Modified Alphablending

What is alphablending?

Alphablending is used in situations where a semi-transparent image is overlaid onto another image. Assuming that the transparency of any given pixel has been defined, the equation for alphabending is simply: NewImage[i] = BaseImage[i] + Alpha[i]*TopImage[i] where i is the pixel index. This process can be repeated as many times as one wishes.

How is transparency determined?

Transparency can be defined in any way one wishes. In the case of the Wildfire ABBA imagery transparency is defined by the albedos and brightness temperatures associated with the radiance measured by the GOES satellites. Visible image pixels with an albedo of less than 6% are considered transparent, while albedos greater than 12% are considered opaque. The transparency between 6% and 12% is linearly scaled from 0% to 100%. For the infrared component (GOES Imager channel 4) brightness temperatures warmer than 280K are transparent and those colder than 250K are opaque.

What is modified alphablending?

The modified alphablending used for the Wildfire ABBA imagery involves doing alphablending twice, once with a GOES visible image and once with a GOES infrared image. The process is not as simple as just repeating a second blend, however. The visible data contains far more detail than the infrared data, and thus high, cold clouds tend to be washed out when both channels are blended together. A second parameter, called beta, is passed to the alphablending routine that reduces the contribution of the IR when the visible component is opaque. A beta value of 0.5 means, for instance, that the IR contribution is reduced by 50%. This allows the details of colder clouds to show through when visible information is available.

How does the modified alphablending handle night?

The same parameters are used for day and night. The infrared band does not change appreciably (as far as clouds are concerned) during that time frame. As for the visible data, at night one generally sees zero albedos, and thus no changes are made. Noise can and does affect the images, but that can happen at any time.

ABBA home | Wildfire ABBA home GOES-8 WFABBA: North America | GOES-8 WFABBA: South America | GOES-10 WFABBA: North America
Click on maps to zoom to regional sectors or use links below

GOES-8 regional sectors:
Western Canada | Northern Ontario/Northern Quebec | Northeastern US/Eastern Canada Western US | Midwest/Ohio Valley | Southwestern US/Northern Mexico Southeastern US | Northern Mexico | Southern Mexico Cuba/Bahamas | Central America | Puerto Rico/Barbados Northern Colombia/Western Venezuela | Eastern Venezuela/Northern Guiana Highlands Southern Colombia/Ecuador | Amazon Basin | Central Peru Southwestern Brazil | Northern Brazilian Highlands | Southern Brazilian Highlands South Central Brazil | Southern Brazil | Southwestern Bolivia Northern Argentina | Central Argentina | Southern Argentina
GOES-10 regional sectors:
Aleutian Islands | Yukon Territory/British Colombia | Alberta/Saskatchewan/Manitoba Pacific Northwest | Great Plains | California/Nevada Southwestern US/Northern Mexico | Central Mexico | Hawaii
The biomass burning products displayed on this web site were
developed and produced with support from the National Aeronautics
and Space Administration and the Space and Naval Warfare Systems
Center, San Diego, CA under contract No. N66001-00-C-0039.

Any opinions, findings and conclusions or recommendations expressed
in this material are those of the author(s) and do not necessarily
reflect the views of the Space and Naval Warfare Systems Center,
San Diego, CA.

Please cite the UW-Madison CIMSS GOES Biomass Burning Monitoring
Program when referencing or displaying any materials presented on this web site.
elaine.prins@ssec.wisc.edu
chris.schmidt@ssec.wisc.edu