reference-http://www.td-grafik.de/ext/xfrog/alpha/index.html
main reason for it's appearance：for the problem ----the alpha value resulting the render color value wrong
1) 1.we have a texture - its' alpha is 0.5 so we just blend the origial color of what we want to show and the background color of the texture.
in this texture , let's set the original color(0,255,0)(green) and the background color (255,255,255)(white)
so we can calculate the result(127,255,127)-----just use the alpha mutiplyied it and add them
(0,255,0)*0.5+(255,255,255)*0.5 = (127,255,127)
the result is the format of the texture stored
2. for the render process
we set the targetresource is (0,0,0),so we can calculate the result
(originalColor.R,originalColor.G,originalColor.B)*0.5 + (TargetColor.R,TargetColor.G,TargetColor.B)
= (127,255,127)*0.5 + (0,0,0)*0.5 = (63,127,63)
we want the result from (0,255,0) to (0,0,0) just (0,127,0), but the result (63,127,63) has Red component and Blue component make the color wrong . we just want make the originalColor and the TargetColor more Efficentive , not be confused by the BackgroundColor
so how can we get rid of it ?
2) pre-Muiltiplying
first of all we must set the texture background color (0,0,0)(black),so we can avoid the influence of the texture background color
so we can conclude the express
(0,255,0)*0.5+(0,0,0)*0.5 = (0,127,0)
then rendering
we can set the Target color (255,255,255)(white)
(0,127,0)+（255,255,255）*0.5 = （127,255,127）
the result is more like the (0,255,0) and feelig so bright
this is the design of the preMutipying