Transcript RGB part 03 - Channel selection and enhancements

APPLICATIONS OF
METEOSAT SECOND
GENERATION (MSG)
RGB IMAGES: PART 03
CHANNEL SELECTION AND
ENHANCEMENTS
Author:
Jochen Kerkmann (EUMETSAT)
[email protected]
Contributors: D. Rosenfeld (HUJ), HP. Roesli (MeteoSwiss)
M. König (EUM)
Version 0.6, 30 June 2004
PART 3:
CHANNEL SELECTION
AND ENHANCEMENTS
Version 0.6, 30 June 2004
Channel Selection & Enhancements
Optimum colouring of RGB image composites depends on:
•
•
•
•
I. Selection of the channels
– Physical properties represented by the channels (e.g. IR10.8 provides cloud top
temperature, VIS0.6 provides cloud optical thickness, VIS0.8 provides "greeness" of
vegetation etc…)
II. Attribution of images to individual colour beams depends on:
a Reproduction of RGB schemes inherited from other imagers;
b Contrast and colours of the resulting RGB composite, which can be more or less
pleasant (depending on personal view);
III. Proper enhancement of individual colour channels requires:
a Conversion from radiances to brightness temperatures/reflectances;
b Selection of display mode (inverted or not inverted);
c Stretching of the intensity ranges (linear stretching of active dynamic range);
d Gamma corection;
e Gamma2 correction;
IV. Final enhancement
– e.g. Gamma correction of final RGB image,
Version 0.6, 30 June 2004
I. (Cloud) Physical Properties represented
by the MSG Channels
VIS0.6:
VIS0.8:
optical thickness and amount of cloud water and ice
optical thickness and amount of cloud water and ice
"greeness" of vegetation
NIR1.6, IR3.9r:
particle size and phase
WV6.2, WV7.3:
mid- and upper level moisture
IR8.7, IR10.8, IR12.0: top temperature
IR8.7 - IR10.8:
IR12.0 - IR10.8:
IR3.9 - IR10.8:
IR13.4 - IR10.8:
WV6.2 - IR10.8:
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phase and optical thickness
optical thickness
optical thickness, phase, particle size
top height
top height, overshooting tops
II.a RGB Schemes inhereted from
NOAA AVHRR
The most popular RGB schemes used for NOAA AVHRR images are:
RGB 1-2-4 (Day)
VIS0.6-VIS0.8-IR10.8
RGB 1-3A-4 (Day)
VIS0.6-NIR1.6-IR10.8
RGB 3B-4-5 (Day/Night)
IR3.7-IR10.8-IR12.0
O. Hyvärinen, FMI, 2003
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II.a RGB Schemes inhereted from MODIS
For MODIS (27 channels, not considering channels 8-16) there are
17550 different ways (this is 27!/(27-3)!) to choose 3 channels from
27 channels. Some popular RGB schemes used for MODIS images
are:
01-04-03
01-02-03
01-06-31
26-06-31
(VIS0.6-VIS0.5-VIS0.4)
(VIS0.6-VIS0.8-VIS0.4)
(VIS0.6-NIR1.6-IR11.0)
(NIR1.3-NIR1.6-IR11.0)
MODIS
5 March 2000
?? UTC
RGB Composite
01-04-03
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II.b Personal Choice/View
RGB 01-02-03
gives bluish surface colours
RGB 03-02-01
gives more "natural colours"
MSG-1, 16 March 2004, 16:00 UTC
Version 0.6, 30 June 2004
II.b Personal Choice/View
RGB 04r-02-09
gives green Cb clouds
RGB 02-04r-09
gives better "warning colours"
MSG-1, 5 May 2003, 13:30 UTC
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III.a Conversion from Radiances to
Brightness Temperatures / Reflectances
The best RGBs are achieved using
brightness temperatures for IR
and reflectances for VIS channels !!!
•
•
•
•
Counts
Radiances
Brightness temp. (IR channels)
Reflectances (VIS channels)
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



III.a Conversion from Rad to BT / Refl.
Example: MSG-1, 5 June 2003, 14:45 UTC, Channel 09 (IR10.8)
Radiance
Range = 14 / 115 mW/m2/sr/cm-1, =1.0
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Brightness Temperature
Range = 200 / 305 K, =1.0
better contrast !
III.a Conversion from Rad to BT / Refl.
Example: MSG-1, 5 June 2003, 14:45 UTC, Channel 02 (VIS0.8)
Radiance
Range = 0 / 17 mW/m2/sr/cm-1, =1.0
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Reflectance
Range = 0 / 100 %, =1.0
better contrast, no sun correction needed !
III.a Conversion from Rad to BT / Refl.
Example: MSG-1, 5 June 2003, 14:45 UTC, RGB 03-02-01
based on Radiances
Version 0.6, 30 June 2004
based on Reflectances
better contrast, no sun correction needed !
no Gamma correction needed !
III.a Conversion from Rad to BT / Refl.
The relation between the SEVIRI radiances and
the equivalent brightness temperatures /
reflectances is described in a separate Powerpoint
file (see conversion.ppt) !
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III.b Selection of Display Mode
(Inverted or not Inverted)
There are no general rules as regards the display mode (in
particular for the IR channels). As described under II.a and
II.b, it is a matter of traditions and personal view to select
the display mode. Traditionally, for AVHRR RGB
composites the IR channels are inverted.
In this MSG Interpretation Guide, if not stated differently, all
RGB composites are created from non-inverted (black = low
energy, white = high energy) images.
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III.c Stretching of Intensity Ranges
The range of interest for the MSG SEVIRI channels
varies, depending on:
• the phenomenon of interest (high clouds, low clouds,
surface features, dust, smoke … )
• the season (winter, summer)
• the time of the day (day / night / twilight)
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III.c Stretching of Intensity Ranges
- Examples OverviewFeature
Channel
Total Range
Range of Interest
Cb Clouds Europe
Cb Clouds Africa
IR10.8
IR10.8
180 K / 340 K
180 K / 340 K
203 K / 233 K
180 K / 220 K
Dust Storm
Dust Storm
IR12.0-IR10.8
IR8.7-IR10.8
-15 K / +5 K
-15 K / +15 K
-4 K / +2 K
-15 K / 0 K
Fog Night
IR3.9-IR10.8
-15 K / +25 K
-10 K / 0 K
Thin Cirrus
IR8.7-IR10.8
-15 K / 15 K
0 K / +7 K
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III.c Stretching of Intensity Ranges
- Example Cb Clouds Africa -
Range = 180 K / 340 K, =1.0
Range = 180 K / 233 K, =1.0
MSG-1, 20 May 2003, 14:00 UTC, Channel 09 (IR10.8)
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III.c Stretching of Intensity Ranges
- Example Cb Clouds Europe -
Range = 180 K / 340 K, =1.0
Range = 203 K / 233 K, =1.0
MSG-1, 5 June 2003, 14:45 UTC, Channel 09 (IR10.8)
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III.c Stretching of Intensity Ranges
- Example Dust Storm -
Range = -15 K / +5 K, =1.0
Range = -4 K / +2 K, =1.0
MSG-1, 3 March 2004, 12:00 UTC, Diff. IR12.0 - IR10.8
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III.c Stretching of Intensity Ranges
- Example Dust Storm -
Range = -15 K / +15 K, =1.0
Range = -15 K / 0 K, =1.0
MSG-1, 3 March 2004, 12:00 UTC, Diff. IR8.7 - IR10.8
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III.c Stretching of Intensity Ranges
- Example Fog at Night -
Range = -15 K / +25 K, =1.0
Range = -10 K / 0 K, =1.0
MSG-1, 9 November 2003, 03:15 UTC, Diff. IR3.9 - IR10.8
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III.c Stretching of Intensity Ranges
- Example Thin Cirrus -
Range = -15 K / +15 K, =1.0
Range = 0 K / +7 K, =1.0
MSG-1, 25 June 2003, 10:00 UTC, Diff. IR8.7 - IR10.8
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III.d Gamma Correction
In a nutshell, gamma correction changes the overall brightness (and color saturation) of
an image as it is displayed on a monitor. The formula to perform a Gamma correction on
a MSG IR (brightness temperature) image, within a range of BTmin and BTmax (see
stretching of intensity ranges) is:
1
 BT  BTmin  
BRIT  255 

 BTmax  BTmin 
where BRIT is the brightness intensity (0-255) of the displayed image. For Gamma <
1.0, the image is darkened, with the biggest effect happening for the dark (low input)
pixel values. If Gamma > 1.0, the image is brightened overall, with the largest changes
happening again for the dark shadows. The formula for the Gamma correction on a
MSG VIS (reflectance) image is analog.
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III.d Gamma Correction
Mapping function for different Gamma corrections
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III.d Gamma Correction: Examples
Reflectance
0
BRIT (=1.0) 0
BRIT (=2.0) 0
BRIT (=0.5) 0
20
40
60
80
100
51 102 153 204 255
114 161 197 228 255
10 41 92 163 255
Examples of mapping functions for different Gamma corrections
for displaying the reflectance (0 - 100%) for the MSG visible
channels.
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III.d Gamma Correction: Examples
Range = 0 - 100%, =0.5
Range = 0 - 100%, =1.0
Range = 0 - 100%, =2.0
MSG-1, 3 March 2004, 12:00 UTC, Channel 01 (VIS0.6)
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III.d Gamma Correction: Examples
Click on the icon to
see the animation
(AVI, 2805 KB) !
Left:
Right:
Channel 01 (VIS0.6), various Gamma corrections (=1.0 … =5.0)
resulting RGB Composite 03,02,01
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III.e Gamma2 Correction
The Gamma2 correction helps to enhance the middle
part of the selected intensity range. This can be done
e.g. with a tanh function.
To be written
Version 0.6, 30 June 2004
IV. Final Enhancement
To be written
Version 0.6, 30 June 2004