How to Draw Radial Raster Lines

I am analyzing a photograph and trying to detect the edge of an ellipse. My strategy is to draw radial lines from approx. the center and then look within a predetermined range. The image is a .jpg, and therefore has pixels. I can draw the lines, but unfortunately, they are not raster, they are vector. Is there a way I can change this so I can plot and analyze the intensity of different colors? This is my code so far:

for i = -70:10:80    
  lineLength = 2000;
  angle = i;
  centerX(1) = 2393;
  centerY(1) = 1690;
  centerX(2) = centerX(1) + lineLength * cosd(angle);
  centerY(2) = centerY(1) + lineLength * sind(angle);
  hold on; % Don't blow away the image.
  plot(centerX, centerY);
  grid on;  
end