Image information tools
Photomatix provides the following tools to analyze image content:
- | Image histograms, including a histogram variant adapted to HDR images.
- | Histogram Comparison that shows 8-bit versions of histograms side by side
- | Image Properties.
- | HDR Viewer intended for viewing the content of HDR images
Image histograms
Images with a bit-depth of 8 or 16 are encoded using a fixed number of tonal values: 256 values (from 0 to 255) in the case of an 8-bit image and 65,536 values (from 0 to 65,535) in the case of a 16-bit image. An histogram displays the occurrences of those tonal levels in a given image.
To view the histogram for an 8- or 16-bit image, click on 8-bit Histograms from the View menu. The Brightness histogram shown on top displays the occurrences of grey levels in the image. The Color channel histograms are for each one of the Red, Green and Blue channels and are drawn in the corresponding color.
Contrary to 8- or 16-bit images, High Dynamic Range Images are encoded as floating point values, i.e. in a decimal format, and not in a fixed range of discrete values. Moreover, HDR images store unprocessed tonal information values, that is, linear values directly proportional to the amount of light captured for each pixel. While linear information are valuable for many image processes, they do not provide a good representation of the way the human eye perceives incoming light.
For HDR images, Photomatix shows a logarithmic variant of the image histogram. Based on the fact that perceived image brightness can be approximated with a logarithm, the HDR histogram displays the occurrences of the logarithm based two of the image values.
To view the histogram adapted to HDR images, click on HDR Histograms from the HDR menu.
Histogram Comparison
This feature lets you view the histograms of two different images side by side. The panel shows the histogram of the active image. To compare it with the histogram of another image, select this image on the combo box and click on the Show button.
For an image with 8 bits per color channel, one unit on the x-axis refers to 1. For an image with 16 bits per color channel, one unit on the x-axis refers to 256.
Image Properties
The Image Properties panel is located under the File menu. The panel lists basic image information such as its dimension and pixel depth. When the file is a source image, it also shows the Exposure, Aperture and ISO settings when those have been found in the Exif data.
In the case of an HDR image, the estimated Dynamic Range of the image is also provided. The Dynamic Range is a ratio of maximum to minimum values. In order to account for pixel outliers due to noise and saturation, the minimum and maximum values are taken at respectively 0.2 and 99.95 percentiles of the logarithmic histogram.
HDR Viewer
The HDR viewer is a small window that pops-up when opening or creating an HDR image (note: you can disable the automatic pop-up in the Preferences from menu Photomatix). Because this window is small, it can show the local content of the image at an exposure which is appropriate to most pixels inside the window.
If you want to view local details of the image in the main window, you will have to adjust the viewing exposure up or down by clicking the items under menu HDR->Adjust View.

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