Menu
The function R*0.2126+ G*0.7152+ B*0.0722 is said to calculate the perceived brightness (or equivalent grayscale color) for a given an RGB color. Assuming we use the interval [0,1] for all RGB values, we can calculate the following: yellow = RGB(1,1,0) => brightness=0.9278.
The typical approach to averaging RGB colors is to add up all the red, green, and blue values, and divide each by the number of pixels to get the components of the final color.
You can calculate subjective brightness of an RGB value by weighting the three channels according to their perceived brightness, e.g. with something like: (0.21 × R) + (0.72 × G) + (0.07 × B) So, for instance, that makes yellow ( #ffff00 ) twice as intense as red ( #ff0000 ).
The standard blending equation is:
out = alpha * new + (1 - alpha) * old
Where out, new and old are RGB colors, and alpha is a floating point number in the range [0,1].
So, you have (for red):
240 = 0.1 * newR + 0.9 * 255
Solving for newR, we get:
newR = (240 - 0.9 * 255) / 0.1
which evaluates to 105. Repeat for the other components, and you're done.
I'm just wondering how come you know its alpha and not its r,g,b... Unless the blending mode used is in every case Normal, you won't be able to calculate the original value. And even if all is ideal, you will only be able to approximate, as some rounding has probably taken place already when going from original to result. Also, you will most likely have to round when going back from result to original.
If the blending mode is Normal, then it's not too difficult :-)
opacity*original + (1-opacity)*background = resulting pixel
original R = (resulting R - ((1-opacity)*background R)) / opacity.
original G = (resulting G - ((1-opacity)*background G)) / opacity.
original B = (resulting B - ((1-opacity)*background B)) / opacity.
in which opacity is (alpha/100).
The following pseudo-code resolves your problem:
CalculateSolidColorFromTransparentColor(Color color, Color background)
{
alpha = color.A / 255;
oneminusalpha = 1 - alpha;
newR = ((color.R * alpha) + (oneminusalpha * background.R));
newG = ((color.G * alpha) + (oneminusalpha * background.G));
newB = ((color.B * alpha) + (oneminusalpha * background.B));
}
Note that input color and backgroud has values expressed in [0..255]
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With