Can we find the transparency of an overlay on an image ?

If I have an image with a transparent overlay on it, can we find the percentage of transparency from the alpha channel of the image?

[Image Analyst]

Since you put the image into the alpha channel, why don't you already know the transparency.

[Adam]

Keep the handles to the things you plot instead of just ignoring the output argument of these functions and all these type of things are trivial.

[Mr.X]

What I have is an image with a line(say red) with a previously set transparency and i need to get the value of transparency of the line from the image. (NB: The line with the transparency is inserted using a separate image processing tool).

[Walter Roberson]

Is the transparent line in a separate color plane? Or do you mean that you had an image and you "etched in" a partly transparent red line that would, by itself, be a single color, but because it was drawn over top of other things, the color over the length of the line varies, and you wish to estimate the transparency? Could you post a sample image ?

[Mr.X]

Yes I have a partly transparent red line etched on the image and I need to find the transparency of the line alone. Not the image transparency

[Walter Roberson]

Could you post a sample image?

Will the line always be straight? Is the line in a known position or does it need to be detected?

Answer 1

[Walter Roberson]

Yes.

if isinteger(YourAlphaData)
  tAD = im2double(YourAlphaData);
else
  tAD = YourAlphaData;
end
percent_transparent = 100 * (1 - tAD);