Count how many pixels of a color the image has

Question

I am working on a solution for tattoo studios. I would like to know how it would be possible to determine how many pixels a certain color exists in the image inserted in pictureBox1.

In this image, for example, would it be possible to count only the diamond, that is, not counting the white space? or count only the white space (so it would be possible to subtract the value of the total area)?

Also show the count result in a textbox.

private void button3_Click(object sender, EventArgs e)
    {

        Form2 form = new Form2();

        form.Show();
        OpenFileDialog file = new OpenFileDialog();
        file.Filter = "jpg|*.jpg|png|*.png";
        if (file.ShowDialog() == DialogResult.OK)
        {
            form.pictureBox1.ImageLocation = file.FileName;
            var count = 0;
            var searchColor = Color.FromName("SlateBlue");
            for (var x = 0; x < form.pictureBox1.Image.Width; x++)
            {
                for (var y = 0; y < form.pictureBox1.Image.Height; y++)
                {
                    Color pixelColor = form.pictureBox1.Image.GetPixel(x, y);
                    if (pixelColor == searchColor)
                    {
                        count++;
                    }
                }
            }

It did not work. He does not accept ". Getpixel"

Answer 1

EDIT: Another question came up with a similar problem, but using Java. So I essentially translated the code I created here (and even used the same example). If by chance any reader eventually need a solution also in the Java language, please reference this answer.

Well, since your image is not captured from the real world (a very important detail that you didn’t put in the original question), it’s much simpler to use a method like the limiarization to do the job automatically. The limiarization (thresholding in English) is a process in which you transform any image into a binary image (containing only two colors) replacing all pixels in the original image with one of the binary colors depending on their brightness being greater or less than a defined threshold.

As in your case the tattoo region is very distinct from the background, an average threshold value (0,5) is enough for an interesting result. But note that you may need to adjust it for a better effect on other images. The comparison is made with the brightness in the image, and the lower the brightness the closer the color is to black (so the code only counts the pixels in which the brightness is minor than the threshold).

Here is an example program:

using System;
using System.Drawing;
using System.Windows.Forms;

namespace Teste
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog file = new OpenFileDialog();
            file.Filter = "jpg|*.jpg|png|*.png";
            if (file.ShowDialog() == DialogResult.OK)
            {
                pictureBox1.ImageLocation = file.FileName;
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            var limiar = 0.5;

            var count = 0;
            Bitmap img = new Bitmap(pictureBox1.Image);

            for (var x = 0; x < img.Width; x++)
            {
                for (var y = 0; y < img.Height; y++)
                {
                    Color pixelColor = img.GetPixel(x, y);
                    if (pixelColor.GetBrightness() < limiar)
                    {
                        count++;
                        img.SetPixel(x, y, Color.Red);
                    }
                }
            }
            pictureBox1.Image = img;

            var percent = ((float) count / (img.Width * img.Height)) * 100;
            label1.Text = String.Format("A tattoo contém {0:d} pixels (ou seja, uma área equivalente a {1:f}%)", count, percent);
        }
    }
}

The home screen has the Picture Box, two buttons (one to load the image and display it and one to process it) and a label to display the result:

After processing, the code not only counts the pixels (and also the percentage, a value that is more effective because it is normalized independently to the dimensions of the image) but also changes the pixels of the tattoo to red to effectively demonstrate what has been calculated. Upshot:

Note: The processing uses the class Bitmap not the Picture Box to access the raw pixels.

Answer 2

As the question has no details, I will answer without details. It would be some more or less like this.

var count = 0;
var searchColor = Color.FromName("SlateBlue");
for (var x = 0; x < pictureBox1.Image.Width; x++) {
    for (var y = 0; y < pictureBox1.Image.Height; y++) {
        Color pixelColor = pictureBox1.Image.GetPixel(x, y);
        if (pixelColor == searchColor) count++;
    }
}

I put in the Github for future reference.