Methods for in-scene shadow compensation using sunlit and skylit illumination factors
Methods are provided for automatically performing atmospheric compensation of a multi or hyper spectral image. One method comprises transforming at least two endmembers extracted from an image into at-ground reflectance. The transformation may be approximate and/or only in certain spectral bands in order to reduce processing time. A matching component is then located in a spectral library for each of the at least two extracted endmembers. Gain and offset values are then calculated using the at least two matched extracted endmember and spectral library component pairs. At least part of the image is then compensated using the calculated gain and offset values. Another method uses at least one endmember extracted from the image and a black level. Methods for atmospheric compensation using water vapor content of pixels are also provided. In addition, methods for shadow correction of hyper and multi spectral images are provided.
1. A method for use in shadow compensation of a multi or hyper spectral image, the method comprising:
(a) identifying a brightest pixel in the image in term of its norm and assuming the pixel is fully lit by both the sun and the sky;
(b) determining a direct flux and a diffuse flux of the pixel;
(c) determining a sunlit only surface signature and a skylit only surface signature of the pixel using the direct and diffuse fluxes;
(d) for each pixel of the image, determining an abundance of the sunlit surface signature and an abundance of the skylit surface signature, and removing the sunlit surface and skylit surface signatures from pixels of the image;
(e) repeating (a) through (d) a predetermined number of times or until the norm of the pixel last identified in (a) is below a predetermined threshold value, where the pixel identified in (a) in the next iteration is the next brightest pixel in the image after the previous iteration sunlit and skylit only surface signatures have been removed; and
(f) compensating at least some selected pixels of the image, where a given pixel from the selected pixels is compensated using the direct and diffuse fluxes of the given pixel as well as a sun illumination factor and a sky illumination factor of the given pixel, the sun illumination factor being the sum of the abundances of the sunlit surface signatures and the sky illumination factor being a sum of the abundances of the skylit surface signatures of all pixels selected in the iterative process for the given pixel.
2. The method of claim 1 , wherein the compensating of a given pixel in (f) involves multiplying the given pixel by a value proportional to:
(
I
dif
+
I
diff
f
sun
I
dir
+
f
sky
I
diff
)
where I dir is the direct flux, I diff is the diffuse flux, f sun is the solar illumination factor and f sky is the sky illumination factor of the given pixel.
3. The method of claim 1 , wherein the determining of the skylit only surface signature involves multiplying a fully lit pixel signature, ρ, by a value proportional to:
(
I
diff
I
dir
+
I
diff
)
where I dir is the direct flux and I diff is the diffuse flux, and wherein the determining of the sunlit only surface signature involves multiplying the fully lit pixel signature, ρ, by a value proportional to:
(
1
-
I
diff
I
dir
+
I
diff
)
.
4. A non-transitory computer-readable storage medium comprising instructions for execution on one or more electronic devices, the instructions for a method for use in shadow compensation of a multi or hyper spectral image, the method comprising:
(a) identifying a brightest pixel in the image in term of its norm and assuming the pixel is fully lit by both the sun and the sky;
(b) determining a direct flux and a diffuse flux of the pixel;
(c) determining a sunlit only surface signature and a skylit only surface signature of the pixel using the direct and diffuse fluxes;
(d) for each pixel of the image, determining an abundance of the sunlit surface signature and an abundance of the skylit surface signature, and removing the sunlit surface and skylit surface signatures from pixels of the image;
(e) repeating (a) through (d) a predetermined number of times or until the norm of the pixel last identified in (a) is below a predetermined threshold value, where the pixel identified in (a) in the next iteration is the next brightest pixel in the image after the previous iteration sunlit and skylit only surface signatures have been removed; and
(f) compensating at least some selected pixels of the image, where a given pixel from the selected pixels is compensated using the direct and diffuse fluxes of the given pixel as well as a sun illumination factor and a sky illumination factor of the given pixel, the sun illumination factor being the sum of the abundances of the sunlit surface signatures and the sky illumination factor being a sum of the abundances of the skylit surface signatures of all pixels selected in the iterative process for the given pixel.
5. The non-transitory computer readable medium of claim 4 , wherein the compensating of a given pixel in (f) involves multiplying the given pixel by a value proportional to:
(
I
dir
+
I
diff
f
sun
I
dir
+
f
sky
I
diff
)
where I dir is the direct flux, I diff is the diffuse flux, f sun is the solar illumination factor and f sky is the sky illumination factor of the given pixel.
6. The non-transitory computer readable medium of claim 4 , wherein the determining of the skylit only surface signature involves multiplying a fully lit pixel signature, ρ, by a value proportional to:
(
I
diff
I
dir
+
I
diff
)
where I dir is the direct flux and I diff is the diffuse flux, and wherein the determining of the sunlit only surface signature involves multiplying the fully lit pixel signature, ρ, by a value proportional to:
(
1
-
I
diff
I
dir
+
I
diff
)
.