All points above a line

edit

Nice piece of code @Tetragramm !

I modified your code by "randomly" swapping the elements until it works as dealing with the 2 frame origins gave me some headaches and I didn't manage to make work your original code with the different line equations.

I also added my "brute force approach".

The result with your (modified) approach:

The result with pt1(20, 15), pt2(600, 385) in normal 2D frame:

The result with pt1(50, 421), pt2(621, 77) in normal 2D frame:

The result with pt1(0, 600), pt2(621, 77) in normal 2D frame:

The result with pt1(0, -50), pt2(621, 253) in normal 2D frame:

The corresponding code:

#include <iostream>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>


//@link: http://answers.opencv.org/question/87979/all-points-above-a-line/
//@author: Tetragramm
void drawPoints(cv::Mat &img, const double m, const double b)
{
    //Optimal way
    int rows = img.rows, cols = img.cols;
    if(m > 0)
    {
        for(int y = 0; y < MAX(b,rows); ++y)
        {
            for(int x = MAX(0, (y-b)/m); x<cols; ++x)
            {
                //Do Stuff Here
                img.at<cv::Vec3b>(y, x) = cv::Vec3b(255,0,0);
            }
        }
    }
    else if(m == 0) {
        //Trivial case
    }
    else
    {
        for(int y = 0; y < MIN(b, rows); ++y)
        {
            for(int x = 0; x < MIN(cols, (y-b)/m); ++x)
            {
                //Do Stuff Here
                img.at<cv::Vec3b>(y, x) = cv::Vec3b(255,0,0);
            }
        }
    }
}

void calculateLineEquation(const cv::Point &pt1, const cv::Point &pt2, double &a, double &b)
{
    a = (pt2.y - pt1.y) / (double) (pt2.x - pt1.x);
    b = pt1.y - a*pt1.x;
}

void myDrawPoints(cv::Mat &img, const double a, const double b)
{
    //BruteForce
    for(int i = 0; i < img.rows; i++)
    {
        for(int j = 0; j < img.cols; j++)
        {
            double x = j, y = img.rows - i;
            if(a * x + b - y > 0)
            {
                img.at<cv::Vec3b>(i, j) = cv::Vec3b(255,0,0);
            }
            else if(a * x + b - y < 0)
            {
                img.at<cv::Vec3b>(i, j) = cv::Vec3b(0,255,0);
            }
        }
    }
}

int main()
{
    //Coordinates in regular 2D frame
    //int x1 = 20, x2 = 600;
    //int y1 = 15, y2 = 385;
    //int x1 = 50, x2 = 621;
    //int y1 = 421, y2 = 77;
    //int x1 = 0, x2 = 621;
    //int y1 = 600, y2 = 77;
    int x1 = 0, x2 = 621;
    int y1 = -50, y2 = 253;
    cv::Mat img1 = cv::Mat::zeros(480, 640, CV_8UC3), img2 = cv::Mat::zeros(480, 640, CV_8UC3), 
        img3 = cv::Mat::zeros(480, 640, CV_8UC3);
    cv::Point pt1(x1, img1.rows - y1), pt2(x2, img1.rows - y2);

    cv::line(img3, pt1, pt2, cv::Scalar(0,0,255), 1);

    double a = 0.0;
    double b = 0.0;

    calculateLineEquation(pt1, pt2, a, b);
    std::cout << "Line equation in image space, slope a=" << a << " ; ordinate at origin b=" << b << std::endl;
    drawPoints(img1, a, b);

    calculateLineEquation(cv::Point(x1,y1), cv::Point(x2,y2), a, b);
    std::cout << "Regular 2D frame, slope a=" << a << " ; ordinate at origin b=" << b << std::endl;
    myDrawPoints(img2, a, b);

    cv::imshow("img1", img1);
    cv::imwrite("points_above_line.png ...