Image dynamic range control for visual display

1. A system for mapping high dynamic range image data derived from a high dynamic range imaging device to render a corresponding image on a lower dynamic range display device, the high dynamic range image data including representations of relatively low intensity contrast, high spatial frequency detail and relatively low spatial frequency intensities, comprising:

a filtering unit receiving the high dynamic range image data and producing high-pass filtered image data that include representations of high spatial frequency data of the high dynamic range image data, the filtering unit including:

a blurring unit receiving as an input the high dynamic range image data and producing an output representing a blurred version of the high dynamic range image data;

an inversion unit receiving the output from the blurring unit and producing an output representing an inverted version of the blurred high dynamic range image data; and

a summing unit combining the high dynamic range image data with the output of the inversion unit while maintaining the same dynamic range as that of the high dynamic range image data to thereby produce the high-pass filtered image data;

a conversion unit implemented with an intensity transform for converting the high-pass filtered image data to produce an output signal characterized by a dynamic range that is compatible with the lower dynamic range of the display device and the low intensity contrast, high spatial frequency detail of the high dynamic range image data;

a reference image unit producing an output signal representing a gain and level adjustment of the high dynamic range image data; and

a combining unit for combining the output signals of the conversion and the reference image units to produce a low dynamic range image with visually perceptible local area detail for display on the display device.

2. The system of claim 1 , further comprising a statistics unit for determining a global mean intensity level of the high dynamic range image data.

3. The system of claim 2 , wherein: the gain adjustment is determined by a minimum intensity extent, a maximum intensity extent, and the global mean intensity level of the high dynamic range image data; and the reference image unit sets the global mean intensity level of the high dynamic range image data in the lower dynamic range as approximately a mid-range intensity level of the lower dynamic range.

4. The system of claim 1 , wherein the intensity transform is a non-linear intensity transform and minimizes visible artifacts generated by the filtering unit.

5. The system of claim 1 , wherein the intensity transform is selected from a plurality of transform curves, the selection dictated by the dynamic range of the high dynamic range image data.

6. The system of claim 1 , wherein the conversion unit optimizes the gain for converting the high-pass filtered image data to the lower dynamic range.

7. The system of claim 1 , wherein the output signal of the reference image unit represents a clamped version of the high dynamic range image data.

8. The system of claim 7 , in which the output of the blurring unit is applied to the reference image unit.

9. The system of claim 7 , further comprising a statistics unit for determining a global mean intensity level of the high dynamic range image data, wherein the clamped version of the high dynamic range image data is limited by the global mean intensity level calculated by the statistics unit.

10. The system of claim 1 , wherein the combining unit is controllable to selectively adjust a ratio of the output signals of the conversion and reference image units.

11. The system of claim 1 , wherein the high dynamic range imaging device comprises a thermal infrared camera.

12. The system of claim 1 , wherein the system is used in conjunction with aircraft.

13. A method of mapping high dynamic range image data to render on a lower dynamic range display device a corresponding image characterized by stable global intensity levels and visually perceptible local area detail, comprising:

providing from a high dynamic range imaging device high dynamic range image data including representations of relatively low intensity contrast, high spatial frequency detail and relatively low spatial frequency intensities;

deriving in a filtering unit high-pass filtered image data from the high dynamic range image data by:

blurring the high dynamic range image data;

inverting the blurred high dynamic range image data; and

combining the high dynamic range image data with the inverted blurred high dynamic range image data; and

performing in a processing unit the steps of:

applying to the high-pass filtered image data a nonlinear intensity transform to produce a first set of low dynamic range image data, the nonlinear intensity transform substantially preserving or enhancing the low intensity contrast, high spatial frequency detail and maintaining a visually perceptible representation of the relatively low spatial frequency intensities represented in the high dynamic range image data;

applying to data derived from the high dynamic range image data a gain and level adjustment to produce a second set of low dynamic range image data; and

combining the first and second sets of low dynamic range image data to produce a low dynamic range image, the first set of low dynamic range image data providing visually perceptible local area detail within the low dynamic range image.

14. The method of claim 13 , in which the high-pass filtered image data include representations of visible artifacts, and the nonlinear intensity transform is applied to the high-pass filtered image data to reduce perception of the visible artifacts.

15. The method of claim 13 , further comprising: calculating a mean value of the high dynamic range image data; and presenting the mean value of the high dynamic range image data as a mid-range intensity of the low dynamic range image.

16. The method of claim 13 , further comprising: selecting the nonlinear intensity transform from a plurality of transform curves, the selection dictated by the dynamic range of the high dynamic range image data.

17. The method of claim 13 , wherein the high dynamic range image data are provided by a thermal infrared camera.

18. The method of claim 13 , wherein the method is used in conjunction with aircraft.