Is it possible to revert preapplied alpha blend and find original color?

Question

I have a color 0xADADAD, it was originally white, now it's grey, because it's been alpha blended with 30%(0.3 alpha) black + the unknown original value. In this case I know from lots of trial and error that the original color was 0xF8F8F8

Problem is I have over 20 other colors that need to be fixed like this, but I have no accurate way of finding the original colors. Is there a formula I can do to figure this out? Is it even possible?

Answer 1

Yeah its possible i made a quick change to the code in this post and came up with following very sloppy python 2.X code:

from Tkinter import *
import struct


def ablend(a, fg, bg):
    return ((1-a)*fg[0]+a*bg[0],
            (1-a)*fg[1]+a*bg[1],
            (1-a)*fg[2]+a*bg[2])

def aunblend(a, result, bg):
    return ((result[0]-a*bg[0])/(1-a),
            (result[1]-a*bg[1])/(1-a),
            (result[2]-a*bg[2])/(1-a))


class findUnblendedColor(object):

    def __init__(self, root):
        self.fg=(0xAD, 0xAD, 0xAD)
        self.bg=(255,255,255)
        self.alpha = 0.3

        l = Label(root, text="alpha:")
        l.grid(row=0, column=0)
        label = '0.3'
        self.a = Text(root, height=1,
                       relief=FLAT,
                       width=20,)
        self.a.insert(1.0, label)
        self.a.grid(row=0, column=1)
        self.a.bind("<Return>", self.update)

        l = Label(root, text="Result:")
        l.grid(row=1, column=0)
        label = '#%02x%02x%02x' % self.fg
        self.e = Text(root, height=1,
                       relief=FLAT,
                       width=20,
                       background=label)
        self.e.insert(1.0, label)
        self.e.grid(row=1, column=1)
        self.e.bind("<Return>", self.update)

        l = Label(root, text="BG:")
        l.grid(row=2, column=0)

        label = '#%02x%02x%02x' % self.bg
        self.b = Text(root, height=1,
                       relief=FLAT,
                       width=20,
                       background=label)
        self.b.insert(1.0, label)
        self.b.grid(row=2, column=1)
        self.b.bind("<Return>", self.update)


        l = Label(root, text="FG:")
        l.grid(row=3, column=0)

        col = aunblend(self.alpha, self.fg, self.bg)
        label = '#%02x%02x%02x' % col
        self.f = Text(root, height=1,
                       relief=FLAT,
                       width=20,
                       background=label)
        self.f.insert(1.0, label)
        self.f.configure(state='disabled')
        self.f.grid(row=3, column=1)


        l = Label(root, text="verify:")
        l.grid(row=4, column=0)

        col = ablend(self.alpha, col, self.bg)
        label = '#%02x%02x%02x' % col
        self.g = Text(root, height=1,
                       relief=FLAT,
                       width=20,
                       background=label)
        self.g.insert(1.0, label)
        self.g.configure(state='disabled')
        self.g.grid(row=4, column=1)



    def update(self, event):
        data = self.e.get(1.0,END)
        self.e.delete(1.0, END)
        self.e.insert(END, data[:-1])

        data2 = self.b.get(1.0,END)
        self.b.delete(1.0, END)
        self.b.insert(END, data2[:-1])

        try:
            self.alpha = float(self.a.get(1.0,END)) 

            self.fg = struct.unpack('BBB',data[1:-1].decode('hex'))
            self.e.configure(background=data[:-1])

            self.bg = struct.unpack('BBB',data2[1:-1].decode('hex'))
            self.b.configure(background=data2[:-1])

            col = aunblend(self.alpha, self.fg, self.bg)
            self.f.configure(state='normal')
            self.g.configure(state='normal')

            if all(i <= 256 and i >= 0 for i in col):
                label = '#%02x%02x%02x' % col
                self.f.configure(background=label)

                col = ablend(self.alpha, col, self.bg)
                label2 = '#%02x%02x%02x' % col
                self.g.configure(background=label2)
            else:
                label = "out of range"
                label2 = "out of range"
            self.f.delete(1.0, END)
            self.f.insert(END, label)
            self.g.delete(1.0, END)
            self.g.insert(END, label2)

            self.f.configure(state='disabled')
            self.g.configure(state='disabled')
        except Exception as ex:
            print "error "+repr(type(ex))
            pass

        return 'break'

root = Tk()
root.title("unblend")
findUnblendedColor(root)

root.mainloop()

And you get a window that allows you to type alpha, resulting color and background.

Image 1: Dialog of program