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The current 2.8 and 2.9 documentations state that “<GammaValue> is a scaling factor that adjusts the brightness of the image. A value less than one (1) darkens the image, and a value greater than one (1) brightens it. The default is 1 (no change).” but I observe an inverted behaviour using this custom raster style on common imagery rgb-rasters:
<?xml version="1.0" encoding="ISO-8859-1"?>
<StyledLayerDescriptor version="1.0.0"
xsi:schemaLocation="http://www.opengis.net/sld http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd"
xmlns="http://www.opengis.net/sld"
xmlns:ogc="http://www.opengis.net/ogc"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<NamedLayer>
<Name>raster_contrast</Name>
<UserStyle>
<Title>Raster Contrast Enhancement</Title>
<Abstract>Raster style with custom contrast adjustment</Abstract>
<FeatureTypeStyle>
<Rule>
<Name>rule1</Name>
<Title>Opaque Raster</Title>
<Abstract>A raster with 100% opacity and custom contrast</Abstract>
<RasterSymbolizer>
<Opacity>1.0</Opacity>
<ChannelSelection>
<RedChannel>
<SourceChannelName>1</SourceChannelName>
<ContrastEnhancement>
<Normalize>
<VendorOption name="algorithm">StretchToMinimumMaximum</VendorOption>
<VendorOption name="minValue">33</VendorOption>
<VendorOption name="maxValue">227</VendorOption>
</Normalize>
<GammaValue>0.6</GammaValue>
</ContrastEnhancement>
</RedChannel>
<GreenChannel>
<SourceChannelName>2</SourceChannelName>
<ContrastEnhancement>
<Normalize>
<VendorOption name="algorithm">StretchToMinimumMaximum</VendorOption>
<VendorOption name="minValue">44</VendorOption>
<VendorOption name="maxValue">225</VendorOption>
</Normalize>
<GammaValue>0.6</GammaValue>
</ContrastEnhancement>
</GreenChannel>
<BlueChannel>
<SourceChannelName>3</SourceChannelName>
<ContrastEnhancement>
<Normalize>
<VendorOption name="algorithm">StretchToMinimumMaximum</VendorOption>
<VendorOption name="minValue">46</VendorOption>
<VendorOption name="maxValue">227</VendorOption>
</Normalize>
<GammaValue>0.6</GammaValue>
</ContrastEnhancement>
</BlueChannel>
</ChannelSelection>
</RasterSymbolizer>
</Rule>
</FeatureTypeStyle>
</UserStyle>
</NamedLayer>
</StyledLayerDescriptor>
To brighten the image I have to use 0.6 which is less than 1.
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