ImageMagick: High Dynamic-Range Images

Introduction to High Dynamic-Range Images

High dynamic-range imaging (HDRI) permits a far greater dynamic range of exposures (i.e. a large difference between light and dark areas) than standard digital imaging techniques. HDRI accurately represents the wide range of intensity levels found in real scenes ranging from the brightest direct sunlight to the deepest darkest shadows. The HDR imaging approach includes:

render/capture floating-point color space
encompass the entire perceivable gamut (extend values outside [0,1] range)
post-process in extended color space
apply tone-mapping for specific display

Enabling HDRI in ImageMagick

By default image pixels in ImageMagick are stored as unsigned values that range from 0 to the quantum depth which is typically 16-bits (Q16). With HDRI enabled, the pixels are stored in a floating-point representation and can include negative values as well as values that exceed the quantum depth. A majority of digital image formats do not support HDRI so any pixels outside the quantum range are clamped before they are stored. The mostly promising HDR image format is EXR. You will need the OpenEXR delegate library to read or write this format. Other HDR formats include TIFF 48-bit integer and 96-bit float formats, HDR, PFM, and ImageMagick's own MIFF format.

To enable the HDRI version of ImageMagick, use this Unix/Linux command:

  ./configure --enable-hdri

Under Windows, set the UseHDRI definition in the magick-config.h file and build.

To verify HDRI is properly configured, look for "HDRI" in the version string:

  -> identify -version
  Version: ImageMagick 6.3.7 11/01/07 Q16 HDRI http://www.imagemagick.org