confused with elementary maths in compositing

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Old 01 January 2009   #1
confused with elementary maths in compositing

Hi dear friends.
I'm trying to learn some compositing basics, and there's a multiplication operation. To me this means: rgb multiplied is let's say 2.2.2 x 2 = 4.4.4. But why the image becomes darker? If I multiply the image by alpha, it becomes black where the alpha is black and remains the same where alpha is white. Why is that? Isn't 4 is brighter value than 2?
And will addition, subtraction and division do the same weird stuff?
Addition 2.2.2 + 2.2.2 = 4.4.4 brighter
subtraction 2.2.2 - 1.1.1 = 1.1.1 darker
division 4.4.4 / 2 = 2.2.2 darker Right?
 
Old 01 January 2009   #2
The operation multiply multiplies every pixel in image A with the corresponding pixel in image B.

So if you have a color photo (image A) and you multiply it with an alpha image (image B), then wherever there is a pixel value of 1 in your alpha (white) it will return the value that was in image A. Because 1 * any RGB value = any RGB Value.

On the other hand, if you multiply anything by 0, then you get 0 (black). That is why in your new multiplied image you get black where it is black in the alpha.
 
Old 01 January 2009   #3
Thank you Theta.waves, I think I got it: it multiplies not 0-255 of an alpha channel value, but a normalised 0-1, so the multiplied value never exceeds 1 (the initial A image values) and only can be darkened by a smaller normalised alpha value.
 
Old 01 January 2009   #4
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