GML Getting the % of the drawn area of a surface

[Dawn]

Let's say

1) There's an empty (or white) area/surface.

2) Clicking on any point inside it will draw a circle (or any shape) of a certain size on it.

3) The screen shows (filled area/whole area)

So it's like a paint brush and the system need to get the % of the painted area. Through what method can I achieve this?

 

[NightFrost]

You know the positions and other measurements, so you can calculate the size of the drawn area, and from there the percentage it takes of the entire area. A rectangle draw to (10, 10, 20 ,20) is 10x10 or 100 pixels, so on a 100x100 draw area that would be 100 / 10000 = 0.01 or 1%.

 

[chamaeleon]

Let's say

1) There's an empty (or white) area/surface.

2) Clicking on any point inside it will draw a circle (or any shape) of a certain size on it.

3) The screen shows (filled area/whole area)

So it's like a paint brush and the system need to get the % of the painted area. Through what method can I achieve this?

Copy the surface to a buffer and iterate through it, counting the number of times a byte in a group of 4 (to account for RGBA/BGRA/whatever) is not zero (or whatever kind of threshold you intend to apply to a give pixel). No idea how inefficient it would be to run continuously (assuming you want to update the % every step, etc.) Percentage calculation should be trivial if you make that work.

Another option could be to use a shader to count for you (https://stackoverflow.com/questions/29352965/shader-for-counting-number-of-pixels, for inspiration).