Natural color representation of Sentinel-2 data: additional resources

This repository contains additional resources for our paper:

     B. Sovdat, M. Kadunc, M. Batič, G. Milčinski, Natural color representation of Sentinel-2 data. Submitted.

Introduction

The true color image — defined as an image that strives to offer a natural color rendition of a scene — is one of the most widely used Earth observation products. In the context of satellite imagery, the true color should represent spectral responses of the satellite bands so as to match the color perceived by the human eye. However, most true color satellite products fail to achieve this.

When it comes to Sentinel-2 — an optical mission part of European Space Agency's Copernicus Program — the state-of-the-art true color products map Sentinel-2 MSI bands B4, B3, and B2 directly to sRGB components R, G, and B, respectively (sometimes with additional gain and gamma corrections in the sRGB space). This results in images with overly saturated and sometimes unrealistic colors.

Highlights of our contributions:

• We define the natural color product as the one matching the color perceived by the human eye.
• We propose two approaches for designing efficient algorithms for computing natural color.
• We implement and demonstrate several efficient algorithms for computing the natural color.
• We advocate correct sRGB encoding when preparing images for digital display.
• We urge that levels adjustment always be applied in the CIE LAB space.

The images below showcase the default true color (left) and the natural color (right) that we propose in our work.

True color	Natural color
Carrizo Plain National Monument	Carrizo Plain National Monument
Izmir, Turkey	Izmir, Turkey
Amposta, Spain	Amposta, Spain
Venice, Italy	Venice, Italy

List of locations

The table belows is a listing of Sentinel-2 data used throughout the paper. Locations, their acquisition dates and links to EO Browser where user can reproduce the results from the article using JavaScript algorithms provided in the supplementary material.

Location name	Level	Date	Coordinates	EO Browser
Mount Bromo (Indonesia)	1C	August 9, 2016	7.96° S 113.00° E	URL
Sand dunes (Saudi Arabia)	1C	September 7, 2017	19.57° N 48.63° E	URL
Venice (Italy)	1C	July 3, 2017	45.50° N 12.20° E	URL
Carrizo Plain (California, USA)	1C	March 28, 2017	35.05° N 120.00° W	URL
Amposta (Spain)	1C	August 4, 2017	40.67° N 0.58° E	URL
Izmir (Turkey)	1C	September 7, 2017	38.54° N 27.08° E	URL
Venice (Italy)	2A	July 3, 2017	45.50° N 12.20° E	URL
Amposta (Spain)	2A	August 4, 2017	40.67° N 0.58° E	URL

Interactive examples

You can try out the algorithms we've proposed via Sentinel Playground and EO Browser applications which run atop our interactive Sentinel-Hub platform.

All natural-color algorithms have been implemented as ready-to-use custom scripts (under ./scripts/) that you can copy-and-paste into Sentinel Playground and/or EO Browser. Needless to say, you can also fiddle with the code!

For convenience we've collected the algorithms into a table with links where you can view them interactively in Sentinel Playground and/or EO Browser.

List of procedures

The table below summarizes each of the procedures used in the paper.

Procedure	Shorthand description	Custom script
I	DN -> sRGB	true_color.js
II	DN -> S2 -> sRGB*_lin -> sRGB_8bit	aster_3x4_naive_gamma.js
III	DN -> S2 -> XYZ -> LAB* -> XYZ -> sRGB_lin -> sRGB_8bit	aster_3x4_proper_gamma.js
IV	DN -> S2 -> XYZ -> sRBG_lin -> sRGB_8bit	aster_3x4_no_gamma
V	DN -> S2 -> XYZ -> sRGB*_lin -> sRGB_8bit	aster_3x3_naive_gamma_I.js
VI	DN -> S2 -> XYZ -> sRGB*_lin -> sRGB_8bit	aster_3x3_naive_gamma.js
VII	DN -> S2 -> XYZ -> LAB* -> XYZ -> sRGB_lin -> sRGB_8bit	aster_3x3_proper_gamma.js
VIII	DN -> S2 -> XYZ -> sRGB_lin -> sRGB_8bit	aster_3x3_no_gamma.js
IX	DN -> S2 -> XYZ -> sRGB*_lin -> sRGB_8bit	basis_3x4_naive_gamma.js
X	DN -> S2 -> XYZ -> LAB* -> XYZ -> sRGB_lin -> sRGB_8bit	basis_3x4_proper_gamma.js
XI	DN -> S2 -> XYZ -> sRGB_lin -> sRGB_8bit	basis_3x4_no_gamma.js
XII	DN -> S2 -> XYZ -> sRGB*_lin -> sRGB_8bit	default_3x3_naive_gamma.js
XIII	DN -> S2 -> XYZ -> LAB* -> XYZ -> sRGB_lin -> sRGB_8bit	default_3x3_proper_gamma.js
XIV	DN -> S2 -> XYZ -> sRGB_lin -> sRGB_8bit	default_3x3_no_gamma.js

Data

We used the ASTER spectral library extensively for our experiments. The file used_aster_spectra.txt contains the list of ASTER spectra used for fitting the mappings with least-squares method (described in detail in Subsection 2.1 of the paper).