confused with elementary maths in compositing

 01 January 2009 mister3d Expert portfolio Oleksandr Aleksandrov Kiev, Ukraine 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? share quote
 01 January 2009 th3ta Veteran   portfolio 3d Artist In a dark office, USA 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. share quote
 01 January 2009 mister3d Expert portfolio Oleksandr Aleksandrov Kiev, Ukraine 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. share quote
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