How to differentiate between a pixel with no color and a black pixel?

Question

I am using ChunkyPNG in order to get the colors of pixels on images. When an image has no background color at all, and is rendered into a browser, the background color will become white (usually, depending on your browser configuration).

My problem is that, when analyzing the pixels with ChunkyPNG, the values of a pixel that has no color at all is rgb(0,0,0), and the value of a pixel that is black is rgb(0,0,0). Is there any way I can differentiate between those? Checking opacity or... What am I missing?

Here is my code:

chunky_image = ChunkyPNG::Image.from_file("test.png")

width = chunky_image.dimension.width
height = chunky_image.dimension.height
(0..width - 1).each do |x|
  (0..height- 1).each do |y|
    red = ChunkyPNG::Color.r(chunky_image[x,y])
    green = ChunkyPNG::Color.g(chunky_image[x,y])
    blue = ChunkyPNG::Color.b(chunky_image[x,y])
    p "red: " + red.to_s
    p "green: " + green.to_s
    p "ble: " + blue.to_s
  end
end

Answer 1

You need to the rgba value instead of the rgb value.

if r,g,b is 0 and a is 255 it is solid black

if r,g,b is 0 and a is > 0 but < 255 it is still black but more or less transparent

if a is 0, no matter what r,g,b it is full transparent (no color in your words)

So what you miss is the Color.a(pixel) value. Hopefully your picture has COLOR_TRUECOLOR_ALPHA