How is calculated the resulting color's alpha?

[guix]

Hello all, sorry I am new and need some help, it's not really related to Paint.NET usage, but how it calculate the resulting color:

 

For example I make red background, call it C1:

 

RGBA = 255, 0, 0, 255

 

Then I place a yellow pixel, call it C2:

 

RGBA: 220, 216, 0, 200

 

Then I use the Color Picker tool on this pixel, I get C3:

 

RGBA: 227, 169, 0, 255

 

 

I already know how to calculate R, G, and B of C3

 

R = ( (C2.A / 255) * C2.R ) + ( (1 - (C2.A / 255)) * C1.R ) = 227

G = ( (C2.A / 255) * C2.G ) + ( (1 - (C2.A / 255)) * C1.G ) = 169

B = ( (C2.A / 255) * C2.B ) + ( (1 - (C2.A / 255)) * C1.B ) = 0

 

 

But for the Alpha channel, it is different and I can't find how to calculate it!

 

Thanks for any help

 

[BoltBait]

If you place a simi-transparent pixel on top of a fully opaque pixel, the result will be a fully opaque pixel.

[guix]

Of course, that was a bad example then... Ok another example.

 

C1 alpha is 128, C2 alpha is also 128, the resulting alpha is 192.

 

Another

 

C1 alpha is 250, C2 alpha is 200, the resulting alpha is 254.

 

How to calculate this?

 

 

Edit: is this correct:

 

floor( 255 - ( ((255 / C1.A) * C2.A) + ((1 - (C2.A / 255)) * C1.A) ) )

 

But now I realized the other calcualtions are wrong...

 

Edit again: solved.

Edited January 23, 2013 by guix

[BoltBait]

Of course, that was a bad example then... Ok another example.

 

C1 alpha is 128, C2 alpha is also 128, the resulting alpha is 192.

Alpha 128 is half opaque.

So, half opaque plus another half (of what's left) is three quarters opaque. 192 is three quarters of 255.

Clear?

[guix]

No but problem solved, the whole thing was wrong because I used values between 0 and 255 when I should have used values between 0 and 1.

 

Thanks anyway

Edited January 23, 2013 by guix