Why imread makes 3D array from simple png?

I want to count squares and circles on the following png picture:

I begun my adventure with writing:

    obrazek=imread('trial.png');

I can see it created an array, but I have no idea why does it have three dimensions. How to interpret this last dimension (155x298 x3)? I guess that first two (155x298) are the size of a picture (in pixels).

Answer 1

[Image Analyst]

Because it's a color image. The only two colors in it are black and white, but it's still color (with 2 colors). If you want a gray scale image, simply extract one of the color channels:

obrazek = obrazek(:, :, 2); % Extract green channel.  Resulting image will be grayscale.

[Marcin Zyskowski]

So third dimension represents red, blue or green? And the numbers in the array represent "amount" of those colors in one picture?

[Image Analyst]

Correct.