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[MrAksel]

I'm making this image editing library, and I thought of making an "anti grayscale" filter. The problem is, I just can't find the right formula to get the red, green and blue values from the gray value. Lets say I have the color R=128, G=191, B = 191. To get the gray value, I use the following function:

Code: Select all

byte grayScale = (byte)((b * 0.114) +
                        (g * 0.587) +
                        (r * 0.299));

Since this is basic math, there should be a somewhat easy way to reverse it. Though my math grades are good , I have failed in every attempt to calculate the red, green and blue components from the value that is returned by the grayscale function, in this case 172.

[Cheatmasterbw]

You can't do it easily...
Basically grayscale is averaging the RGB values of each pixel.

say the original RGB values of the pixel were 126, 127, 128. This in grayscale is 127, 127, 127. There are also many other pixel colors that can average to 127, for example, 27, 127, 227. These two colors are completely different, even though they average to the same grayscale color.

The method you are using is similar to averaging the values together, so this applies o what you are doing.

So basically, this can't be (easily) done.

[MrAksel]

My method isn't averaging. Average would be (R + G + B) / 3. I have (R * 0.299 + G * 0.587 + B * 0.114) which makes the result the way the human eye would see it, so I thought I could use those constants to convert the grayscale value back to its original colors.

[Cheatmasterbw]

My method isn't averaging. Average would be (R + G + B) / 3. I have (R * 0.299 + G * 0.587 + B * 0.114) which makes the result the way the human eye would see it, so I thought I could use those constants to convert the grayscale value back to its original colors.

I know you are using a different grayscaling method, but the concept is the same.

[comathi]

Well, seeing as you are multiplying it by those values to get the grayscale, wouldn't it be logical that you divide your grayscale by those same values? It would cancel the math operation, and theoretically restore the color to it's original tone.

[Cheatmasterbw]

Well, seeing as you are multiplying it by those values to get the grayscale, wouldn't it be logical that you divide your grayscale by those same values? It would cancel the math operation, and theoretically restore the color to it's original tone.

The problem with that is that you don't know what to divide by.

[Codex]

Take a look at this: http://stackoverflow.com/questions/8357 ... esentation

[MrAksel]

Im trying to restore the original colors Codex, not making a Color from a grayscale value (Color.FromArgb(gray, gray, gray)).
The problem is where I add the three values together to make the gray. That is ruining my possibilities to restore the colors. You are right Cheatmasterbw :(

[Cheatmasterbw]

If you have 2 of the 3 original RGB values, you can find out the third. You can probably make it so the user chooses 2 of the original values of each pixel, and then paint over part of the image. The user could adjust the 2 values until the part of the image looks right.

Edit:
like this
http://planetphotoshop.com/colorizing-a ... image.html

[MrAksel]

That looks pretty good, thanks. Ill probably implement a "key color" that will fill the grayscale image as in the PS tutorial