In image processing, computer graphics, and photography, high dynamic range imaging (HDRI or just HDR) is a set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods. This wide dynamic range allows HDR images to represent more accurately the range of intensity levels found in real scenes, ranging from direct sunlight to faint starlight, and is often captured by way of a plurality of differently exposed pictures of the same subject matter.
In simpler terms, HDR is a range of techniques geared toward representing more contrast in pictures. Non-HDR cameras take pictures at a single exposure level with a limited contrast range. This results in the loss of detail in bright or dark areas of a picture, depending on whether the camera had a low or high exposure setting. HDR compensates for this loss of detail by taking multiple pictures at different exposure levels and intelligently stitching them together so that we eventually arrive at a picture that is representative in both dark and bright areas.
Tone mapping is a technique used in image processing and computer graphics to map one set of colors to another in order to approximate the appearance of high dynamic range images in a medium that has a more limited dynamic range. Print-outs, CRT or LCD monitors, and projectors all have a limited dynamic range that is inadequate to reproduce the full range of light intensities present in natural scenes. Essentially, tone mapping addresses the problem of strong contrast reduction from the scene values (radiance) to the displayable range while preserving the image details and color appearance important to appreciate the original scene content.
The first low dynamic range (LDR) image taken normally with a camera isn’t able to capture much of the detail found in the highlight and shadow areas of the scene. Two (or more) photographs are then taken at different exposure values to capture a wider range (the bracketing step) and subsequently combined into a single image with a high dynamic range (HDR). Since most displays aren’t capable of displaying this full range, the image needs to be tone mapped to have its appearance approximated on LDR screens.