Work with High Dynamic Range Images

Dynamic range refers to the range of brightness levels, from dark to light. The dynamic range of real-world scenes can be quite high. High dynamic range (HDR) images attempt to capture the whole tonal range of real-world scenes (called scene-referred), using 32-bit floating-point values to store each color channel. HDR images contain a high level of detail, close to the range of human vision. The toolbox includes functions for reading, creating, and writing HDR images. The toolbox also includes tone-map operators for creating low dynamic range (LDR) images from HDR images for processing and display.

Read HDR Image

To read an HDR image into the MATLAB^® workspace, use the hdrread function.

hdr_image = hdrread("office.hdr");

The output image hdr_image is an m-by-n-by-3 image of data type single.

whos

  Name        Size            Bytes    Class     Attributes

  hdr_image   665x1000x3      7980000  single

The range of data exceeds the range [0, 1] expected of LDR data.

hdr_range = [min(hdr_image(:)) max(hdr_image(:))]

hdr_range =

  1×2 single row vector

         0    3.2813

Display and Process HDR Image

Many toolbox functions assume that images of data type single and double are LDR images with data in the range [0, 1]. Since HDR data is not bound to the range [0, 1] and can contain Inf values, you must examine the behavior of each function carefully when working with HDR data.

• Some functions clip values outside the expected range before continuing to process the data. These functions can return unexpected results because clipping causes a loss of information.

• Some functions expect data in the range [0, 1] but do not adjust the data before processing it. These functions can return incorrect results.

• Some functions expect real data. If your HDR image contains values of Inf, then these functions can return unexpected results.

• Some functions have no limitations for the range of input data. These functions accept and process HDR data correctly.

To work with functions that require LDR data, you can reduce the dynamic range of an image using a process called tone mapping. Tone mapping scales HDR data to the range [0, 1] while attempting to preserve the appearance of the original image. Tone mapping functions such as tonemap, tonemapfarbman, and localtonemap give more accurate results than simple linear rescaling such as performed by the rescale function. However, note that tone mapping incurs a loss of subtle information and detail.

To display HDR images, you must perform tone mapping. For an example, see Display High Dynamic Range Image.

Create High Dynamic Range Image

To create an HDR image from a group of low dynamic range images, use the makehdr function. Note that the low dynamic range images must be spatially registered and the image files must contain EXIF metadata. Specify the low dynamic range images in a cell array.

hdr_image = makehdr(files);

Write High Dynamic Range Image to File

To write an HDR image from the workspace into a file, use the hdrwrite function.

hdrwrite(hdr,"filename");