if (brightness < 0.0) 
    value = value * ( 1.0 + brightness);
else
    value = value + ((1.0 - value) * brightness);

 value = (value - 0.5) * (tan ((contrast + 1) * PI/4) ) + 0.5;

 where value is the input color value in the 0..1 range and brightness and contrast are in the -1..1 range;

From http://en.wikipedia.org/wiki/Image_editing#Contrast_change_and_brightening.

The expression comes from GIMP, free software raster graphics editor.

Less bright and less contrast
b = -0.3
c = -0.2     



Original


     
Brighter and more contrast
b = 0.2
c = 0.5

       

Note that the transformer only works with RGB interpretations such as RGB24 and RGB48. The RasterExpressionEvaluator cannot take expressions from an attribute. If it could (or will be able in the future), we could analyze a raster with RasterBandPropertiesExtractor, and construct an expression suitable for a particular raster interpretation.

This transformer contains one of the longest conditions I've ever written for the RasterExpressionEvaluator, check it out:
 

if(A:_brightness<0.0, (((1/(A:_num_of_colors)*A[0]*(1.0 + A:_brightness)) - 0.5) * (tan((A:_contrast + 1) * 3.14159/4.0))+0.5)*A:_num_of_colors, ((((1/A:_num_of_colors*A[0] + ((1.0- 1/A:_num_of_colors*A[0])*A:_brightness))) - 0.5) * (tan((A:_contrast + 1) * 3.14159/4.0))+0.5)*A:_num_of_colors);if(A:_brightness<0.0, (((1/A:_num_of_colors*A[1]*(1.0 + A:_brightness)) - 0.5) * (tan((A:_contrast + 1) * 3.14159/4.0))+0.5)*A:_num_of_colors, ((((1/A:_num_of_colors*A[1] + ((1.0- 1/A:_num_of_colors*A[1])*A:_brightness))) - 0.5) * (tan((A:_contrast + 1) * 3.14159/4.0))+0.5)*A:_num_of_colors);if(A:_brightness<0.0, (((1/A:_num_of_colors*A[2]*(1.0 + A:_brightness)) - 0.5) * (tan((A:_contrast + 1) * 3.14159/4.0))+0.5)*A:_num_of_colors, ((((1/A:_num_of_colors*A[2] + ((1.0- 1/A:_num_of_colors*A[2])*A:_brightness))) - 0.5) * (tan((A:_contrast + 1) * 3.14159/4.0))+0.5)*A:_num_of_colors)