[Gimp-developer] GIMP's layer math disagrees with mine

Hello GIMP developers -

I have experience with image processing, but I am fairly new to GIMP.  I was experimenting with combining 
layers that have transparency, and the pixel values that GIMP computes do not agree with the equation I've 
seen posted on GIMP and photoshop web sites.

Here's a specific example.  The math here applies to a single pixel, but for convenience I'll assume that I 
have images with the same pixel value everywhere.  Start with a 4-band image, where the RGB values are all 
128, and the alpha layer is all 255, or as a fraction 1.0.  Use that as your base layer.  Now put on top of 
that another layer that has RGB values of 0 and an alpha value of 191, or as a fraction, 191/255 = 0.749.  
When those layers get combined in GIMP, I thought it was done mathematically using the following equation:

output value = basePixel*(1 - topAlpha) + topPixel*topAlpha = 128*(1 - 0.749) + 0*0.749 = 32.

That's not what I get with GIMP.  I get a value of 66.  Apparently I'm using the wrong equation.  Does anyone 
know the answer?

For what it's worth, the blend mode is set to normal.

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