How do I draw a line?

You can use the Windows GDI to draw a line as DirectDraw doesn't provide a line drawing function. If you want to draw a line using DirectX then use Breshenham's Line Drawing Algorithm to draw lines by joining Pixels.

The pseudocode to draw a line between (x1, y1) and (x2, y2)

a) Assume x=x1 and y=y1. Plot (x, y) Pixel

b) Find the largest among (x2-x1) and (y2-y1) (To find the line is predominant on which axis)

c) If(x2-x1) is the biggest (Line is predominant on X Axis)

   {    
     Loop till x becomes x2
         a. if (x2-x1) is positive then Increment x by 1 
            else Decrement x by 1
         b. Find corresponding y as (int)((y2-y1)/(x2-x1))*x) 
         c. Plot (x, y) Pixel
   }
   else /* The line is Predominant on Y Axis */
   {
     Loop till y becomes y2
         a. if (y2-y1) is positive then Increment y by 1 
            else Decrement y by 1
         b. Find corresponding x as (int)((x2-x1)/(y2-y1))*y) 
         c. Plot (x, y) Pixel
   }

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