hanspeter roesli, introrgb 2002-12-24 / 1 introduction to rgb image composites hanspeter roesli...

HansPeter Roesli, IntroRGB 2002-12-24 / 1

Introduction to RGB image composites

HansPeter RoesliMeteoSwiss Locarno


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 ofRGB colour intensities


Simple display of individual SEVIRI channels

4 solar (on black), 1 solar + IR (on cream), 6 IR (on whitish)

Adequate for viewing information of 3 MFG channels;

Not very practical for 12 MSG/SEVIRI channels.


Rendering of individual SEVIRI channels

Proper choice of grey wedgeSolar 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.



Proper choice of grey wedgesolar: reflectivity

(P mode only)high

low

clouds

land / sea



Proper choice of grey wedgeIR channels rendered either in P or S

mode: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;

S mode: P mode inverted: traditional “solar-like” rendering, allowing for easy comparison to images from solar channels.



Proper choice of grey wedgeIR: emission / brightness temperature

P mode

strong / warm

weak / cold

clouds / more absorption

land / sea / less absorption



Proper choice of grey wedgeIR: emission / brightness temperature

S mode

strong / warm

weak / cold

clouds / more absorption

land / sea / less absorption


Differences/ratios of 2 channels

Simply displaying a larger set of single channels for comparison is neither efficient in mining useful information nor particularly focussed on phenomena of interest;

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;

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).


Differences of 2 channels – examples

night - dark day - bright

04 – 09fog

03 – 01ice clouds

day (only)- dark


Some recommended differences

Clouds 03-01 04-09 05-06 05-09 06-09

Thin cirrus 07-09 04-09 10-09

Fog 04-09 07-09

Snow 03-01

Volcanic ash (SO2) 06-11

Dust 04-09 07-09 10-09

Vegetation 02-01

Fire 04-09

Smoke 03-01


Quantitative image products using multi-spectral

algorithms 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;

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).


Quantitative image products using multi-spectral

algorithms – an example

SAFNWC/MSG PGE03Cloud Top Temperature/Height (CTTH)

checkerboardboundary

green fringe around blue

feature


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;

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


RGB image composites – additive colour scheme

R red beam

B bl

ue b

eam

G green beam

Click Color Selector.exe

• 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


RGB image composites – pros and cons

Drawback: Much more subtle colour scheme compared

to discrete LUTs used for quantitative image products interpretation more difficult;

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.


RGB image composites – inside

+

+

Channel 03

Channel 02

Channel 01

Color Selector.exe


RGB image composites – inside

Optimum (and stable) colouring of RGB image composites depends on some manipulations:

Proper enhancement of individual colour channels requires: Some stretching of the intensity ranges; Selection of either P or S mode for IR channels;

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.


RGB image composites – 3 examples out of many

Reveals some cloud properties Channel attribution:

R 01 G 04 B 09 For 04 and 09 beams P mode is used!

Reveals fog and cirrus/snowChannel attribution

R 03 G 02 B 01

Reveals atmospheric and surface features

Channel attributionR 06-05 G 04-09 B 03-01

Color Selector.exe


RGB image composites – using HRV (channel 12)

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);

Attributing it to beams R and G is preferred rendering close to natural colours for surface features;

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


RGB image composites – using HRV (channel 12)

Reveals fine details of snow cover, fog patches and higher clouds

R 12 G 12 B 09 (09 in P mode!)


Recommended schemes for RGB image composites

Convection 01,03,09

01,03,10 01,04,09

01,04,10 03,04,09

03,04,10

HRV (channel) 12,12,04 12,12,09

Dust 01,03,04 03,02,01

Vegetation 03,02,01

Fire/Smoke 03,02,01 04,02,01

Channel differences 06-05,04-09,03-01


Summary of RGB image composites

Fast technique for feature enhancement exploiting additive colour scheme of RGB displays;

May require simple manipulation to obtain optimum colouring (choice of P or S mode for IR channels!);

More complex RGB schemes may require some time to get acquainted with;

Some RGB schemes may be inherited from other imagers (e.g. AVHRR or MODIS);

Combination of an IR channel with HRV feasible and much informative;

RGB image composites retain natural texture of single channel images;

RGB image composites remain coherent in time and space, i.e. ideal for animation of image sequences.

hanspeter roesli, introrgb 2002-12-24 / 1 introduction to rgb image composites hanspeter roesli...

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