photography is a field in, and of, itself. Much like specializing in B&W or 35mm slides, or wide format (4x4) photography.
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