Line drawing algorithm

A line drawing algorithm is a graphical algorithm for approximating a line segment on discrete graphical media. On discrete media, such as pixel-based displays and printers, line drawing requires such an approximation (in nontrivial cases).

On continuous media, by contrast, no algorithm is necessary to draw a line. For example, oscilloscopes use natural phenomena to draw lines and curves.

The Cartesian slope-intercept equation for a straight line is Y= mx+b With m representing the slope of the line and b as the y intercept. Given that the two endpoints of the line segment are specified at positions (x1,y2) and (x2,y2). we can determine values for the slope m and y intercept b with the following calculations,

m=(y2-y1)/(x2-x1)

so, b=y1-m.x1

A naïve line-drawing algorithm

dx = x2 - x1
dy = y2 - y1
for x from x1 to x2 {
        y = y1 + (dy) * (x - x1)/(dx)
        plot(x, y)
}

It is assumed here that the points have already been ordered so that x₂ > x₁. This algorithm works just fine when dx > = dy (i.e., slope is less than or equal to 1), but if dx < dy (i.e., slope greater than 1), the line becomes quite sparse with lots of gaps, and in the limiting case of dx = 0, only a single point is plotted.

The naïve line drawing algorithm is inefficient and thus, slow on a digital computer. Its inefficiency stems from the number of operations and the use of floating-point calculations. Line drawing algorithms such as Bresenham's or Wu's are preferred instead.

List of line drawing algorithms

The following is a partial list of line drawing algorithms:

• Digital Differential Analyzer (graphics algorithm) — Similar to the naive line-drawing algorithm, with minor variations.
• Bresenham's line algorithm — optimized to use only additions (i.e. no divisions or multiplications); it also avoids floating-point computations.
• Xiaolin Wu's line algorithm — can perform anti-aliasing

References

Fundamentals of Computer Graphics, 2nd Edition, A.K. Peters by Peter Shirley