HDR
From ProPHOTO WIKI
|
|
This article is a stub. You can help ProPhotoWiki by expanding it. |
Contents |
Introduction
High Dynamic-Range (HDR) imaging is a photographic technique used to capture images with significantly larger tonal ranges than conventional imaging processes are capable of. By using floating point numbers to represent the colour values in an image, HDR image files can store highlight and shadow detail that would normally be sacrificed when stored using integer values used in typical image files. This allows Photographers to capture images that would otherwise be impossible, and allows a much wider latitude in exposure corrections after-the-fact.
Due to limitations in capture equipment, however, the creation of these images typically requires the photographer to take multiple bracketed exposures and then combine them together in post processing. As such, HDR Imaging is generally limited to special purpose applications whose subject material will remain still for extended periods of time.
Storing Colour Values
Conventional 8-bit Image Files
In a conventional image file, the colour of each pixel is typically stored as a set of three integer values (one for each colour channel) between 0 and 255. A value of 255 represents the maximum contribution of the specified primary colour possible within the colour space, whereas a value of 0 represents no contribution. As such, a colour value of 255,255,255 indicates white and a value of 0,0,0 represents pure black.
In most situations, this configuration works well. The colour spaces that are commonly used generally have a wide enough gamut that the values fitting within these bounds are sufficient to meet our reproduction needs. Further, a range of 255 values for each of the colour channels is generally sufficient to exceed the sensitivity of the human eye.
The main problem, however, arises when significant adjustments have to be performed on the image files. When applying significant changes using the curves control within Photoshop, for instance, a number of defects can become apparent in the resultant image. Firstly, a steep adjustment curve can create banding effects as a formerly large range of tonal values must be compressed into a much smaller set. Secondly, areas of the image that were near the top or bottom end of the tonal range may be pushed beyond the edges of that scale, leaving the system with no way to store the resultant values.
Conventional 16-bit Image Files
To solve the first problem, we can simply increase the precision that we use to store the numbers. Rather than the 8-bit range of 0 to 255, we can use 16-bit integers to store colours in the range of 0 to 65,535. As there are more graduations to go around, those rounding errors created by these adjustments are greatly reduced. As such, the use of 16-bit image files provides photographers with a safer domain to perform significant changes to their image files.
The problem with this, however, is that 16-bit images do not address the second problem. If, for instance, the adjustment curve results in a value of 65,536 the image file has no way to store it in the resulting image. As such, any colour that exceeds the maximum value is simply clipped at 65,535. The result of this problem is that extreme highlights (like the Corona of the Sun) may become devoid of any details when brightening up the other areas of the image.
HDR Image Files
To work around the latter problem, floating point numbers can be used in place of the integers in conventional image files. That is, instead of using the range of 0 to 255, these images use the value of 0 to 1.0 to store the normal range of colours. As such, incremental values are stored as a decimal value with their precission determined by the specific format being used (typically 24-bits).
Using this format, if a processing step resulted in a value outside of the 0 to 1.0 range it could then be stored accurately (eg a value of 1.1 is permissable in this system). As such, HDR images using floating point representation can work with colours that may fall outside of the normal gamut of the colour space that is in use. While these out of range values may not be directly reproducable, their values remain intact so further processing can accurately bring them back into range.
Support
Photoshop CS2
Adobe Photoshop CS2 first added native support for 32-bit HDR image files. Like early support for 16-bit images, however, many functions within the program were not available when used in this mode. More comprehensive support was later added to Photoshop CS3, which provides nearly full functionality for HDR images.
File Formats
Photoshop PSD
As Adobe has done in the past, Photoshop's native PSD format added support for HDR images along with CS2.
JPEG XR/HD Photo
Microsoft's HD Photo format was designed from the start to support HDR imaging. Naturally, the program opening the images must still have support for handling HDR images to load these files.
See Also
- HD Photo - Image file format specifically designed to contain HDR images.
- scRGB - Colour space designed to expand the sRGB colour space when used with HDR images.
Categories: Stubs | Computers | Colour | Technology
