clouds module

A module providing facilities to darken clouds in images.

Images taken with the camera at the Kitt Peak National Observatory are taken with either 0.3-second or 6-second exposure times. This module provides support for finding and darkening clouds for each of the two exposure times.

clouds.cloud_contrast(img)

Darken cloud pixels in an image.

This is a convenience method that will determine the appropriate method to use.

Parameters:img (image.AllSkyImage) – The image. Returns:A higher contrast version of the original image. Return type:numpy.ndarray

See also

zero_three_cloud_contrast()

Darken cloud pixels in images with 0.3 second exposure times.

six_cloud_contrast()

Darken cloud pixels in images with 6 second exposure times.

clouds.six_cloud_contrast(img)

Darken cloud pixels in an image taken with an exposure time of 6 seconds.

Parameters:img (image.AllSkyImage) – The image. Returns:A higher contrast version of the original image. Return type:numpy.ndarray

Notes

At the start of this method, the dead pixels and horizon objects are masked out. The image is inverted, and subtracted from itself four times. This highly increases the contrast between the clouds (which fall close to 0 in the original pixel value) and the background, which will get reduced to 0. A copy of this image is used later in a separate calculation. Meanwhile, a greyscale closing is performed on this resulting image, which smooths out stars that were turned into small black dots in the inversion process.

This result then gets thresholded, which creates a two tone, black and white version of the image. This is done by making each pixel with a value above 10 as white and anything below as black. A binary closing is performed to remove any created singular white pixels. The horizon items are once again masked out, and a buffer circle of black pixels is created around the image content. As a result, the image is filled with white regions that correspond to the original clouds, with the rest of the image being black.

In some images, however, the center of the Milky Way is bright enough to be recorded at a pixel value approximately equal to the value at which the clouds appear. To account for this, for each white region in the binary image, count the number of stars in the original image that would appear within that region. Removes any region where the density of stars is too high to be a cloud. This leaves a binary image with clouds in white, and everything else in black.

From here, a scaling darkness fraction is determined by the original inversion image. Cloud pixels that are close to white in the inversion, from the darkest regions of the clouds, are scaled to 0, while the rest of the pixels are scaled less dark. This preserves the large scale structure of the clouds, but reduces them in brightness to nearly 0. The exact formula used to calculate this scaling darkness is 0.6 - (inverted pixel value) / 255.

clouds.zero_three_cloud_contrast(img)

Darken cloud pixels in an image taken with an exposure time of 0.3 seconds.

Parameters:img (image.AllSkyImage) – The image. Returns:A higher contrast version of the original image. Return type:numpy.ndarray

Notes

In order to first determine which pixels should be considered clouds this method first finds the difference between the pixel at position (510, 510) in the given image and the image taken at 05:29:36 on November 8, 2017. This difference is then subtracted from all the pixels, normalizing the image to have the same background pixel value. A greyscale closing is then performed, smudging out white pixel noise.

The average value of all the pixels in this new normalized image is calculated and any pixel that is above this average value is considered a cloud pixel. This is because light reflected off the moon illuminates the clouds, raising them above the average pixel value.

Once the cloud pixels are found, all non-cloud pixels are raised in value by 40, while the cloud pixels are reduced to 0.