The raw HDR output image from Predict Engine must be converted to a displayable LDR RGB image. The output image can be either :

• • the raw image converted to LDR RGB using a given gamut and a gamma correction,

  • the raw image with a gamma correction only, or

  • the raw image as a representation of the measured physical quantity in false colors.

Depending on the sensor you defined, the color system will be automatically set to the right type. The gamut and gamma settings are then available in the Color System section.

Gamma Correction

For sensors requiring an RGB Color System, you can choose between Preset values that define both the gamut and the transfer function applied to the image, or use a Gamma correction only and define the gamma value manually.

You can find a good explanation of what the gamma correction is needed for on this page. Here is an extract :

"The need for gamma arises for two main reasons: The first is that screens have a non-linear response to intensity. The other is that the human eye can tell the difference between darker shades better than lighter shades. This means that when images are compressed to save space, we want to have greater accuracy for dark intensities at the expense of lighter intensities. Both of these problems are resolved using gamma correction, which is to say the intensity of every pixel in an image is put through a power function. Specifically, gamma is the name given to the power applied to the image.

Most images are stored with a gamma of 0.45 (1/2.2) applied to them [...]. Images stored like this are in “gamma space” [...]. This is the default behavior of nearly every digital camera, as well as image editing applications and so on. In fact, nearly every image you see on your computer has that gamma applied to it.

You may be wondering why images display correctly, and don’t look too bright. This is where the non-linear response of displays comes in. CRT screens, simply by how they work, apply a gamma of around 2.2, and modern LCD screens are designed to mimic that behavior. A gamma of 2.2, the reciprocal of 0.45, when applied to the brightened images will darken them, leaving the original image."

In order to display on screen an image corrected for your specific screen, Predict Engine needs the exact value of this gamma correction : this is the gamma defined in the color system.

The gamma value defined in the color system should match your display. Follow the next steps to find the right gamma for your display :

• Download the image on the right on your computer (unless you have a truecolor video system, do not attempt to use the image while viewing it online, since your browser will be changing the colors to compensate for other colors on the screen),

• Open the image with a viewer that displays the correct intensities (ensure that the image is not enlarged or reduced in any way by the viewing program, as this will also ruin its effectiveness),

• Darken the room and set the monitor brightness and contrast to maximum,

• While viewing a black screen, lower the brightness gradually until the "background" (ie the scanlines that you see across what should be a black screen) is no longer noticeable (when it just fades from view). This may be difficult on monitors that normally cover the whole visible portion of the display, unless you change the viewable area settings until you can see the actual border where the monitor isn't displaying anything,

• While viewing the reference image, lower the contrast until the alternating white and black bars on the left edges of each column are equal in width. This is trying to get a 50% gray by using half white and half black. If this is not possible, choose a contrast setting which is about in the middle of the available range,

• While viewing the image from a distance, or with squinted eyes, one of the numbered "swatches" will best match the gray value approximated by the white and black bars. The number in this "swatch" is your display's actual gamma value (usually 2.2).