Hi,
In an image (0,0) co-ordinate is at top-left and in cartesian co-ordinate system it is at bottom left. That is why you are getting different orientation when you plot with same co-ordinates in both. In images 'y' co-ordinate grows in opposite direction to that of normal cartesian co-ordinate system. Just flip the y co-ordinate values while plotting in cartesian plot and you will get the correct orientation.
canvas = zeros(320, 320);
I = uint8(canvas);
imshow(I)
a_x = 122.6544;
a_y = 234.9782;
b_x = 165.9290;
b_y = 126.9200;
hold on
plot([a_x,b_x],[a_y,b_y] )
%Plot cartesian coordinate system
figure()
plot([a_x, b_x], [-a_y, -b_y])
xlim([0 320])
ylim([-320 0])
axis equal
grid on
Hope this helps
