This post-processor is also responsible for converting the input texture from an HDR format (usually HdrBlendable) to an LDR format. Therefore, the surface format in DefaultTargetFormat is set to Color.

This post-processor stores luminance information of a frame for use in the next frame. The information is stored with the currently active camera node. When there is a cut in the scene (i.e. a new level is loaded or the view changes significantly), the method InvalidateViewDependentData of the camera node needs to be called to reset the luminance information. Further, this post-processor expects that it is called once per frame for a certain camera node. It might not work as expected if it is called several times per frame (e.g. to process different views using the same camera).

Exposure and Middle Gray
The HdrFilter computes the average brightness of the scene and applies a brightness scale factor ("exposure") so that the average brightness of the final image is equal to MiddleGray. That means, if you are in a dark cave, the filter will make the image a lot brighter. If you are in a white dessert, it will make the image darker. Both images will have the same average brightness defined by MiddleGray. The brightness scale factor is limited by MinExposure and MaxExposure. That means, if you set MinExposure = MaxExposure = 1, then there will be no brightness change. If your scene is very bright or very dark, then you have to adjust MiddleGray. Use MinExposure and MaxExposure to limit the allowed brightness changes.

Night Tone mapping with Blue Shift (Scotopic Vision):
The human visual system perceives low luminance scenes (e.g. night scenes) different than well-lit scenes. Under daylight the cones in the human eye dominate and create the normal color image (photopic vision). The cones start to lose sensitivity at about 3.4 cd/m², and the rods start to get more dominant. A dark scene perceived by the rods (scotopic vision) is monochrome with a blue shift. This HdrFilter supports physically-based night tone mapping. Per default it is disabled. The blue shift is controlled using the properties EnableBlueShift, BlueShiftColor, BlueShiftCenter and BlueShiftRange. Depending on the lighting in the scene, BlueShiftColor might need to be scaled. It is also possible to use a different color for a more dramatic effect. BlueShiftCenter and BlueShiftRange might also need to be changed if the scene is not lit using physically-based light intensities and lighting models.

Reference