Count black pixels from a region of the image

Question

I have the following question:

I have a script that can select the coordinates of an area of an image:

https://jsfiddle.net/zauk0qLk/2/

Whereas I have an image binária (black and white), how do I count the amount of black pixels in a given area of that image.

For example, I want to know the amount of black pixels in the following image area:

{"se":[175,145],"sd":[229,145],"ie":[175,221],"id":[229,221]}

Any idea how I can do that?

Answer 1

If you’re going to do it in php try something like:

$numeroDePixels = abs(($se[0]-$id[0])*($se[1]-$id[1]));

In this code we have:

($if[0]-$id[0]) as the width of the area.

($if[1]-$id[1]) as area height.

Area = Width * Height

And we use asb() to prevent the result from being negative.

Answer 2

If I understand correctly, to count only the pixels niggas from the image area, you need to scan each pixel of the area by checking whether it is black or not. To manipulate the image, it needs to be on a canvas. https://stackoverflow.com/questions/8751020/how-to-get-a-pixels-x-y-coordinate-color-from-an-image

var canvas = document.createElement('canvas');
canvas.width = img.width;
canvas.height = img.height;
canvas.getContext('2d').drawImage(img, 0, 0, img.width, img.height);

var contador = 0;
for(var i = pos.top; i < pos.bottom; i++)
    for(var j = pos.left; j < pos.right; j++){
        var pixelData = canvas.getContext('2d').getImageData(i, j, 1, 1).data; //valor de cores do pixel i,j em RGBA 
        if(pixeldata[0] == 0 && pixeldata[1] == 0 && pixeldata[2] == 0) //R, G e B == 0
            contador++;
    }

PS: I did not test, I appreciate if you can test and give feedback