Histogram equalisation

By default, Sanchez applies histogram equalisation to satellite imagery in order to maximise contrast. Depending on the source of the image, the image channel and Sanchez composition mode, this equalisation may be turned off or adjusted.

The images below show, in order, no histogram equalisation (via -L / --noadjustlevels), default histogram equalisation, and adaptive histogram equalisation (-A / --adaptivelevels) for Himawari-8 and Electro-L N2. Click to enlarge.

Applying adaptive levels to a multi-satellite stitched image generally improves the overall image quality, however can result in areas of bloom. This is particularly noticeable when creating animations.

Histogram equalisation on images with low bit-depth can also increase noticeable colour banding and, when applied to visible-spectrum images where the Earth is only partially-illuminated, can result in image-wide artifacts.

In general, images from Electro-L N2, EWS-G1 and GK-2A benefit from no histogram equalisation.