Retrieving pixel color

OpenGL OpenGL: Basic Coding
1

I am trying to get the value of each rbg value of each pixel. But each value returns the exact same thing. The code I am using is seriously not optomised, considering for each pixel it checks every pixel every time. But I was just using this to try the concept. But like i said, it is constantly returning the same value, any one possibly know why, or what i am doing wrong.

int width = 800;
	int height = 600;
	for (int i = 0; i < height; i++)
	{
		for (int j = 0; j < width; j++)
		{
			unsigned char pick_col[3];
			glReadPixels(j, i, screenWidth, screenWidth, GL_RGB, GL_UNSIGNED_BYTE, 
pick_col);`
			cout << (float)pick_col[0]/255 << " " << (float)pick_col[1]/255 << " " << 
(float)pick_col[2]/255 << endl;
		}
	}

I have also tried using

GLubyte pick_col[800 * 600 * 4];
 int width = 800;
	int height = 600;
	for (int i = 0; i < height; i++)
	{
		for (int j = 0; j < width; j++)
		{
			glReadPixels(j, i, screenWidth, screenWidth, GL_RGBA, GL_UNSIGNED_BYTE, 
pick_col);
			cout
				<< (float)pick_col[i*width * 4 + j * 4] << " "
				<< (float)pick_col[i*width * 4 + j * 4 + 1] << " "
				<< (float)pick_col[i*width * 4 + j * 4 + 2] << endl;
		}
	}
2

The third and fourth parameters (width and height) should both be 1 if you’re reading a single pixel.

But if you’re reading the entire screen, there’s no need for a loop, just one call to glReadPixels with a large enough buffer.

3

it is the entire screen so changed it to work with that, thank you for that. But now getting some strange values. Think that has something to do with getting the value and converting it to a float though.