Introduction to RGB image composites Hans Peter Roesli

Introduction to RGB image composites Hans. Peter Roesli Meteo. Swiss Locarno Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 1

Basics of displaying MSG/SEVIRI images Four processing and rendering methods: 1. Images of individual channels, using a simple grey wedge or LUTs for pseudo colours (typical for MFG channels); 2. Differences/ratios of 2 channels, using a simple grey wedge or LUTs for pseudo colours (e. g. fog, ice/snow or vegetation); 3. Quantitative image products using multi-spectral algorithms (e. g. SAFNWC/MSG software package) and discrete LUTs; 4. RGB composites by attributing 2 to 3 channels or channel combinations to individual colour (RGB) beams classification by addition of. RGB colour intensities Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 2

Simple display of individual SEVIRI channels 4 solar (on black), 1 solar + IR (on cream), 6 IR (on whitish) Ø Adequate for viewing information of 3 MFG channels; Ø Not very practical for 12 MSG/SEVIRI channels. Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 3

Rendering of individual SEVIRI channels Proper choice of grey wedge Solar channels rendered similar to black & white photography (channel 03 with particular response from ice/snow) physical rendering using lighter shades for higher reflectivity and darker shades for lower reflectivity. Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 4

Rendering of individual SEVIRI channels Proper choice of grey wedge high solar: reflectivity (P mode only) clouds land / sea low Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 5

Rendering of individual SEVIRI channels Proper choice of grey wedge IR channels rendered either in P or S mode: Ø P mode: grey shades follow intensity of IR emission: physical rendering with lighter shades for stronger IR emission and darker shades for weaker IR emission; Ø S mode: P mode inverted: traditional “solar-like” rendering, allowing for easy comparison to images from solar channels. Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 6

Rendering of individual SEVIRI channels Proper choice of grey wedge IR: emission / brightness temperature P mode weak / cold clouds / more absorption strong / warm Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 7 land / sea / less absorption

Rendering of individual SEVIRI channels Proper choice of grey wedge IR: emission / brightness temperature S mode weak / cold clouds / more absorption strong / warm Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 8 land / sea / less absorption

Differences/ratios of 2 channels Ø Simply displaying a larger set of single channels for comparison is neither efficient in mining useful information nor particularly focussed on phenomena of interest; Ø Displaying specific channel differences or ratios, a simple operation though, improves the situation awareness by enhancing particular phenomenon of interest (e. g. fog or ice clouds) in a particular situation; Ø Grey-scale rendering (small values in dark or light shades – large values in light or dark shades) is not standardised; mode may be inherited from similar products based on data of other imagers (e. g. AVHRR or MODIS). Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 9

Differences of 2 channels – examples 04 – 09 fog night - dark day - bright 03 – 01 ice clouds day (only)- dark Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 10

Some recommended differences Ø Clouds § 03 -01 § 04 -09 § 05 -06 § 05 -09 § 06 -09 Ø Thin cirrus § 07 -09 § 04 -09 § 10 -09 Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 11 Ø Fog § 04 -09 § 07 -09 Ø Snow § 03 -01 Ø Volcanic ash (SO 2) § 06 -11 Ø Dust § 04 -09 § 07 -09 § 10 -09 Ø Vegetation § 02 -01 Ø Fire § 04 -09 Ø Smoke § 03 -01

Quantitative image products using multi-spectral algorithms Ø Quantitative algorithms (thresholding or pattern recognition techniques) extract specific features from multi-spectral images and code them into a single-channel image quantitative image products; Ø Using discrete LUTs quantitative images are easy to read due to relation between identified features and colour values, but may have some drawbacks: § Feature boundaries appear very artificial (e. g. checker board due to use of ancillary data of different spatial scale); § Extracted features show unclassified or misclassified fringes; § Natural texture of features is lost (“flat” appearance); § Depending on robustness of feature extraction, time evolution of images is not necessarily very stable animated sequences somewhat confusing (e. g. erratically jumping classification boundaries). Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 12

Quantitative image products using multi-spectral algorithms – an example green fringe around blue feature checkerboard boundary SAFNWC/MSG PGE 03 Cloud Top Temperature/Height (CTTH) Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 13

RGB image composites – additive colour scheme Attribution of images of 2 or 3 channels (or channel differences/ratios) to the individual colour (RGB) beams of the display device; Ø RGB display devices produce colours by adding the intensities of their colour beams optical feature extraction through result of colour addition. FAST BUT QUITE EFFICIENT SURROGATE FOR QUANTITATIVE FEATURE EXTRACTION Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 14

G RGB image composites – additive colour scheme n ee gr am be R red beam B bl ue be am Click Color Selector. exe Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 15 • Tool reveals individual colour intensities adding to the colours shown in the circle; • Close tool after use (also when calling it later on again).

RGB image composites – some RGB colours/values Examples of colours (names) and 8 -bit (octal and decimal) values loaded to the RGB beams: § Red 255, 0, 0 § Fuchsia 255, 0, 255 § Skyblue 153, 206, 235 Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 16

RGB image composites – pros and cons Ø Drawback: § Much more subtle colour scheme compared to discrete LUTs used for quantitative image products interpretation more difficult; Ø Advantages: § Processes “on the fly”; § Preserves “natural look” of images by retaining original textures (in particular for clouds); § Preserves spatial and temporal continuity allowing for smooth animation RGB image sequences. Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 17

RGB image composites – inside Channel 03 + Channel 02 + Channel 01 Color Selector. exe Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 18

RGB image composites – inside Optimum (and stable) colouring of RGB image composites depends on some manipulations: Ø Proper enhancement of individual colour channels requires: § Some stretching of the intensity ranges; § Selection of either P or S mode for IR channels; Ø Attribution of images to individual colour beams depends on: § Reproduction of RGB schemes inherited from other imagers; § Permutation among colour beams and individual images more or less pleasant / high-contrast appearance of RGB image composite. Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 19

RGB image composites – 3 examples out of many Ø Reveals fog and cirrus/snow Ø Channel attribution R 03 G 02 B 01 Ø Reveals some cloud properties Ø Channel attribution: R 01 G 04 B 09 Ø For 04 and 09 beams P mode is used! Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 20 Ø Reveals atmospheric and surface features Ø Channel attribution R 06 -05 G 04 -09 B 03 -01 Color Selector. exe

RGB image composites – using HRV (channel 12) Ø In order to preserve high resolution of HRV channel assign it to 2 colour beams (using only one colour beam blurs the image too much); Ø Attributing it to beams R and G is preferred rendering close to natural colours for surface features; Ø Beam B is then free for any other SEVIRI channel properly downscaled (factor of 3) to HRV. Assigning an IR window channel in P mode to beam B (as a temperature profile surrogate) adds height information to a detailed cloud view Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 21

RGB image composites – using HRV (channel 12) Ø Reveals fine details of snow cover, fog patches and higher clouds Ø R 12 G 12 B 09 (09 in P mode!) Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 22

Recommended schemes for RGB image composites Ø Convection § 01, 03, 09 01, 03, 10 § 01, 04, 09 01, 04, 10 § 03, 04, 09 03, 04, 10 Ø HRV (channel) § 12, 04 § 12, 09 Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 23 Ø Dust § 01, 03, 04 § 03, 02, 01 Ø Vegetation § 03, 02, 01 Ø Fire/Smoke § 03, 02, 01 § 04, 02, 01 Ø Channel differences § 06 -05, 04 -09, 03 -01

Summary of RGB image composites Ø Fast technique for feature enhancement exploiting additive colour scheme of RGB displays; Ø May require simple manipulation to obtain optimum colouring (choice of P or S mode for IR channels!); Ø More complex RGB schemes may require some time to get acquainted with; Ø Some RGB schemes may be inherited from other imagers (e. g. AVHRR or MODIS); Ø Combination of an IR channel with HRV feasible and much informative; Ø RGB image composites retain natural texture of single channel images; Ø RGB image composites remain coherent in time and space, i. e. ideal for animation of image sequences. Hans. Peter Roesli, Intro. RGB 2002 -12 -24 / 24